ABRHS PHYSICS (CP) NAME: ____________________ Chapters 6 & 7: Newton’s 3rd Law & Momentum

2014-15

Text: Chapter 6 Chapter 7 Think and Explain: 1-15 Think and Explain: 1-9 Think and Solve: --- Think and Solve: 1-6 Vocabulary: Newton’s 3rd law, action force, reaction force, momentum, vector, impulse, change in momentum, law of conservation of momentum, recoil, elastic collision, inelastic collision Equations:

mvp = J = Ft Δp = pf − pi Key Objectives: Concepts State Newton’s 3rd law and identify action/reaction pairs. State why action/reaction forces never cancel. Define momentum and state the units of momentum. Recognize the affect of force and time on change in momentum (impulse). (Egg Drop Activity) Describe situations where it is beneficial to have a large force and a small time and vice versa. Relate bouncing to impulse. Understand conservation of momentum and distinguish between the different types of

conservation of momentum problems: recoil, inelastic, elastic. Recognize action/reaction forces in collisions. Problem Solving Calculate momentum when given mass and velocity. Determine the change in momentum using mass and change in velocity or force and time. Use impulse equation to solve for an unknown variable. Solve using conservation of momentum for the three different types of problems: recoil, inelastic

and elastic.

ABRHS PHYSICS (CP) NAME: _________________

Newton's Third Law

side 1

1. What is Newton’s Third Law? 2. What is another way of saying Newton’s Third Law? 3. According to Newton’s Third Law, forces are interactions between two objects and that forces

always come in _________________. 4. In Newton's Third Law, the action/reaction forces are always equal and opposite. Why don't they

simply cancel each other out? 5. How does Newton’s Third Law apply to a hammer hitting a nail? 6. How does Newton’s Third Law apply to a propeller on a plane? 7. How does Newton’s Third Law apply to a person trying to walk? 8. If you are leaning and pushing against a wall, what is the reaction force? 9. If you pull a heavy bag to the right, what is the reaction force?

Even though the ACTION and REACTION forces are always equal and opposite, the effect of each force can be quite different, depending on the ______________ of each object.

A B

hit this pin to release plunger

The picture above shows 2 different carts, one with a plunger, set on a level track. In each of the situations below, the carts start off touching, and the plunger is quickly released. The mass of each empty cart is 500 grams, and the mass of a black bar is also 500 grams. Change the masses of each cart, set up the carts and answer each question.

ABRHS PHYSICS (CP) NAME: _________________

Newton's Third Law

side 2

10. A = 500 grams & B = 500 grams.

a. When the plunger is released, A pushes B to the right. What is the reaction force? b. What happens to each cart? c. What is true about the acceleration of each cart?

11. A = 500 grams & B = 1000 grams.

a. When the plunger is released, A pushes B to the right. What is the reaction force? b. What happens to each cart? c. What is true about the acceleration of each cart?

Just because two forces are equal and opposite does not mean they are part of a Newton's Third Law action/reaction pair. Here is a tricky example of that:

12. A backpack that weighs 50 N is at rest on a table. a. Draw a force diagram showing the two forces that are acting on the backpack. b. What are the reactions to each of those forces?

13. In general, what is the reaction to the weight of an object? 14. A bug collides with a car on the highway.

a. Who experiences the greater impact force? b. Who experiences the greater change in motion? c. Defend your answers.

Chapter 7 Newton’s Third Law of Motion—Action and Reaction 41

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CONCEPTUAL PHYSICS

Newton’s Third Law

1. In the example below, the action-reaction pair is shown by the arrows (vectors), and the action-reaction described in words. In (a) through (g) draw the other arrow (vector) and state the reaction to the given action. Then make up your own example in (h).

Example:

Fist hits wall Head bumps ball Windshield hits bug

Wall hits fi st a. b.

Bat hits ball Hand touches nose Hand pulls on fl ower

c. d. e.

Athlete pushes bar Compressed air pushes h. upward balloon surface outward

f. g.

