the brilliant book of physics for elementary teachers
TRANSCRIPT
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The Brilliant Book
of Physics
for
Elementary Teachers
Scott Ziglinski
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An Elementary Engineering Production
Presents
The Brilliant Book of Physics
for Elementary Teachers
Scott Ziglinski (Title by Cian Ziglinski)
All Rights Reserved. Copyright 2012.
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Contents
Introduction 4
The Basic Approach – What? Why? 5
Part 1. Forces 6
The Ball 7
Forces 9
Three Types of Forces 9
Go-Carts 11
Unbalanced Forces 13
The Big Misconception 18
Part 2. Mechanical Energy 20
What is energy? 21
Energy: A General Definition 22
Mechanical Energy: Two Forms 22
The Pen 23
Potential and Kinetic Energy:
The Relationship 25
The Slide 26
Roller-Coasters 27
A Few More Things You Should Know 31
Bibliography 32
The Team 33
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And when I read Feynman’s description of a rose—in which he explained how he could experience the fragrance and the beauty of the flower as fully as anyone, but how his knowledge of physics enriched the experience enormously because he could also take in the wonder and magnificence of the underlying molecular, atomic, and subatomic processes—I was hooked for good. I wanted what Feynman described: to assess life and to experience the universe on all possible levels, not just those that happened to be accessible to our frail human senses. Brian Greene, The Fabric of the Cosmos
Introduction
In 2001, I began working with elementary students after spending nine years in the middle and high school classroom. In the summer of 2002, I “officially” began what has become a lifelong pursuit—getting kids excited about engineering. Why? For the simple reason that I think it’s important to get kids excited about learning and few things get kids more excited than designing and building something that moves.
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The Basic Approach -- What? Why?
Generally speaking, there are two questions that you ask students to help them
become better engineers and better scientific thinkers:
1. What happened?
2. Why did it happen?
To ask students what happened doesn’t require any background knowledge. The
why questions do. To be able to ask open ended questions, direct questions, and all those
that fall in between, it’s important that you have a fundamental understanding of two key
concepts in physics: forces and energy.
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Part 1.
Forces
Looking through the Lens
of Newton’s 1st Law
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It Mostly Revolves Around Newton’s 1st Law of Motion
90% of the questions that I ask students regarding forces have to do with
Newton’s 1st Law of Motion.* If you understand Newton’s 1st Law and what forces have
to do with it, you’ll be equipped to ask elementary students about forces.
*The other ten percent of the questions stem from Newton’s other two laws, and things like momentum, impulses, rotational motion—all material that I’ll cover in subsequent books for elementary teachers. In other words, we’ll get to those in due time.
________________________________________________________________________
Newton’s 1st Law of Motion is also known as the Law of Inertia. Inertia is an
object’s resistance to change.
Newton’s 1st Law of Motion in every day language:
1. Things tend to keep doing what they are doing.
2. Things will change what they are doing if they are pushed or pulled.
I have no doubt that at some point that you’ve heard the more scientific sounding
definition and we will get to that soon enough. First, let’s take a look at an example of
how something—a ball—continues doing what it is doing; that is, until it is pushed.
The Ball
Let’s say there’s a ball lying on the playground on Friday afternoon. On Saturday
morning, the ball is still in the same place. On Sunday morning, the ball is still in the
same place. On Monday morning the ball is still in the same place. During recess, a boy
sees the ball and kicks it. The ball rolls across the grass and eventually stops.
Friday to Monday
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What happened?
For three days the ball stayed in the same place.
Why did that happen?
Because objects have a tendency to keep doing what they are doing. The ball
was not moving and continued not to move.
Recess
Part 1. The Kick
What happened?
The ball was kicked and it rolled across the grass.
Why did that happen?
A foot pushed the ball. When it was pushed, its state of motion—not moving—
changed to moving.
Part 2. The Ball Stops
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What happened?
The ball rolled on the ground and eventually stopped. Since it was moving and
then stopped, the ball’s motion changed (from moving to not moving). Since there was a
change, we can conclude something pushed or pulled the ball.
Why did that happen?
Since the ball was moving and came to a stop, it is clear the motion of the ball
changed. Since the ball changed its motion, something must have pushed or pulled it. It
was the friction between the grass and the ball.
