ps notes forces
TRANSCRIPT
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Newton’s Laws of Motion
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Force Notes
Newton’s Laws
So far, we have discussed displacement, velocity, and acceleration (all ways to describe
motion). In this unit we will discuss forces, which explain why objects move.
In the 17th century, Sir Isaac Newton discovered animportant relationship in physics – the relationship
between force, mass, and acceleration.
His 3 laws (Newton’s Laws of Motion) explain why
objects move (or don’t move) as they do.
Force – A push or a pull.
Ex: gravity, magnetic, electrical, or a physical push/pull. Whenever there is an interaction between
two objects, there is a force upon each of the objects.
Force is a vector quantity (it has direction). So, wecan represent forces with arrows.
Types of Forces
All forces (interactions) between objects can be placed into two broad categories:
• contact forces (push, pull, friction, etc)• forces resulting from action-at-a-distance (gravity, magnetism)
Type of Force Description of Force
Force by a person or another object (A push
or pull)
(also known asWeight)
Fgrav
Force of the earth attracting another object towardsitself. Objects on Earth experience a force of gravity,
which pulls "downward" towards the center of the
earth.
5NUnit for force is
Newtons (N)5N5N
Multiple forces mayact upon the same
object5N1N
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Newton’s Laws of Motion
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Fnorm
The support force exerted upon an object which is incontact with another stable object. Often balances out
gravity.
Ffrict
Force exerted by a surface as an object moves across it
or tries to move across it.
Fair
Special frictional force which acts upon objects as theytravel through the air.
Ftens
Force transmitted through a string, rope, or wire when
it is pulled tight by forces acting from both ends.
Fspring
Force exerted by a compressed or stretched spring
upon any object which is attached to it.
Free Body Diagram – Drawing that shows all forces acting on an object.(Object may be represented as a “dot”
Draw the following free-body diagrams in your notes:
1. book is at rest on a table top.2. A girl is suspended motionless from the ceiling by two ropes.
3. An egg is free-falling from a nest in a tree. Neglect air resistance.
4. A flying squirrel is gliding from a tree to the ground at constant velocity.Consider air resistance.
5. A rightward force is applied to a book in order to move it across a desk with a
rightward acceleration. Consider frictional forces.
6. A rightward force is applied to a book in order to move it across a desk atconstant velocity.
Name forces based upon what is cau
the force.
Fgrav = Force of Gravity
F p = Force of Push
Fnorm = Normal Force (opposes gravit
F pull
Fg
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Newton’s Laws of Motion
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Determining Net Forces
Add forces just like all other vectors.- Same direction: add
- Opposite direction: subtract
Q: What is the net force (Fnet) acting on each of these blocks?
6N
4N
3N
Free-body diagrams for four situations are shown below. The net force is known for each situation.
However, the magnitudes of a few of the individual forces are not known. Analyze each situationindividually and determine the magnitude of the unknown forces.
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Newton’s Laws of Motion
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Newton’s First Law: An object at rest tends to stay at rest and an object in motion tends
to stay in motion with the same speed and in the same direction unless acted upon by
an unbalanced force.
This is often called the law of inertia. Inertia is the tendency of an object to resistchange in its state of motion (velocity).
Balanced Forces _________ Unbalanced Forces
Fnet = 0 Fnet ≠ 0
Velocity remains the same Velocity changes (objectaccelerates)
Example: When driving in a car, how do you feel (which way do you lean) when the
car…
Action of Car Passenger Observations Why?
Goes straight, at constantspeed
Turns left
Speeds up
Slows down
Q: So why does a ball rolling along the ground eventually come to a stop? Shouldn’t it
keep going because of inertia?
Try this: Push your textbook across the surface of your desk.
What happens when you stop pushing? Why?
Q: Imagine a place in the cosmos far from all gravitational and frictional influences.Suppose that you visit that place (just suppose) and throw a rock. The rock will
a. gradually stop.
b. continue in motion in the same direction at constant speed.
Q: A 4.0-kg object is moving across a friction-free surface with a
constant velocity of 2 m/s. Which one of the following horizontalforces is necessary to maintain this state of motion?
a. 0N b. 0.5N c. 2.0 N d. 8.0 N
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Newton’s Laws of Motion
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Is a net force displayed in the following situations?