2. Draw arrows to show the chain of at least six pairs of action-reaction forces below.

7-2Concept-DevelopmentPractice Page

42 Chapter 7 Newton’s Third Law of Motion—Action and Reaction

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CONCEPTUAL PHYSICS

3. Nellie Newton holds an apple weighing 1 newton at rest on the palm of her hand. The force vectors shown are the forces that act on the apple.

a. To say the weight of the apple is 1 N is to say that a downward gravitational force of 1 N is exerted on the apple by (Earth) (her hand).

b. Nellie’s hand supports the apple with normal force n, which acts in a direction opposite to W. We can say n(equals W) (has the same magnitude as W).

c. Since the apple is at rest, the net force on the apple is (zero) (nonzero).

d. Since n is equal and opposite to W, we (can) (cannot) say that n and W comprise an action-reaction pair. The reason is because action and reaction always (act on the same object) (act on different objects), and here we see n and W(both acting on the apple) (acting on different objects).

e. In accord with the rule, “If ACTION is A acting on B, then REACTION is B acting on A,” if we say action is Earth pulling down on the apple, reaction is (the apple pulling up on Earth) (n, Nellie’s hand pushing up on the apple).

f. To repeat for emphasis, we see that n and W are equal and opposite to each other (and comprise an action-reaction pair) (but do not comprise an action-reaction pair).

g. Another pair of forces is n [shown] and the downward force of the apple against Nellie’s hand [not shown]. This force pair (is) (isn’t) an action-reaction pair.

h. Suppose Nellie now pushes upward on the apple with a force of 2 N. The apple (is still in equilibrium) (accelerates upward), and compared to W, the magnitude of n is (the same) (twice) (not the same, and not twice).

i. Once the apple leaves Nellie’s hand, n is (zero) (still twice the magnitude of W), and the net force on the apple is (zero) (only W) (still W – n, which is a negative force).

ABRHS PHYSICS (CP) NAME: ___________________

Momentum Concepts

Answers: 1) 24,000 kg•m/s 2) 0.6 kg•m/s 3) 14 kg•m/s 4) 150 kg person 5) same! 6) 15 m/s 7) 1900 kg 8) 5.33 m/s 9) also doubles 10) also triples 11) smaller one is faster 12) sure - if masses different 13) 1200 kg car (25 m/s vs 20 m/s)

Concepts

A. What is momentum?

B. What are the units of momentum?

C. Is momentum a vector or a scalar?

Equation

1. What is the momentum of a 1200 kg car moving at 20 m/s? 2. What is the momentum of a 0.5 kg cart moving at 1.2 m/s? 3. What is the momentum of a 40 gram bullet traveling at 350 m/s? 4. Who has more momentum, a 75 kg person running at 3 m/s or a 150 kg person running at 3 m/s? 5. Who has more momentum, a 75 kg person running at 3 m/s or 225 kg person walking at 1 m/s? 6. How fast is a 5 kg ball moving if it has a momentum of 75 kg•m/s? 7. What is the mass of a car that has a momentum of 38,000 kg•m/s when it is moving at 20 m/s? 8. How fast does a 75 kg person have to travel to have a momentum of 400 kg•m/s ? 9. What happens to your momentum if you double your speed? 10. What happens to your momentum if you triple your speed? 11. Two objects have the same momentum, but different masses. How could this be? 12. Is it possible for two things to have different speeds, yet have the same momentum? 13. Which is going faster, a 1200 kg car with a momentum of 30,000 kg•m/s or a 1750 kg car with a

momentum of 35,000 kg•m/s?

ABRHS PHYSICS (CP) NAME: ____________________ Impulse Problems

Side 1

Concept Review: A. What is momentum and what are its units? B. What is impulse and what are its units? C. How are momentum and impulse related to each other? D. What is meant by the phrase "change in momentum?" How about "change in velocity?"