Forces
By definition a force is a push or pull. Let’s restate Newton’s 1st Law with the
term force:
Newton’s 1st Law of Motion:
1. Things tend to keep doing what they are doing.
2. Things will change what they are doing if a force acts on them.
From here on out, we’ll use the word force. Before going into the physics of the
go-cart, I want to take a moment to go over some different types of forces. For now, I’m
going to focus on the three types of forces involved with the gravity go-carts.
Three Types of Forces
Applied Force
An applied force is a force that is applied to an object by another object or by a
person. It is important to understand that the term “object” is being used in a very broad
sense. An object can be a solid, liquid, or gas. In other words, if a solid, liquid, or gas
pushes or pulls an object, it is an applied force.
Examples:
1. Wind (gas) pushes a leaf.
2. A wave (liquid) pushes a surfboard.
3. You pull a grocery cart.
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Use Subcategories with Elementary Students
. When I discuss applied forces with students, I break them down into
subcategories:
Applied Muscle Force
When a person pulls or pushes an object, I refer to that as an “applied muscle
force” (since it is the muscles that are actually doing the pulling or pushing).
Applied Air Force
When a student blows on an object, I refer to that as an “applied air force” (since
the air is doing the pushing).
Gravity
Simple: Gravity is that force that pulls things down.
A bit more: The force of gravity is the force with which the earth attracts an object
towards itself. By definition, this is the weight of the object.
Friction
Simple: Friction is the force that occurs when two surfaces rub against each other.
Friction is a slow down force.
A bit more: A friction force is the force exerted by a surface as an object moves across it
or makes an effort to move across it. The friction force opposes the motion of the object.
Examples:
1. It’s more difficult to run through water than it is to run through air. Why? Friction
between the water and your body slows you down. (Yes, air is also rubbing against you,
but at low speeds—such as running—there isn’t much air friction. Now, if you are
traveling on a plane, that’s a whole different story.)
2. When you push a book across a table, the surface of the table rubs against the surface
of the book and slows the book down.
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Go-Carts
Let’s look at the forces involved with the go-carts the students made. (The
assumption here is you’ve seen me work with the kids on go-carts. If you haven’t, I still
think you’ll be able to follow along.) Three things happened at the table with the track
on it.
What happened?
1. Student lifted the go-cart from the table and placed it on top of the ramp.
Why did that happen?
The go-cart was at rest on the table. A student used an applied muscle force to
lift the go-cart to the top of the ramp. Had a force not acted on the go-cart, it would have
remained motionless on the table. A force did act on the go-cart and that force caused the
go-cart to change its motion.
What happened?
2. Student released the go-cart and it started moving down. As it moved down the ramp,
it sped up.
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Why did that happen?
The go-cart was at rest at the top of the ramp. When the student released the go-
cart, it started moving. Since the motion of the go-cart changed, we know that a force
made it change its motion. That force was gravity.
Note: If you’re thinking that frictional forces are at work here, you are absolutely right. I’m not ignoring that. I’m just waiting until we go over unbalanced forces to discuss what’s going on when gravity is working in one direction and friction is working in the other. I’m trying to keep it simple for now by thinking about one force at a time.
What happened?
3. On the table, the go-cart slowed down and stopped.
Why did that happen?
As the go-cart went down the ramp, it sped up. On the table, the go-cart began to
slow down and eventually stopped. Let’s recall what Newton’s 1st Law says. It says that
things will keep on doing what they are doing unless a force changes their motion. When
the go-cart is moving on the table, it’s going to keep moving at the same speed unless a
force causes it to change its motion. The go-cart slowed down and stopped. We can
definitely say it changed its motion. What caused it to change its motion? Friction.
With the materials the students make the go-carts with, there are three places
where two surfaces rub together:
1. Axles and straws rub against each other.
2. Straws and wheels rub against each other.
3. Wheels and table rub against each other.
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Newton’s 1st Law: Textbook Definition (and why it’s important)
Let’s take a look at how Newton’s 1st Law of Motion is presented in a textbook:
1. An object at rest will stay at rest unless acted upon by an unbalanced force.
2. An object in motion will stay in motion in a straight line at a constant speed unless
acted upon by an unbalanced force.
