Linear Momentum
Q
uestions
1.

T he longer the impact time of an impulse, the smaller the force can be for the same momentum change, and hence the smaller the deformation of the object on which the force acts. On this basis, explain the value of air bags, which are intended to inflate during an automobile collision and reduce the possibility of fracture or death.

2.

Cars used to be built as rigid as possible to withstand collisions. Today, though, cars are designed to have
“crumple zones” that collapse upon impact. What is the advantage of this new design?

3.

Is it possible for an object to receive a larger impulse from a small force than from a large force? Explain.

4.

A light object and a heavy object have the same kinetic energy. Which has the greater momentum? Explain.

5.

Describe a collision in which all kinetic energy is lost.

6.

At a hydroelectric power plant, water is directed at high speed against turbine blades on an axle that turns an electric generator. For maximum power generation, should the turbine blades be designed so that the water is brought to a dead stop, or so that the water rebounds?

Problems
1.

What is the magnitude of the momentum of a 28­g sparrow flying with a speed of 8.4 m/s?

2.

A constant friction force of 25 N acts on a 65­kg skier for 20 s. What is the skier’s change in velocity?

3.

A 0.145­kg baseball pitched at 39.0 m/s is hit on a horizontal line drive straight back toward the pitcher at 52.0
­3
m/s. If the contact time between bat and ball is 3.00 x 10 s, calculate the average force between the ball and bat

during contact.
4.

A child in a boat throws a 6.40­kg package out horizontally with a speed of
10.0 m/s. Calculate the velocity of the boat immediately after, assuming it was initially at rest. The mass of the child is 26.0 kg, and that of the boat is 45.0 kg. Ignore water resistance.

5.

A 95­kg halfback moving at 4.1 m/s on an apparent breakaway for a touchdown is tackled from behind. When he was tackled by an 85­kg cornerback running at 5.5 m/s in the same direction, what was their mutual speed immediately after the tackle?

6.

A 12,600­kg railroad car travels alone on a level frictionless track with a constant speed of 18.0 m/s. A 5350­kg load, initially at rest, is dropped onto the car. What will be the car’s new speed?

7.

A 9300­kg boxcar traveling at 15.0 m/s strikes a second boxcar at rest. The two stick together and move off with a speed of 6.0 m/s. What is the mass of the second car?

8.

A 3800­kg open railroad car coasts along with a constant speed of 8.60 m/s on a level track. Snow begins to fall vertically and fills the car at a rate of 3.50 kg/min. Ignoring friction with the tracks, what is the speed of the car after 90.0 min?

9.

A 23­g bullet traveling 230 m/s penetrates a 2.0­kg block of wood and emerges cleanly at 170 m/s. If the block is stationary on a frictionless surface when hit, how fast does it move after the bullet emerges?

10.

A golf ball of mass 0.045 kg is hit off the tee at a speed of 45 m/s. The golf club was in contact with the ball for
­3
3.5 x 10
. Find ( a ) the impulse imparted to the golf ball, and ( b ) the average force exerted on the ball by the golf

club.
11.

A 12­kg hammer strikes a nail at a velocity of 8.5 m/s and comes to rest in a time interval of 8.0 ms. ( a ) What is the impulse given to the nail? ( b ) What is the average force acting on the nail?

12.

A 95­kg fullback is running at 4.0 m/s to the east and is stopped in 0.75 s by a head­on tackle by a tackler running due west. Calculate ( a ) the original momentum