Change in Momentum 1. A 2 kg ball at rest is kicked and has a final velocity of 15 m/s. What was the change in

momentum of the ball? 2. A 2 kg ball is moving with a velocity of 15 m/s when someone catches it. What is the change in

momentum of the ball? 3. A 1500 kg car speeds up from 10 m/s to 25 m/s. What is its change in momentum? 4. A 1200 kg starts at rest, and undergoes a change in momentum of 20,000 kg•m/s. What is its

final velocity? 5. A 25 kg rock has an initial velocity of 10 m/s. What is its final velocity if its change in

momentum was 350 kg•m/s? 6. A 25 kg rock has an initial velocity of 10 m/s. What is its final velocity if its change in

momentum was –350 kg•m/s? Impulse 7. A box is pushed with a force of 150 N for 2 seconds. What impulse was exerted on the box? 8. A box is pushed with a force of 150 N for 10 seconds. What impulse was exerted on the box?

ABRHS PHYSICS (CP) NAME: ____________________ Impulse Problems

Side 2

9. 0.15 kg ball is hit with a force of 170 N for 0.25 seconds. What impulse was exerted on the ball? 10. How much force is needed to exert an impulse of 50 N•s in 15 seconds? How about in 2 seconds?

How about 0.003 seconds? 11. How much time is needed for a 4.5 N force to create an impulse of 12 N•s? All Together 12. What impulse is needed to make a 2.5 kg ball go from rest to 23 m/s? 13. What impulse is needed to stop a 0.15 kg egg traveling at 1.3 m/s? 14. A 2.5 kg ball is traveling with a velocity of 20 m/s to the right. What impulse is needed to make

the ball move at 20 m/s to the left? 15. What is the change in momentum of an object if a force of 75 N is exerted on it for 3 seconds? 16. A 0.5 kg cart is at rest on a table. An explosion gives the cart a speed of 1.2 m/s in only 0.2

seconds. What was the force on the cart from the explosion? 17. A 1200 kg car traveling is traveling at 30 m/s when it hits the brakes. If the braking force of

2000 N lasts for 7 seconds, what is the final velocity of the car? Answers: 1) 30 kg•m/s 2) –30 kg•m/s 3) 22,500 kg•m/s 4) 16.7 m/s 5) 24 m/s 6) –4 m/s 7) 300 N•s 8) 1500 N•s 9) 42.5 N•s 10) 3.3 N, 25 N, 16,700 N 11) 2.67 s 12) 57.5 N•s 13) –0.2 N•s 14) –100 N•s 15) 225 kg•m/s 16) 3 N 17) 18.3 m/s

ABRHS PHYSICS (CP) NAME: ___________________

Egg Drop Challenge

Using only straws and some tape, you will have to figure out a way for an egg to survive a 2.5 meter fall. This is a 10 point challenge activity. You can earn the 10 points through the survival of your egg or by a writing a brief description of the physics involved. Challenge Rules

Materials: 25 straws & 1 meter of masking tape & 1 egg. That's it.

Drop Height: 2.5 meters. (Set up is probably on the demo table.)

Construction: Do whatever you like using only the materials listed.

Testing: When you are ready, go over to the testing station. Hold your egg device above the indicated line, and drop it. Your teacher will check the egg, and you will receive the points indicated below. If necessary, your device will be cut up to verify that the egg has survived.

Points Egg Condition

10 Egg totally unharmed.

9 Egg is cracked, but not leaking.

0 Anything else.

+2 Egg survives a drop from the 2nd floor to the 1st floor. (Checked only if egg survives unharmed in the classroom.)

Write-up Option If your egg broke, you are not stuck with a 0/10 grade! Hand in a paragraph explaining the physics of why an egg may survive the fall. This is worth 10 points. To do this you have to:

• In terms of impulse and momentum, explain why a cushioned egg can survive a fall while an uncushioned egg will not.

• Make sure you correctly use the following terms: o Force o Time o Change in Momentum o Impulse.

• Exactly what does the cushioning change in the collision so that the egg survives? • What stays the same in the collision, even if the egg survives?

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-1: Momentum and Recoil

side 1

Purpose: To determine if momentum is conserved in an “explosion" by measuring the velocities of two carts immediately after a spring is released and they quickly push apart.

Materials: 1 track 1 cart with plunger 1 cart with no plunger 2 500 gram bars 2 motion detectors Procedure:

A B

1. As always, make sure the track is level on the lab table. 2. Start up Logger Pro and open the file "18 Momentum Energy Coll." The motion detector

plugged into “Dig 1” is Position 1 (the red line) and the motion detector plugged into “Dig 2” is Position 2 (the blue line).