Unbalanced Forces
So far, we’ve been thinking about Newton’s 1st Law like this: If an object’s
motion changes, a force is the reason. In other words, “Blame the force for causing the
change.” As you can see in the textbook definition, it isn’t a force that causes the
change—it is an unbalanced force.
Unbalanced Forces -- The Bigger One Wins
Let’s say there are two twin brothers, Padraig and Eamon, competing in a tug-of-
war event. There’s a flag in the middle of the rope. If both brothers pull with equal
force, will the flag move? No. Why? Because they are pulling in the opposite direction
with an equal amount of force. Here we have two forces, but because they cancel each
other out, the flag keeps doing what it is doing. When forces cancel each other out, they
balance each other out. Forces that balance each other out have no effect on an object’s
motion.
Note: Even though it may not appear so, the applied muscle force arrows are the same
size—really. ☺
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Let’s say Darragh joins in to help Padraig. What will happen? The flag will be
pulled towards Darragh and Padraig. Why? Because the forces are no longer balanced.
Padraig and Darragh together apply a bigger force than Eamon.
Objects at Rest and Unbalanced Forces
The Book
Balanced Forces
Let’s say there’s a book on a table. It’s at rest. Does this mean no forces are
acting on the book? No. It just means that there are balanced forces acting on the book.
Gravity is pulling down and what is called a “normal” or “support” force is pushing up—
the upward force applied by the table. The forces are balanced and therefore cancel each
other out.
Applied Muscle vs. Friction
Let’s say you gently tap the book with a rightward force. The book doesn’t move.
It continues doing what it is doing—it stays at rest. Wait a second. You just pushed it
with your finger. Why didn’t it move? Because there was a friction force pushing it
leftward. How strong was the friction force? It was equal to the applied muscle force.
The forces balanced each other out so the book continued to stay at rest.
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Unbalanced Forces
Let’s say you push the book as hard as you can in a rightward direction. Does it
move? Oh yes. Was there friction? Yes, but not enough to balance out the applied
muscle force. Because there is an unbalanced force, the book changes its motion—from
not moving to moving.
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Objects in Motion and Unbalanced Forces
Pushing a Grocery Cart
Let’s say you and your friend go out to the desert and find a stretch of abandoned
road that is a mile long. The road is perfectly straight and there is no wind. (For the
point of this example, let’s ignore air resistance since it would be incredibly small.) You
have a speed gun and your friend is going to push the cart.
1. At the beginning, the grocery cart is not moving. It is at rest and will remain at rest
unless an unbalanced force acts on the go-cart.
2. Your friend starts pushing the grocery-cart. Two forces are in play here. The friction
force (between the road and the wheels) and the applied muscle force. Since the applied
muscle force is greater than the friction force, the grocery cart moves in the direction
your friend is pushing it—the forces are unbalanced.
3. Your friend speeds up and hits a maximum speed of 7 mph. Your friend is in great
shape and is able to keep running as fast as she can for the next half mile. What forces
are in play here? Friction and applied muscle.
Important things to consider:
a. The grocery cart is moving at a constant speed of 7 mph.
b. The grocery cart is moving in a straight line.
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How does the friction force compare to the applied muscle force in terms of magnitude
(how big each is each one)? They are equal. They balance each other out. Think of it
like this:
a. If the applied muscle force was greater than the friction force, the forces wouldn’t be
balanced; consequently, the go-cart would speed up.
b. If the friction force was greater than the applied muscle force, the forces wouldn’t be
balanced; consequently, the grocery cart would slow down.
c. Because they are equal and acting in opposite directions, they are balanced;
consequently, there is no change in the motion of the go-cart. The go-cart is moving at a
constant speed of 7 mph. As long as the two forces are equal, the motion of the grocery
cart won’t change—meaning, it will keep going in the same direction at the same speed.
Unbalanced Forces: Go-Cart
Now let’s think about the go-cart in terms of unbalanced forces.
What happened?
Student released the go-cart and it started moving down. As it moved down the
ramp, it sped up.
Why did that happen?
There were two forces in play here: gravity and friction. Because the gravity
force was stronger than the friction force, the go-cart went down (the direction gravity
pulls).
What happened?
On the table, the go-cart slowed down and stopped.
