PHYSICS STUDY GUIDE
name:
Finding the momentum of an object
Strategy: plug the object’s mass and velocity into p=mv
1) What is the momentum of a 1 kg baseball traveling at 10 m/s?
2) What is the momentum of a 22 kg baby carriage traveling at 2 m/s?
3) What is the momentum of a 4,000 kg truck traveling at -8 m/s?
4) What is the momentum of a 65 kg hotdog salesman if he is sitting still?
Find mass, velocity or momentum given the other two variables using p=mv
Strategy: plug the two numbers you are given into p=mv, then do the necessary algebra to solve for the
third variable.
5)
Object
Mass (kg)
A
10
40
B
15
45
C
D
E
Velocity (m/s)
2
3
Momentum (kg
m/s)
30
8
6
12
Predict velocity after a change in mass using the conservation of momentum
Strategy: recall that in a closed system, momentum DOES NOT CHANGE. This means that if mass
suddenly changes in a closed system, velocity must change as well since p=mv.
6) If the momentum of an object stays the same after an increase in mass, does velocity increase
or decrease?
7) If the momentum of an object stays the same after an decrease in mass, does velocity increase
or decrease?
8) A truck filled with 10,000 kg of liquid nitrogen is driving at 2 m/s. Suddenly, the truck springs
a leak and the nitrogen begins to leak, decreasing the truck’s mass. Assuming the truck’s
momentum stays the same, does velocity increase or decrease?
9) A cart with an empty bucket on top is traveling at a constant velocity. The bucket is gradually
filled with water.
a. How does the cart’s total mass change?
b. if momentum is conserved, how must velocity change?
Find an unknown velocity after an elastic collision

Strategy: You will solve this problem in 2 steps.
-First, plug the two objects’ masses and their velocities BEFORE the collision into
pt  m1v1  m2v2 to find the momentum of the system.
-Second, plug the two masses, the velocity you know AFTER the collision, and total
momentum into pt  m1v1  m2v2 , and do some algebra to solve for the variable you don’t have
a number for.
10 )A 2000 kg elephant has an initial velocity of 5 m/s. It strikes a second elephant with a mass of 2000 kg and
 of 0 m/s. After the collision, the first elephant comes to rest.
an initial velocity
-What it the total momentum of the system before the collision?
G m1=
m2=
v1=
v2=
U
E
pT= m1v1+m2v2
S
S
pT= 10,000 kg m/s
G
U
E
S
S
-What is the velocity of the second elephant after the collision?
m1=
m2=
v1=
pT=
pT=m1v1+m2v2
v2= 5m/s
10) Jerome rolls an 8 kg bowling ball at 5 m/s at a 1 kg baseball that is at rest. After the collision, the
bowling ball is at rest. What is the final velocity of the baseball?
11) Ericka drives a truck (1500 kg) with an initial velocity of 20 m/s. She crashes into a taco wagon (500
kg) that has an initial velocity of 5 m/s in the opposite direction. After the collision, Ericka’s truck has a
velocity of -10 m/s. (The taco wagon guy is fine, don’t worry)
Find the velocity after an inelastic collision
Strategy: You will solve this problem in 2 steps.
-First, plug the two objects’ masses and their velocities BEFORE the collision into
pt  m1v1  m2v2 to find the momentum of the system.
-Second, find the total mass of the stuck-together objects. You now have p and m, just solve for
v! (you only have one object, so DON’T USE pt  m1v1  m2v2 , instead use p=mv)
 12) A 750 kg walrus slides across the ice at 1 m/s. He strikes a 250 kg penguin who is traveling 1 m/s in the
opposite direction. The two freeze together upon impact

-What it the total momentum of the system before the collision?
G m1=
m2=
v1=
v2=
U
E
pT= m1v1+m2v2
S
S
pT= 500 kg m/s
G
U
E
S
S
-What is the velocity of the stuck-together creatures after the collision?
m=
pT=
pT=mv
v2= 0.5m/s
13) Cierra throws a chunk of play-dough with a mass of 1 kg and a velocity of 10 m/s. It hits Keisha, who
weighs 29 kg and is at rest. The gum sticks to Keisha. What is the final velocity of the Keisha-play dough
object?
14) A bullet (1 kg) is fired with velocity 100 m/s into a wooden block (9 kg) which is initially at rest. The bullet
gets stuck in the block and they both continue on together. What is the final velocity of the bullet-block object?
Identifying 3rd law reaction pairs
Strategy: Remember, no matter what the following is true: “For every action force there is a reaction
force with equal size and the opposite direction.” So, ANY time you have two objects, and one is
pushing or pulling on the other, the other one is pushing or pulling with EXACTLY the same force in
the opposite direction no matter what their relative sizes.
15) Jasmine hits a softball with her bat. According to Newton’s 3rd law, what is the reaction to
jasmine’s bat hitting the ball?
16) Trayvon pushes on Travon with a 10 N force. What is the magnitude of Travon’s reaction force?
17) The Earth’s gravity pulls Mr. Dufficy down with a 700 N force. Is the force with which Mr.
Dufficy pulls up the Earth greater, smaller or equal to 700 N?
18) An astronaut on the moon hits a golf ball. The ball sails out of Earth’s orbit, and eventually hits the
sun. Which of the following best describes the situation when the golf ball hits the sun?
a. The golf ball strikes the sun with the same force as the sun strikes the golfball
b. The sun strikes the golf ball with a force, but the golf ball is too small to exert a force on the
sun.
c. The golf ball strikes the sun with a force, but the sun doesn’t move so it does not exert a force.
d. The golf ball strikes the sun with a very large force, and the sun exerts a small force on the golf
ball.
e. The golf ball strikes the sun with a small force, and the sun strikes the golf ball with a much
larger force.
20) Consider a basketball hitting the ground.
-What is the action force, and what is the reaction force?
-Why does the basketball bounce?