3. Call cart A the one with the plunger in it and push in the plunger so that it locks in place. (Push it in and lift it up a little so that the notch in the plunger hooks on the cart. Ask your teacher for help if needed. This is surprisingly difficult.)

4. Place both empty carts on the track. Start recording data by clicking on “Collect”. Using a meter stick, tap the little knob on the cart to release the spring plunger, pushing the carts apart. Record the velocities for both carts after the spring is released. This is probably easiest to do by using the "Examine" button and just finding the biggest velocities from the velocity vs. time time graphs. Include any negative signs!

5. Place a 500 gram black bar mass in cart B and repeat. 6. Place the second 500 gram black bar oin cart B and repeat.

Data and Results:

Mass (kg)

Initial Velocity

(m/s)

Final Velocity

(m/s)

Initial Momentum

(kg•m/s)

Final Momentum

(kg•m/s)

Cart A 0.5 0

Cart B 0.5 0

Totals --- --- --- Cart A 0.5 0

Cart B 1.0 0

Totals --- --- ---

Cart A 0.5 0

Cart B 1.5 0

Totals --- --- --- Conclusions: 1. In terms of Newton’s Third Law, why do the two carts go in opposite directions? 2. For each trial calculate the momentum of each cart before and after the explosion and record in

the table above. Include any negative signs. What is the equation for finding momentum? 3. For each trial, calculate the total momentum before and after the explosion by adding up the

momentums of each cart. Record your results in the table above.

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-1: Momentum and Recoil

side 2

4. In all three trials, what was the total momentum of the carts before the explosion? Why does this make sense?

5. In all three trials, the total momentum of the carts after the explosion should also have been

zero. Did your results show that? How can the total momentum be zero if the carts were moving?

6. When we say that something is conserved in physics, we mean that a quantity does not change

during an interaction or over time. In this lab, we are curious about whether momentum was conserved before and after the explosion. a. Was the momentum of cart A conserved? b. Was the momentum of cart B conserved? c. Was the total momentum of cart A plus cart B conserved?

7. In the second and third trials, the masses of the carts were not the same.

a. Which cart experienced a greater change in velocity? b. Which cart experienced a greater force during the explosion? c. Which cart experienced a greater change in momentum?

Follow-up Questions: 1. Imagine you do the experiment again, but this time the masses of the carts are 2 kg and 1 kg. If

the 1 kg cart has a speed of 1 m/s after the “explosion,” how fast is the other cart going? 2. This time the masses of the carts are 3 kg and 2 kg. If the 2 kg cart has a speed of 1.5 m/s after

the “explosion,” how fast is the other cart going? 3. This time the speeds of the carts after the “explosion” are 5 m/s and 2.5 m/s. If the cart going 5

m/s has a mass of 2 kg, what is the mass of the other cart? 4. This time the speeds of the carts after the “explosion” are 4 m/s and 1 m/s. If the cart going 1 m/s

has a mass of 2 kg, what is the mass of the other cart? 5. This time the speeds of the carts after the “explosion” are 1.75 m/s and 1.25 m/s. If the cart going

1.75 m/s has a mass of 2 kg, what is the mass of the other cart? 6. A 1.5 kg gun can fire a 0.005 kg bullet with a speed of 500 m/s. What is the “recoil velocity” of

the gun? 7. Why would a super-light gun that has a mass of only 0.010 kg, yet could a fire a 0.005 kg bullet

with a speed of 500 m/s be really dangerous for the person firing the gun?

ABRHS PHYSICS (CP) NAME: ____________________ Recoil Problems

1. A 500 kg cannon fires a 5 kg ball forward at 80 m/s. What was the final velocity of the cannon? 2. John is sitting in a boat and throws a rock north at 30 m/s. John and the boat (total mass 130

kg) move south at 2 m/s. What is the mass of the rock? 3. Mary (40 kg) and Larry (60 kg) are competing in the Ice Dancing Championship. They begin

their routine with Mary pushing off on Larry. If Larry travels at 0.85 m/s, what is Mary’s velocity?