Description of MotionNet Force: Yes or
No?
Mass and Inertia
What is mass?
- Mass is a measure of the inertia of the object.- Related to the amount of matter in an object
- Measured in grams, kilograms
Mass is different from weight. Weight is _______________________________________
____________________________________________________ (measured in Newtons).
Mass and weight are related because the earth pulls more strongly on objects with more
mass, but the numbers are not the same.
Practice This:
Q: What is the weight (in Newtons) of a 7.0-kilogram bowling ball on Earth’s surface?
Objects with greater mass have greater inertia. (Picture the friction created when you push a
Styrofoam brick and a cement brick across the table with equal force.)
Weight (Fgrav) = mass x g
where g = 9.8 m/s2 on earth
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Newton’s Laws of Motion
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Q: What is the weight of a 7.0-kg bowling ball on the surface of the moon (where g=2.6)?
Q: What is the mass of a 7.0-kg bowling ball on the surface of the moon?
Q: Does a 2-kg iron brick have twice as much inertia as a 1-kg iron brick? Twice asmuch mass? Twice as much weight? Twice as much volume?
Q: Does a 2-kg iron brick have twice as much inertia as a 1-kg wooden brick? Twice as
much mass? Twice as much weight? Twice as much volume?
Newton’s Second Law
Unbalanced forces acting on an object result in acceleration. Newton’s second law tells
us the relationship between the force and the resulting acceleration.
Law 2: The acceleration of an object is directly proportional to the net force
acting on the object, is in the direction of the net force, and is inversely proportional to
the mass.
**Remember, net force is the sum of all forces acting on an object
Use the equation a = Fnet/m to fill in the chart below.
Net Force Mass Acceleration
Acceleration = =
Net Force
Mass
Fnet
m
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Newton’s Laws of Motion
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(N)(kg)
(m/s/s)
1. 10 2 ?
2. 20 2 ?
3. 20 4 ?
4. ? 2 5
5. 10 ? 10
Q: What (2) changes result in the acceleration being doubled?
Q: In a physics lab, Kate and Rob use a hanging mass and pulley system to exert a 2.45 N
rightward force on a 0.500-kg cart to accelerate it across a low-friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart's
acceleration?
Q: An applied force of 50 N is used to accelerate an
object to the right across a frictional surface. The object
encounters 10 N of friction. Use the diagram to determinethe normal force, the net force, the mass, and the
acceleration of the object. (Neglect air resistance.)
Q: An applied force of 20 N is used to accelerate an object to the right across a frictional
surface. The object encounters 10 N of friction. Use the diagram to determine the normal
force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)
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Newton’s Laws of Motion
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When acceleration is zero – Equilibrium
Balanced Forces
Object is either at rest (static) or moving at a constant speed: Dynamic EquilibriumFill in the chart below:
Test your understanding:Two students are discussing the object shown at the right. Anna Litical
suggests to Noah Formula that the object could be moving. Can you think of a scenario in which this object is moving vertically? Horizontally?
When acceleration is zero – Equilibrium
Balanced Forces
Object is either at rest (static) or moving at a constant speed: Dynamic Equilibrium
Fill in the chart below:
Q: A jumbo jet cruises at constant velocity of 1000 km/h when the thrusting force of itsengines is at a constant 100,000 N. What is the acceleration of the jet? What is the force
of air resistance on the jet?
Acceleration = __________________ Fair = __________________
Movement Static or Dynamic
Equilibrium?A hockey puck slides at
constant speed
A pen sits motionless on a
desk
Two friends push with
equal force on each other’s
hands.
Two equally strong teams
play tug-of-war. The ropedoes not move
Movement Static or Dynamic
Equilibrium?
A hockey puck slides at
constant speed
A pen sits motionless on adesk
Two friends push withequal force on each other’s
hands.Two equally strong teams
play tug-of-war. The rope
does not move
Summarize: When is zero
acceleration NOT zero
velocity?
Summarize: When is zeroacceleration NOT zero velocity?
