Momentum
1. Define the following terms:
a. Momentum – product of mass and velocity
b. Impulse - product of force and time of interaction
c. Conservation of momentum – in a closed and isolated system momentum is conserved
d. Elastic collision – both momentum and KE are conserved
e. Inelastic collision – only momentum is conserved
f. Closed system – nothing enters, nothing leaves
g. Isolated system – no outside forces acting
2. What causes the momentum of an object to change? A change in velocity caused by a force acting
on the object for some period of time which means an impulse acted on the object.
3. How are the impulse and the change in momentum related? Impulse equals the change in
momentum
4. How do you create the largest change in momentum of an object? Large force acting over a long
time
5. Why do most cars have air bags that deploy during a collision? To increase the time you are
stopping so the force is less
6. Why does it hurt less if you punch a pillow rather than a brick wall? Both have to create the same
change in momentum but since the pillow takes longer to stop your hand, it exerts less force on
your hand.
7. Suppose that a cannonball’s mass is somehow doubled and its velocity is also doubled, what happens
to its momentum? quadrupled
8. Why does a super ball that bounces off of the wall with same speed it struck the wall have a greater
change in momentum than a piece of clay with the same mass that sticks to the wall? Change in
direction so greater change in velocity
9. List at least 3 things that occur in equal but opposite pairs in a collision. Forces, impulses, and
changes in momentum
10. In an elastic collision, the objects will never stick together
11. When a moving object collides with a stationary object inelastically, what happens to their speed?
The object that was moving will slow down
12. In an explosion, if you have a closed and isolated system, the total momentum before and the total
momentum after are equal to zero. How is that possible since the objects in the system are moving?
They are vectors and cancel each other out
13. A 2 kg mass moving north at 2 m/s collides head - on with a 6 kg mass moving south at 1 m/s. After the
collision what is the sum of the momenta of both masses? -2 kg m/s
14. How much time is needed for a net force of 20 N to change the velocity of a 5 kg mass by 4 m/s? 2 seconds
15. Object A has a mass of 8 kg and a velocity of 10 m/s. Object B, which has a mass of 2 kg, is standing
motionless. Object A strikes object B and continues in the same direction at a speed of 4 m/s. What is the
velocity of object B after the collision? What is the total kinetic energy before the collision? What is the total
kinetic energy after the collision? What is the change in kinetic energy? Is the collision elastic or inelastic?
24 m/s, 400 kg m/s, 592 kg m/s, 192 kg m/s, inelastic
This is an impossible situation, it says that kinetic energy was created in the collision. This could only
happen if one of the objects also had potential energy that was converted to kinetic energy during the
collision.
16. A 0.014 kg baseball moving horizontally at 15 m/s is struck by a bat and rebounds off at 18 m/s in
the opposite direction. What impulse acted on the ball? If the bat and ball were in contact for 0.05
sec, what force did the bat exert on the baseball? -0.462 kg m/s, -9.24 N
17. A life raft of mass 250 kg carries two swimmers of mass 80 kg and 90 kg, respectively. The raft is
initially floating at rest. Then the swimmers simultaneously dive off opposite ends of the raft, each
with a horizontal velocity of 2 m/s. With what speed does the raft start to move? (Pre-AP only)
0.08 m/s