4. A 50 g bullet is fired from a 1.2 kg gun with a speed of 150 m/s.

a. What is the recoil velocity of the gun?

b. Which experiences a greater force, the gun or the bullet?

c. Which experiences a greater change in velocity?

d. Which experiences a greater acceleration?

e. Which experiences a greater impulse? 5. Bonnie (60 kg) and Ronnie (45 kg) are on a row boat (25 kg) in the middle of a lake. For some

unknown reason, Bonnie jumps off the front of the boat with a speed of 2 m/s and Ronnie jumps off the back with a speed of 1.5 m/s. What is the resultant speed of the boat?

Answers: 1) 0.8 m/s, backwards 2) 8.67 kg 3) 1.28 m/s, opposite direction 4. a) 6.25 m/s b) same c) bullet d) bullet e) same 5) –2.1 m/s

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-2: Inelastic Collisions

side 1

Purpose: To determine if momentum is conserved in a collision in which two objects stick to each other after the collision.

Materials: 1 track 1 cart with plunger 1 cart with no plunger 2 500 gram bars Procedure:

velcro sides

this car at rest

BA

1. Arrange the empty carts so that the velcro ends face each other as shown in the diagram.

Start the motion detector and give the cart with the index card (A) a little push. Make sure the resulting graph shows the velocity both before and after the collision.

2. Record the velocity of cart A alone, and the velocity of the carts combined. Measure the velocities by measuring the slope of the position vs time graphs just before the collision and just after the collision.

3. Place a 500 gram bar in cart A (the pushed cart) and repeat. 4. Remove the 500 gram bar from cart A, and put it in cart B (the target cart) and repeat. 5. Place the second 500 gram bar in cart A and repeat.

Calculations: 1. For each trial, calculate the momentum of the individual carts and then the momentum of the

carts when they are stuck together. Show an example of your calculations here, and record all the results in the table below.

Data: Trial 1 Trial 2

Mass (kg)

Velocity (m/s)

Momentum (kg•m/s)

Mass (kg)

Velocity (m/s)

Momentum (kg•m/s)

Cart A 0.5 1.0

Cart B 0.5 0 0.5 0 stuck together

Trial 3 Trial 4

Mass (kg)

Velocity (m/s)

Momentum (kg•m/s)

Mass (kg)

Velocity (m/s)

Momentum (kg•m/s)

Cart A 0.5 1.0

Cart B 1.0 0 1.0 0 stuck together Conclusions: 1. In general, how did the total momentum before the carts crashed compare to the total

momentum after the carts crashed?

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-2: Inelastic Collisions

side 2

Follow-up Questions: 1. A 1 kg cart traveling at 3 m/s crashes and sticks to another 1 kg cart initially at rest. How fast

are the two carts going when they are stuck together? 2. A 2 kg cart traveling at 4 m/s crashes and sticks to a 1 kg cart initially at rest. How fast are the

two carts going when they are stuck together? 3. A 2 kg cart traveling at 2.5 m/s crashes and sticks to a 3 kg cart initially at rest. How fast are

the two carts going when they are stuck together? 4. A 1 kg cart crashes and sticks to another 1 kg cart initially at rest. If they are going at 2 m/s

when they are stuck together, how fast was the first cart going by itself? 5. A 2 kg cart crashes and sticks to a 1 kg cart initially at rest. If they are going at 2 m/s when they

are stuck together, how fast was the first cart going by itself? 6. A 2 kg cart crashes and sticks to a 3 kg cart initially at rest. If they are going at 1.75 m/s when

they are stuck together, how fast was the first cart going by itself?

ABRHS PHYSICS (CP) NAME: ___________________

Momentum Problems

side 1

1. Carl (mass 70 kg) is somehow stranded on some frictionless ice, and cannot walk at all. He has a heavy backpack of mass 10 kg. a. How could he get off the ice? b. If he can throw the backpack with a speed of 5 m/s, how fast would he go?

2. A 500 kg cannon fires a 10 kg cannonball with a speed of 300 m/s.

a. What is the recoil speed of the cannon? b. Which object has a greater change in momentum? c. Which object has a greater impulse? d. Which object experiences a greater force? e. Which object has a greater change in velocity? f. Which object has a greater acceleration?

