### Critical Thinking Items

#### 8.1 Linear Momentum, Force, and Impulse

Cars these days have parts that can crumple or collapse in the event of an accident. How does this help protect the passengers?

- It reduces injury to the passengers by increasing the time of impact.
- It reduces injury to the passengers by decreasing the time of impact.
- It reduces injury to the passengers by increasing the change in momentum.
- It reduces injury to the passengers by decreasing the change in momentum.

How much force would be needed to cause a 17 kg ⋅ m/s change in the momentum of an object, if the force acted for 5 seconds?

- 3.4 N
- 12 N
- 22 N
- 85 N

#### 8.2 Conservation of Momentum

A billiards ball rolling on the table has momentum **p**_{1}. It hits another stationary ball, which then starts rolling. Considering friction to be negligible, what will happen to the momentum of the first ball?

- It will decrease.
- It will increase.
- It will become zero.
- It will remain the same.

A ball rolling on the floor with momentum **p**_{1} collides with a stationary ball and sets it in motion. The momentum of the first ball becomes **p**'_{1}, and that of the second becomes **p**'_{2}. The directions of **p**_{1}, **p**'_{1}, and **p**'_{2} are all the same. Compare the magnitudes of **p**_{1} and **p**'_{2}.

- Momenta
**p**_{1}and**p**'_{2}are the same in magnitude. - The sum of the magnitudes of
**p**_{1}and**p**'_{2}is zero. - The magnitude of
**p**_{1}is greater than that of**p**'_{2}. - The magnitude of
**p**'_{2}is greater than that of**p**_{1}.

Two cars are moving in the same direction. One car with momentum **p**_{1} collides with another, which has momentum **p**_{2}. Their momenta become **p**'_{1} and **p**'_{2} respectively. Considering frictional losses, compare (**p**'_{1} + **p**'_{2} ) with (**p**_{1} + **p**_{2}).

- The value of (
**p**'_{1}+**p**'_{2}) is zero. - The values of (
**p**_{1}+**p**_{2}) and (**p**'_{1}+**p**'_{2}) are equal. - The value of (
**p**_{1}+**p**_{2}) will be greater than (**p**'_{1}+**p**'_{2}). - The value of (
**p**'_{1}+**p**'_{2}) will be greater than (**p**_{1}+**p**_{2}).