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Newton’s Laws of Motion
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Friction: Force exerted by a surface as an object moves across it or makes an effort to
move across it.
• Opposes motion
• Depends on characteristics of 2 surfaces in contact
2 Types of Friction
1. Sliding (________________) Friction: results when an object slides across asurface (ex – pushing a box across the floor. The floor surface exerts a friction
force upon the box.)
2. _____________________ Friction: the surfaces of two objects are at rest relative
to one another. A force exists on one of the objects to set it into motion.
Ex: you push on a heavy box and it does not move. The static friction force balances
the force you exert on the box and the box remains at rest.
The harder you push an object, the greater the static friction force becomes. (Your push
must = friction if the object is not moving). This is true until the object finally beginsmoving, then the friction is kinetic.
1. A rightward force is applied to a 6-kg object to move it across a rough surface at
constant velocity. The object encounters 15 N of frictional force. Use the diagramto determine the gravitational force, normal force, net force, and applied force.
(Neglect air resistance.)
2. In a Physics lab, Ernesto and Amanda
apply a 34.5 N rightward force to a 4.52-kg cart to accelerate it across a horizontal
surface at a rate of 1.28 m/s/s. Determine the friction force acting upon the cart.
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Newton’s Laws of Motion
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Terminal velocity (Nonfree-Fall): The velocity at which acceleration terminates for
falling objects.
Air resistance acts in the direction opposite the object’s motion. (For falling objects, air resistance pushes __________)
2 Factors influence air resistance:
1. Frontal area of object How much surface area is in contact with the air. For
a feather, large frontal area and little weight slow down movement.
2. Speed of falling object The greater the speed, the more air molecules per
second the object must push through.
So, air “drag” depends on size and speed of a falling object.
An object will only accelerate when the downward force is larger. Since increased speed
= increased drag, the upward force gets bigger and bigger. Eventually, it is the samemagnitude as Fgrav.
Time
Q: Describe the changing forces acting on a skydiver from the moment they leave the
plane until they reach terminal velocity.
Terminal velocity for a skydiver ~ 200 km/h
Terminal velocity for a feather ~ few cm/s
Recall that all objects on Earth accelerate at ________________whenfalling because of gravity. This is true for heavy and light objects, as long
as they are not too influenced by air resistance (ex – a feather).
Note: a rock and a feather will fall at the same speed in a vacuum. Why?
Recall, acceleration depends on the______________________
that is acting upon an object.
1. Gravity pulls down
2. Air resistance (drag) pushes up
1. Fg > FairObject
accelerates
2. Fg > FairAccelerates more
slowly
Fg = FairNet Force = 0, constant speed
Terminal Velocity Reached
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Newton’s Laws of Motion
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Q: A man and a woman are parachuting together. The man weighs twice as much as the
woman and they have the same sized chutes. Who reaches the ground first? Why?
Q: A skydiver jumps from a high-flying helicopter. As she falls faster and faster through
the air, does her acceleration increase, decrease, or remain the same?
Newton’s 3rd Law of Motion
Force can be defined as a push or pull. More specifically, a force is an
interaction between 2 things. All forces exist in pairs.
Ex: When your book sits on a desk, it doesn’t move. Why? Are the forces balanced or
unbalanced?
Draw a free body diagram of the book. What forces are equal and opposite?
Q: On a cold, rainy day, your car battery is dead and you must push the car to get itstarted. Why can’t you push the car by remaining comfortably inside and pushing on the
dashboard?
Q: We know the Earth pulls on the Moon. Does the moon also pull on the Earth?
Law 3: Whenever one object exerts a force on a second object, thesecond object exerts and equal and opposite force on the first.
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Newton’s Laws of Motion
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Q: You hit a baseball with a bat. If we call the force exerted on the ball by the bat theaction force, identify the reaction force.
Q: When a rifle fires a bullet, a force pair exists between the bullet and the rifle. The
force exerted on the bullet is equal to the force exerted on the rifle, which is why the rifle“kicks back” when fired. Why doesn’t the rifle recoil with the same speed as the bullet?
Touching your finger to
your nose
Walking
Holding hand out the
window of a moving car
Two bumper cars
collide
A bird flaps its wing