3. Two astronauts are floating motionless in space. They push off each other. The first astronaut

has a mass of 150 kg, and after the push has a speed of 2 m/s. The other astronaut has a mass of 250 kg. a. How fast is the other astronaut going after the push? b. What is true about the directions of the two astronauts after the push?

4. A 5 kg cart traveling at 2.5 m/s crashes and sticks to a 3 kg cart initially at rest. How fast are

the two carts going when they are stuck together? 5. A 1500 kg car traveling at 25 m/s skids on some ice, and crashes and sticks to a car of mass 2000

kg. a. How fast are the two cars going right after the crash? b. Which car experiences a greater change in momentum?

ABRHS PHYSICS (CP) NAME: ___________________

Momentum Problems

side 2

6. A 1.5 kg cart moving at 3 m/s crashes and sticks to a second cart initially at rest. If they are going at 2 m/s when they are stuck together, what is the mass of the second cart?

7. An archer shoots a 0.1 kg arrow with a speed of 60 m/s at an 0.5 kg apple. If the arrow sticks in

the apple, what is the final speed of the apple and arrow? 8. Peggy (50 kg) and Bill (70 kg) are having an argument standing on an icy surface. Peggy pushes

Bill so that Bill moves backward with a speed of 2 m/s? a. How fast does Peggy move after she pushes Bill? b. Who experiences more force?

9. Barry runs with a speed of 5 m/s and jumps on a stationary 7 kg skateboard. If Barry’s mass is

60 kg, how fast are Barry and the skateboard moving? 10. Darlene is enjoying a relaxing day on her 20 kg canoe, when she drops her cell phone into the

lake. Darlene jumps off the boat and into the lake with a speed of 2.5 m/s and the boat moves in the opposite direction with a speed of 4 m/s. What is Darlene’s mass?

*11. A car traveling at 30 m/s skids on some ice and crashes and sticks to a car of mass 1500 kg. The

two cars move at 21 m/s when they are stuck together. What was the mass of the first car? 12. You are walking down the hall and you randomly over hear one student tell another student

“Everyone knows that when two cars collide the smaller car always experiences more force.” Is this statement correct?

Answers: 1. a) throw backpack b) 0.71 m/s 2) 6 m/s b, c, d.) same e, f) cannonball 3. a) 1.2 m/s b) opposite 4) 1.56 m/s 5) a)10.7 m/s b) same 6) 0.75 kg 7) 10 m/s 8. a) 2.8m/s b) same 9) 4.5 m/s 10) 32 kg 11) 3500 kg 12) Incorrect, Newtonʼs 3rd Law

Chapter 8 Momentum 43

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CONCEPTUAL PHYSICS

8-1Concept-DevelopmentPractice Page

Momentum

1. A moving car has momentum. If it moves twice as fast, its momentum

is as much.

2. Two cars, one twice as heavy as the other, move down a hill at the same speed. Compared to the

lighter car, the momentum of the heavier car is as much.

3. The recoil momentum of a cannon that kicks is

(more than) (less than) (the same as)

the momentum of the cannonball it fi res.

4. If a man fi rmly holds a cannon when fi red, then the momentum of the cannonball is equal to the recoil momentum of the

(cannon alone) (cannon–man system) (man alone).

5. Suppose you are traveling in a bus at highway speed on a nice summer day and the momentum of an unlucky bug is suddenly changed as it splatters onto the front window.

a. Compared to the force that acts on the bug, how much force acts on the bus?

(more) (the same) (less).

b. The time of impact is the same for both the bug and the bus. Compared to the impulse on the bug, this means the impulse on the bus is

(more) (the same) (less).

c. Although the momentum of the bus is very large compared to the momentum of the bug, the change in momentum of the bus, compared to the change of momentum of the bug is

(more) (the same) (less).

d. Which undergoes the greater acceleration?

(Bus) (Both the same) (Bug)

e. Which, therefore, suffers the greater damage?

(Bus) (Both the same) (The bug of course!)