Why did that happen?
Once the go-cart was on the table, there was only one force acting on the go-cart:
friction. When there is only one force acting on an object, there must be an unbalanced
force (there’s nothing to balance it out). Since friction is a slow down force, the go-cart
slowed down and stopped.
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The Big Misconception
If we spent part of our lives on earth and part in space, we would very likely have
no problem understanding the following: A force is NOT required to keep an object
moving. A force IS required to slow an object down (just as a force is required to speed
an object up). Many people, especially kids, think that something is needed to keep
something going.
Earth: Grocery Cart
Let’s return to the grocery cart example. Remember, your friend is on a straight
road in Arizona. Let’s say your friend releases the grocery cart. Will it slow down?
Yes. The moment she released it, it began slowing down. Why?
1. Before she released it there were two forces that balanced each other out: friction and
applied muscle.
2. After she released it, the only force acting on the grocery cart was friction.
3. Since only one force was acting on it, there was an unbalanced force.
4. An unbalanced force acting on an object means the object’s motion will change.
5. In this case, the unbalanced force was friction and friction is a “slow down” force, so
the grocery cart slowed down.
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Space: Grocery Cart
Let’s say you and your friend take a trip deep into space. You both get out of the
spaceship, along with your grocery cart. Your friend is standing on the side of the
spaceship and pushes the grocery cart away from herself.
Question: After she releases the grocery cart, what forces are acting on it?
Answer: With the exception of an infinitesimally small amount of gravity—and I mean
small—no forces would be acting on the go-cart.* There is no friction between the
wheels and the road because there is no road. There is no air friction (air drag) because
there is no air. In other words, there is nothing to slow the grocery cart down. *To say there is absolutely no gravity in deep space isn’t true. It may be 1/100,000,000,000 of the gravitational pull on earth, but it still exists. It’s so small, however, you wouldn’t move much in a decade.
Question: So, if the grocery cart is moving at a speed of 5 mph after it was released from
your friend’s hand, what will happen to the grocery cart in terms of its motion?
Answer: It will move in a straight line at 5 mph until a force causes it to change its
motion. It will, in other words, keep doing what it is doing.
______________________________________________________________________
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Part 2.
Mechanical Energy
Two Forms Students
Already Know
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A Small Jog: Eugene to New Orleans
Let’s say you decide to jog from Eugene to New Orleans. Will you need to eat
during your 2,500 mile trek? Most definitely. Why? Because in order to do any
physical task, you need energy.
Now, let’s think about what actually gets you to New Orleans. You’ll get to New
Orleans by taking lots of steps. Each time you take a step, you push against the ground
(and the ground pushes against you—an example of Newton’s 3rd Law of Motion).
When you use your body to push against something, you are using an applied muscle
force. Essentially, the long jog to the Big Easy is a series of applied muscle forces.
Let’s consider what was just said:
1. If you are going to jog from Eugene to New Orleans, you will need energy.
2. It is a series of applied muscle forces that will cause the movement. (Because you
can’t slide on a frictionless surface to New Orleans, you have to keep taking steps.)
The first point—you need energy to exercise—is something you’ve known since
you were a kid. The second point—forces cause the movement—is something that you
now understand after reading part one of the book (at least I hope so).
If you find yourself getting confused about energy, particularly when it comes to
what energy has to do with motion, refer back to the Eugene to New Orleans example.
I’m not suggesting it will clarify everything; however, it will be useful. Why might you
get a bit confused about energy? Because energy is an abstract concept that confuses the
most brilliant physicists.
On that note, let’s talk about energy.
What is energy? No one can say.
In all likelihood you have an idea what energy is, and in all likelihood you aren’t
so confident to bet your house on your understanding of it. There’s a good reason for
that and it’s NOT because of your background in physics. It’s because no one really
understands just what energy is. It’s true. Here’s what Richard Feynman, one of the most
brilliant physicists of the twentieth century, said about energy: “It’s important to
realize that in physics today, we have no knowledge of what energy is. We do not
have a picture that energy comes in little blobs of a definite amount.”
My point here is to let you know that if find yourself getting confused about
energy, you are in good company.
