PH221 recitation
#1) A 10g bullet is loaded into a 3kg gun. Both start
at rest. The bullet is fired to the right with a velocity
of 500m/s.
a) After being fired, what is the speed of the gun
to the left?
b) After being fired, the recoil of the gun is
absorbed over 4cm which brings the gun back to
rest. Assuming a constant acceleration over those
4cm, what was the force applied to the gun while
the recoil was being absorbed?
#2) A 0.2 kg plastic cart and a 20kg lead cart can both roll without friction on a horizontal surface. Equal
forces are applied on each cart in the following scenarios:
a) force applied for 1s
b) force applied for 1m
For each scenario, which cart has the larger change in momentum? How would your answer change if
we looked at the change in speed instead? Assume the carts are initially at rest.
#3) A 2kg object is moving to the right with a speed of 1m/s when it experiences an impulse of 4 Ns in
the up direction. What is the objects speed and direction after the impulse? Suppose the impulse was
caused by a collision lasting 0.01s, what is the average force on the object?
#4) A rubber ball collides head-on with a steel ball of equal mass traveling in the opposite direction.
Which ball receives the larger impulse? Explain.
#5) An object at rest explodes into three
fragments. The figure to the left shows the
momentum vectors of two of the fragments.
What is the momentum of the third
fragment?
PH221 recitation
#6) A 7kg bowling ball initially starts from rest and
is launched at 7m/s in the x-direction. Given the
force vs time graph given to the right, what was
the maximum force exerted on the bowling ball in
the x-direction? (look at the time scale, a human
did not launch this)
#7) Two 500g blocks of wood which are 10cm in length are placed on a frictionless table. The blocks are
separated by 2.0m of empty space. A 10g bullet is fired at 400m/s toward the blocks. It goes through the
first block, and then it embeds itself in the second block. The speed of the first block after the bullet
went through it is 6.0m/s. What is the speed of the second block after the bullet stops in it? Assuming
constant acceleration, what was the force exerted on the first block while the bullet was going through
it?
Modified from last week:
#4) A 10,000 kg railroad car is rolling at 2.0m/s when a 4000kg load of gravel is suddenly dropped in.
What is the car’s speed just after the gravel is loaded? Given the force exerted by the gravel on to the
railroad car increases at a linear rate, and the gravel exerted the force for only 0.25 seconds, what was
the maximum force exerted by the gravel on to the railroad car?
#5) Two ice skaters stand at rest in the center of an ice rink. When they push off against one another the
45-kg skater acquires a speed of 0.62m/s. If the speed of the other skater is 0.89m/s, what is this
skater’s mass? Given constant acceleration for the duration of the push, and the skaters pushed off
each other for one arm length of 0.8m, what was the force exerted by each skater during the push?