44 Chapter 8 Momentum

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6. Granny whizzes around the rink and is suddenly confronted with Ambrose at rest directly in her path. Rather than knock him over, she picks him up and continues in motion without “braking.”

Consider both Granny and Ambrose as two parts of one system. Since no outside forces act on the system, the momentum of the system before collision equals the momen-tum of the system after collision.

a. Complete the before-collision data in the table below.

b. After collision, does Granny’s speed increase or decrease?

c. After collision, does Ambrose’s speed increase or decrease?

d. After collision, what is the total mass of Granny + Ambrose?

e. After collision, what is the total momentum of Granny + Ambrose?

f. Use the conservation of momentum law to fi nd the speed of Granny and Ambrose together after collision. (Show your work in the space below.)

New speed =

BEFORE COLLISION

Granny’s mass 80 kg

Granny’s speed 3 m/s

Granny’s momentum

Ambrose’s mass 40 kg

Ambrose’s speed 0 m/s

Ambrose’s momentum

Total momentum

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-3: Elastic Collisions

side 1

Purpose: To determine if momentum is conserved in a collision in which the two objects bounce off each other.

Materials: 1 track 2 carts with magnets in end 2 500 gram bars Procedure:

this cart at rest

A B

1. Place one of the carts in the middle of the track. Place the second cart at one end of the

track. Place the two motion detectors at either end of the track. (see diagram above.) Make sure that the magnets in the carts are facing each other, so that the carts bounce!

2. Start up Logger Pro and open the file “Experiments/Physics with Vernier/18 Momentum Energy Coll.cmbl”. The motion detector plugged into “Dig 1” is Position 1 (the red line) and the motion detector plugged into “Dig 2” is Position 2 (the blue line).

3. Start the motion detectors and give cart A a small push so that it bounces off of cart B. 4. Record the velocity of cart A before the collision, and the velocities of both carts after the

collision. Measure the velocities by first clicking anywhere on the velocity graph, and then choosing the "Examine" tool from the menu bar. Include any negative signs!

5. Place one 500 gram bar in cart B (the target cart) and repeat. 6. Place the 500 gram bar in cart A instead and repeat. 7. For the last trial, remove all the masses. Give both of the carts a small push so that they

bounce off of each other. Calculations: 1. For each trial, calculate the initial and final momenta of each of the carts. Show work here, and

record your results in the table below. 2. For each trial, calculate the total initial momentum and the total final momentum. Show your

work here, and record your results in the table below. Data and Results:

Mass (kg)

Initial Velocity

(m/s)

Final Velocity

(m/s)

Initial Momentum

(kg•m/s)

Final Momentum

(kg•m/s)

Cart A 0.5

Cart B 0.5 0

Totals --- --- --- Cart A 0.5

Cart B 1.0 0

Totals --- --- ---

ABRHS PHYSICS (CP) NAME: ___________________

Lab 7-3: Elastic Collisions

side 2

Mass (kg)

Initial Velocity

(m/s)

Final Velocity

(m/s)

Initial Momentum

(kg•m/s)

Final Momentum

(kg•m/s)

Cart A 1.0

Cart B 0.5 0

Totals --- --- ---

Cart A 0.5

Cart B 0.5

Totals --- --- --- Conclusions: 1. In general, how did the total momentum before the carts crashed and bounced compare to the

total momentum after the carts crashed? 2. One of the major ideas in science is the Law of Conservation of Momentum. What does this

mean? 3. How does the conservation of momentum apply to the previous lab in which one cart crashed and

stuck to a second cart? 4. How does the conservation of momentum apply to the previous lab in which the two carts both

started at rest, and then pushed each other on opposite directions after the plungers were released?

Questions: 1. A 1.5 kg cart moving at 3 m/s has a collision with a 3 kg cart initially at rest. The 1.5 kg moves

backwards at 1.2 m/s after the collision. What is the velocity of the 3 kg cart after the collision? 2. A 200 kg bumper car has a head on collision with a 300 kg bumper car. The 200 kg car had a

speed of 2.5 m/s and the 300 kg car had a speed of 1.5 m/s right before the crash. If the 200 kg car is stopped by this, what happens to the 300 kg car?