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Energy: A General Definition
Since energy is not completely understood, should we avoid a definition of
energy? Certainly not. Instead, we should provide a general, yet scientifically accurate
definition (as accurate as we can hope to get with something that is not completely
understood). Let’s start with a textbook definition and work our way from there.
1. Textbook definition: Energy is the ability to do work.
What that means: If you want to move something, you need energy.
2. Dave Watson definition: Energy is the ability to make something happen.
What that means: If you want to make something happen, you need energy.
3. Scott Ziglinski definition: Energy is the stuff needed to make a change.
What that means: If want you to change something, you need energy.
All three of these definitions are fine. In class, I normally go with mine or Dave
Watson’s. When I work with sixth class students—11 to 13 year-olds—I do mention the
textbook definition along with the other two.
Mechanical Energy: Two Forms
While energy may not be something that is completely understood, engineers and
physicists agree that energy comes in two forms: potential and kinetic.
At present, 98% of my program focuses on mechanical energy (the other 2% is
electrical and magnetic). Mechanics is the study of motion, so as you might guess
mechanical energy is the type of energy that is needed for motion. With elementary kids,
I focus on the type of mechanical potential energy that has to do with lifting something in
the air. That type of energy is called gravitational potential energy. From here on out,
whenever I say potential energy, please be aware that I mean gravitational potential
energy.*
*Another type of mechanical potential energy is elastic potential energy. Pull a rubber-band back, and you’ve got elastic potential energy.
Potential Energy – Stored energy based on an object’s position.
What that means:
1. When an object is lifted off the ground, it gains potential energy.
2. The higher an object is lifted, the more potential energy it has.
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The Pen
a. Cian sees a pen. It is on the ground. Since it is on the ground, it does not have
potential energy.
b. Cian lifts the pen up to his waist. The pen has potential energy. Why? It’s off the
ground.
c. Cian lifts the pen up to his lips. The pen has more potential energy than when he lifted
it up to his waist. Why? Because the higher something is lifted, the more potential
energy it has.
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d. Cian lifts the pen above his head. The pen has more potential energy than when he
lifted it up to his lips. Why? Because the higher something is lifted, the more potential
energy it has.
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Kinetic Energy -- Energy of Movement.
What that means:
1. Anything that is moving has kinetic energy.
2. The faster something is moving, the more kinetic energy it has.
The Pen Drops
a. Cian sees the pen on the ground. It does not have kinetic energy. Why? Because
it’s not moving.
b. Cian lifts the pen to his waist and drops it. It falls to the ground. While it’s falling, it
has kinetic energy. Why? Because it’s moving.
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c. Cian lifts the pen to his lips and drops it. It falls to the ground. While it’s falling it
has kinetic energy. Why? Because it’s moving.
d. Cian lifts the pen over his head and drops it. It falls to the ground. While it’s falling,
it has kinetic energy. Why? Because it’s moving.
In which case did the pen gain the most kinetic energy? When he dropped it from
above his head. Why? Because the longer something is falling, the more it will speed up
(unless it reaches terminal velocity, but that’s not going to happen in the classroom). Important: Kinetic energy is not speed. Speed (actually velocity, which is speed in a direction) is part of the kinetic energy equation; therefore, the more speed an object has, the more kinetic energy it has and the less speed it has, the less kinetic energy it has. ----------------------------------------------------------------------------------------------------------------
Potential and Kinetic Energy: The Relationship
The Law of Conservation of Energy says,
Energy cannot be created or destroyed; however, it can be converted from one
form to another form.
Let’s consider what happens when Cian goes down the slide to see the Law of
Conservation of Energy in action.
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The Slide
a. Cian is sitting at the top of the slide. Because he is off the ground, his body has
potential energy. Because he is not moving, his body has no kinetic energy.*
Cian’s total mechanical energy:
PE (potential energy): 100%
KE (kinetic energy): 0%
*We’re going to view Cian’s body like we’d view an innate box—meaning, we aren’t going to worry
about the things going on inside of his body.
b. ¼ of the way down the slide, Cian has lost ¼ (or 25%) of his potential energy. Why?
The top of the slide is the highest point; therefore, that is where Cian has the greatest
amount of potential energy. Since the highest point is where Cian has the greatest
amount of potential energy, it follows that as he slides down, he’ll lose potential energy.
