Motion

MYP / Honors Physics

Question?

Am I in motion right now?

What evidence do you have to prove that I am in motion?

A) YesB) No

What is Motion?Motion is a change in position relative to a frame of reference.

A frame of reference is a point or object from which motion is determined.

o A frame of reference is assumed to be fixed.o The surface of the Earth is the most common frame of

reference used by humans to determine motion.

Based on your “new” physics knowledge, are you in motion right now?

It is said that all motion is “relative” because it depends on your frame of reference.

A) Yes B) No

Example 1

Let’s say I am standing on the back of a pickup truck (that is motionless), and I am throwing apples forwards. I know that I can throw an apple at exactly 15m/s every time.

If a person were standing on the sidewalk, how fast would she say the apples are moving? Since she will see them exactly the same way as

me (we're both in the same reference frame), she will say 15m/s.

Now the truck starts to move forwards at 20m/s. I am still throwing apples forwards, exactly the same as I was throwing them before, at 15m/s. If I am really not paying attention to what’s going on

around me (like the fact that I am standing in the back of a moving truck), how fast would I say the apples are moving?

Still 15m/s! Relative to me, I can only make an apple move away from me at 15m/s, so that’s how fast I measure the apple moving away from me.

How fast does my friend on the sidewalk say the apple is moving? Well, even before I throw it, she’ll say that the

apple is moving at 20m/s (the speed of everything on the truck).

When I have thrown the apple forward, adding more velocity to it, she will say it is going at (20m/s + 15m/s) 35m/s!

Now I turn around and start throwing the apples from the rear of the truck, backwards! I will still say that my apples are moving at 15m/s,

because from my way of looking at it, that’s how fast the apple is moving. The only thing I might say that is different is that it is -15m/s, since even I should be able to notice they are going in the opposite direction now.

My friend on the sidewalk will say that the apple is moving at (20m/s + -15m/s) 5m/s!

Frame of reference: When you are standing on the ground, that is your frame of reference. Anything that you see, watch, or measure will be compared to the reference point of the ground. If I am standing in the back of a moving truck, the truck is now my frame of reference and everything will be measured compared to it

Example 2: Sitting at your desk, how fast are you moving?

Relative to the ground: Zero. You’re not moving relative to the frame of

reference of the ground. Relative to the sun:

2.97e4 m/s! That’s a pretty big difference, but since the Earth is orbiting the sun at this speed, an observer standing on the sun (ouch!) would say that you are moving at 2.97e4 m/s.

Both of these answers are correct in their own frame of reference

Example 3: You might have even noticed relative velocity while sitting at a red light…

Have you ever been sitting at a red light with a bus stopped next to you?

You’re kind of daydreaming, looking out the window at the side of the bus, when all of a sudden it feels like your car is rolling backwards!

Then you realize that it was just the bus moving forwards. Your brain knows that the bus was just sitting there on

the road… it became part of the frame of reference of the ground.

When your brain saw the bus moving forwards, it had already “decided” that the bus won’t move. The only option remaining is that you must be moving backwards.

Frames of reference and relative motion is actually the reason that people get car sick. Your brain is getting two different sets of information about your body's motion that might not exactly agree with each other; information from your eyes, and information from your inner ear. Some people are more sensitive to these differences, which causes them to feel car sick as they watch the road "whiz" by. If you are prone to getting car sickness, try to look forward at a point far in the distance and stay focused on that.

DisplacementDisplacement (x or y)   "Change in position"

It is not necessarily the total distance traveled. In fact, displacement and distance are entirely different concepts. Displacement is relative to an axis.

o "x" displacement means you are moving horizontally either right or left.o "y" displacement means you are moving vertically either up or down.o The word change is expressed using the Greek letter DELTA ( Δ ).o To find the change you ALWAYS subtract your FINAL - INITIAL positiono It is therefore expressed as either   Δs = sf - si or Δx = xf - xi or Δy = yf - yi

Distance - How far you travel regardless of direction.

ExampleSuppose a person moves in a straight line from the lockers( at a position  x =

1.0 m) toward the physics lab(at a position x = 9.0 m) , as shown below

The answer is positive so the person must have been traveling horizontally to the right.

What is the student’s displacement?A) 9.0 mB) 8.0 mC) 7.0 mD) None of the above

ExampleSuppose the person turns around and returns to where they started! What is the person’s displacement?

