Momentum, Impulse and Law of Conservation of Momentum

Starter Questions

Starter 1Momentum problems

1. What is the momentum of a 45 kg boulder rolling at 3.5 m/s?

2. What is the mass of a boulder that is rolling at 4.7 m/s and has a momentum of 891 kg x m/s?

3. What is the speed of a 40.1 kg boulder that has a momentum of 342 kg x m/s?

Starter #2—Use the impulse equation—solve on a separate sheet of paper

1. What is the impulse that changes the momentum of a boulder if a force of 356 N is applied to the boulder for 4.5 s?

2. Solve problems 26-27 found on page 140. 3. How much force is applied to a cart if the application time of the

force is 12 seconds and the impulse is 245 N x s (or kg x m/s)4. How long must a force of 65 N be applied to a cart if the impulse

is 455 N x s? 5. Suppose three objects received a force; the first object for 4.5 s,

the second object for 1.5 s and the third object for 3.1 s. The impulse for each was 28.1 N x s. How much force was applied to each object?

Starter 3(reading from pg. 131) Figure 8.10Momentum of cannon and cannonball

Read Page 131

• Read 1st paragraph• What does Newton’s 3rd law have to say about the net

force of the cannon-cannonball system?

• Why is the momentum of the cannon-cannonball system equal to zero before and after the firing?

Read Page 131

• Read 1st paragraph• What does Newton’s 3rd law have to say about the net force of the

cannon-cannonball system?• The net force of this system equals zero because the

action and reaction forces cancel each other out

• Why is the momentum of the cannon-cannonball system equal to zero before and after the firing?

• The momentum in the system must be conserved; so if the system starts with zero momentum, it must end with zero momentum.

Read Page 131

• Read 2nd paragraph• Why is momentum a vector quantity?

• Explain the difference between the momentum of the cannon and the momentum of the cannonball, and the momentum of the cannon-cannonball system.

Read Page 131

• Read 2nd paragraph• Why is momentum a vector quantity?• Momentum is a quantity that expresses both

magnitude and direction.

• Explain the difference between the momentum of the cannon and the momentum of the cannonball, and the momentum of the cannon-cannonball system.

• After the firing occurs, both the cannon and cannonball have the same momentum (big mass, small velocity vs. small mass, big velocity). But since the momentum for each is moving in the opposite direction, the momentums cancel out, causing the cannon-cannonball system’s momentum to equal zero.

Read Page 131

• Read 3rd paragraph• Why do physicists use the word conserved for

momentum?

• State the law of conservation of momentum.

Read Page 131

• Read 3rd paragraph• Why do physicists use the word conserved for

momentum?• The word conserved refers to quantities

that do not change.

• State the law of conservation of momentum.• In the absence of an external force, the

momentum of a system remains the same.

Read Page 131

• What does system mean?

• In terms of momentum conservation, why does a cannon recoil when fired?

Final Thoughts about Page 131

• What does system mean?• The word system refers to a group of interacting

elements that comprises a complex whole.

• In terms of momentum conservation, why does a cannon recoil when fired?

• The cannon must recoil in order for momentum to be conserved. (The momentum of the cannon-cannonball system was zero before the firing, and must remain zero after the firing.)

Read Page 131

• What does conservation of momentum mean?

• Conservation of momentum means that the amount of momentum in a system does not change.

• Why is the momentum cannon-cannonball system equal to zero?

• The momentum of the cannonball cancels out the recoil of the cannon (both move in opposite directions with an equal amount of momentum.