Since he’s ¼ of the way down, he’s lost ¼ of the potential energy. Where did it go? The
Law of Conservation of Energy says it cannot be destroyed—meaning, the energy must
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have gone somewhere. As he slides down, he is moving. What kind of energy does Cian
have if he’s moving? Kinetic energy. Now, ¼ of Cian’s total mechanical energy is
kinetic energy.
Cian’s total mechanical energy:
PE: 75% (3/4)
KE: 25% (1/4) Note: In the slide example—as well as the roller-coasters—a small amount of the energy will be converted into heat energy due to friction. For the purpose of this example, we’ll ignore friction and the heat produced because of it.
c. ½ of the way down the slide, Cian has lost ½ (or 50%) of his potential energy. That
potential energy was converted to kinetic energy. Now, ½ of his total mechanical
energy is kinetic energy.
Cian’s total mechanical energy:
PE: 50% (1/2)
KE: 50% (1/2)
d. ¾ of the way down the slide, Cian has lost ¾ (75%) of his potential energy. The
potential energy was converted to kinetic energy. Now, ¾ of his total mechanical energy
is kinetic energy.
Cian’s total mechanical energy:
PE: 25% (1/4)
KE: 75% (3/4)
Looks like we are ready to move on to roller-coasters.
Roller-Coasters
There are three main types of roller-coasters I have students make:
1. Straight
2. With hills
3. With hills and loops Note: If you’ve witnessed one of my summer workshops you might be asking yourself, “What about wall drop-offs, chair drop-offs, and two or more marbles?” Remember I just said the three “main” types of roller-coasters. These are the ones I mostly do in class. Also, the two or more marble roller-coasters will be addressed in the next book on Transfer of Energy.
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Straight Roller-Coaster
Take a look at the straight roller-coaster. The marble is going to be released from
point A and travel down to the cup.
Question: At which point will the marble have the most potential energy? The answer
is A, the highest point. The higher something is lifted, the more potential energy it has.
Question: At which point will the marble have the most kinetic energy? This one can be
a bit trickier for people. Remember, the faster the marble is going, the more kinetic
energy it has. To answer this question, let’s consider the two forces: gravity and friction.
1. When the marble is released, the gravitational pull is stronger than the friction force
between the marble and track; therefore, there’s an unbalanced force—in this case that
means the marble is going to speed up.
2. Gravity is going to keep pulling on the marble at points B, C, and D and at each of
these points, frictional forces don’t put up much of fight—meaning, there is still an
unbalanced force in the downward direction.
3. Between D and E the track is even with the floor. Once the marble is on the floor,
gravity is no longer causing it to speed up. (On the floor the downward force of gravity
is balanced by the upward support force of the track.) Now, only friction—a slow down
force—is the only force acting on the marble. So, at point D the marble is speeding up
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and at point E the marble is slowing down. Therefore, of all the points given, the marble
is moving fastest at point D.
Law of Conservation of Energy
When Cian slid down the slide, his potential energy was converted to kinetic
energy. The same thing happens when the marble rolls down the track: potential energy
gets converted to kinetic energy.
Kinetic to Potential
So far, we’ve only discussed potential energy being converted to kinetic energy.
By analyzing hills and loops, we’ll see that kinetic energy can be converted to potential
energy.
The Hill
Now, let’s think some more about potential and kinetic energy, but this time with
a hill. We know from our last discussion that the marble will have the most potential
energy at point A and the most kinetic energy at point D. What happens to the potential
and kinetic energy of the marble as it rolls to points F and G? Let’s start with the kinetic
energy this time.
Kinetic Energy—Points F and G
When things go up, they slow down because gravity is pulling on them. At point
F the marble is losing its kinetic energy. At point G—the top of the hill—it has lost even
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more of its kinetic energy. What happened to that energy? It got converted back to
potential energy.
Potential Energy—points F and G
By the time the marble reaches point E, all of its potential energy has been
converted to kinetic energy. When it rolls up to points F and G, some its kinetic energy is
converted back to potential energy.
Potential and Kinetic Energy—point H
When the marble rolls down the hill, it speeds up—meaning, the potential energy
is converted back to kinetic energy.