The answer is negative so the person must have been traveling horizontally to the left

What is the DISPLACEMENT for the entire trip?

What is the total DISTANCE for the entire trip?

mxxx initialfinal 00.10.1

m1688

A) -9.0 mB) -8.0 mC) -7.0 mD) 8.0 m

A) 8.0 mB) 16 mC) 0.0 mD) -16 mA) 8.0 mB) 16 mC) 0.0 mD) -16 m

Speed SPEED is simply the “RATE at which DISTANCE changes”.

A rate is any quantity divided by time.

speed = distance/time

There are three (3) types of speed:

1. Constant Speed:

Speed that does not change; Speed that stays the same.

2. Average Speed:

Total distance divided by total time.

3. Instantaneous Speed:

Speed at any given point in time.

Q: What information is given by the speedometer of car?

A) Constant SpeedB) Average SpeedC) Instantaneous SpeedD) All of the Above

Q: Speed limit signs tell drivers the maximum _____ they can legally drive.

A) constant speedB) average speedC) instantaneous speedD) All of the Above

Speed Vs. Velocity

Is there a difference between speed and velocity? If yes, what is the difference?

Velocity is SPEED in a given DIRECTION.

VelocityVelocity is defined as: “The RATE at which DISPLACEMENT changes”.

Average SPEED is simply the “RATE at which DISTANCE changes”.

ExampleA quarterback throws a pass to a defender on the other team who

intercepts the football. Assume the defender had to run 50 m away from the quarterback to catch the ball, then 15 m towards the quarterback before he is tackled. The entire play took 8 seconds.

Let's look at the defender's average velocity:

Let's look at the defender's speed:“m/s” is the derived unit for both speed and velocity.

Average AccelerationAcceleration is the RATE at which VELOCITY

changes.

A truck accelerates from 10 m/s to 30 m/s in 2.0 seconds. What is the acceleration?

Suppose the same truck then slows down to 5 m/s in 4 seconds. What is the acceleration?

“m/s/s” or “m/s2” is the derived unit for acceleration.

Q: You are driving a car. Which of the following will allow you to accelerate the car?

A) Gas PedalB) BrakeC) Steering WheelD) Cruise ControlE) Both A and B are correctF) A, B, and C are correctG) A, B, C, and D are correct

Acceleration is a change in velocity. This occurs when an object is speeding up, slowing down, or changing direction.

What do the “signs”( + or -) mean?Quantity Positive Negative

Displacement You are traveling north, east, right, or in the +x or +y direction.

You are traveling south, west, left, or in the –x or –y direction.

Velocity The rate you are traveling north, east, right, or in the +x or +y direction.

The rate you are traveling south, west, left, or in the –x or –y direction.

Acceleration Your velocity(speed) is increasing in a positive direction or your speed is decreasing in a negative direction.

Your velocity(speed) is decreasing in a positive direction or your speed is increasing in a negative direction.

True or False, gravity pulls on all things equally?

Gravity causes all things to accelerate at the same rate.

A)TrueB)False

The Acceleration due to gravity “ag or g”The acceleration due to gravity is a special constant that exists

in a VACUUM, meaning without air resistance. If an object is in FREE FALL, gravity will CHANGE an objects velocity by 9.8 m/s every second.

2/8.9 smag g

The acceleration due to gravity:•ALWAYS ACTS DOWNWARD•IS ALWAYS CONSTANT near the surface of Earth

Q: You go to the roof of a tall building downtown and throw a penny off the roof. Would it kill a pedestrian if it hits them as they walk on the sidewalk below?

A) YesB) No

Beware – the signs can confuse!Suppose a ball is thrown straight upwards at 40 m/s. It takes 4 seconds to reach its maximum height,

then another 4 seconds back down to the point where it was thrown. Assume it is caught with the same speed it was thrown. Calculate the acceleration upwards and downwards.

sms

sm

ss

smsm

t

va

Downwards

sms

sm

ss

smsm

t

va

Upwards

/104

/40

04

/0/40

/104

/40

04

/40/0

This negative sign came from using the DELTA

This negative sign came from the DIRECTION of the velocity.

It is no surprise you get a negative answer both ways as gravity acts DOWNWARDS no matter if the ball goes up or down. It is GRAVITY which changes the ball’s velocity.