Point I to J
What happens to the kinetic energy from point I to J? Friction is the only force
acting on the marble so the marble slows down. What happens to the energy? Energy,
after all, cannot be created or destroyed, but it can be converted to another form (it can
also be transferred, which we’ll address later). Rub your hands together and that ought to
give you a big clue. When things rub against each other, some of the kinetic energy is
converted to heat energy.
______________________________________________________________________
The Loop
Let’s consider what happens to the potential and kinetic energy as the marble goes
around the loop.
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Point F
As the marble goes up, some of the kinetic energy is converted to potential
energy.
Point G
More of the marble’s kinetic energy gets converted to potential energy.
Point H
The potential gets converted back to kinetic energy.
Point I
The marble has no potential energy. At this point, all of the mechanical energy is
kinetic.
Point I to J
Friction is the only force acting on the marble. The kinetic energy is converted to
heat energy.
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A Few More Things You Should Know
Now that we’ve gone over potential and kinetic energy, let’s go into a little more
detail.
Potential Energy -- Height and Mass
You know that the higher an object is raised, the more potential energy it has. This
means that potential energy is dependent on the height of an object. Let’s say a bowling
ball was raised three feet in the air and a marble was raised three feet in the air. Do they
have the same amount of potential energy? If you’re thinking the answer is no, you’re
absolutely right. Let’s face it, it takes a lot more energy to lift a bowling ball than it does
a marble. Why? Because the bowling ball has more mass. Potential energy is not only
dependent on height, it’s also dependent on mass. What’s mass? Next topic.
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Mass and Weight – The Lego Tower
Let’s say you have a tower made of ten Legos and you go off to explore nearby
planets. You go to Venus with the tower. How many Legos is the tower made up of
now? Ten. You go to Jupiter. How many Leggos is the tower made up of now? Ten.
You go to Mars. How many Legos is the tower made up of now? Ten. The number of
Leggos doesn’t change.
Mass – The number of parts something is made of. Mass does not change.
If you weighed the tower on all these planets, would it weigh the same on each
planet. No it would not. Weight depends on gravitational pull. Since the gravitational
pull is bigger on Jupiter than Mars, the tower will weigh more on Jupiter than Mars.
Weight -- The gravitational pull on an object’s mass.
Mass and Weight – The Relationship
The more mass an object has at a specific location, the more it weighs. A bowling
ball on earth has more mass than a marble on earth; therefore, the bowling ball weighs
more on earth than a marble on earth. A bowling ball on the moon has more mass than a
marble on the moon; therefore, the bowling ball weighs more on the moon than a marble
on the moon.
Use Weight, Not Mass with Elementary Kids
We live on earth and kids get weight. Leave mass for middle school. It’s fine to
say that a bowling ball has more potential energy than a marble lifted to the same height
because the bowling ball weighs more.
Kinetic Energy -- Speed and Mass
Like potential energy, mass plays a role in kinetic energy. If a bowling ball and a
marble are moving at the same speed, the bowling ball has more kinetic energy. Why?
Because the bowling ball has more mass.
Focus on Height and Speed
Both potential energy and kinetic energy are dependent on mass, but it’s not
something I ever bring up in class. The big idea is to focus on the height, the speed, and
what one has to do with the other when talking about mechanical energy. Keep it simple
because there’s plenty to think about.
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Bibliography
1. Conceptual Physics – 9th Edition, Hewitt
2. The Physics Classroom: www.physicsclassroom.com
3. FT Exploring (Dave Watson): www.ftexploring.com
4. Mr. Stanbrough’s Physics Class: www.batesvilled.k12.us/physics
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The Team
Author: Scott Ziglinski is the founder of Elementary Engineering.
Over the past seventeen years, he has worked with over
approximately 100,000 elementary students and over 1000
teachers.
Science Editor: Quinton Tyler is a long time friend and physics
mentor of the author. He is always willing to receive calls early in
the morning to discuss energy, forces, and anything else Scott has
been thinking about. Quinton teaches physics in Dubai.
General Editor: Joan Mahony is a high school English teacher in
Cork City, County Cork. She claims she knows nothing about
science—that is, until she edits Scott’s books.
.
Website: www.elementary-engineering.com
Facebook: Elementary Engineering
Twitter: Elementay_Eng