1. Guidelines for Solving Conservation of Momentum Problems
Draw a Diagram of the situation.
Make a before & after chart and list the given & unknown information in the correct column.
Must include Directional signs with ALL Velocities and Momentums. Carry the sign with the numbers when you substitute.
Right & Up = positive
Left & Down = negative
Decide if the collision is Elastic or Inelastic, then pick the appropriate equation for the situation.
Substitute the given information into the formula.
Simplify and Solve for the missing variable.

2. Ex. #1 A 2 kg ball traveling to the left with a speed of 24 m/s collides head-on with a 4 kg ball traveling to the right at 16 m/s. Find the resulting velocity if the two balls stick together after impact.
Before
After
v2 = +16 m/s 2
v1 = -24 m/s 1 2 1
vf = ???
m1 = 2 kg
m2 =4 kg
v1 = -24 m/s
v2 = +16 m/s
m1 = 2 kg
m2 =4 kg
They stick together so . . .
vf = v1 = v2 =???
Inelastic Collision
Substitute
Simplify
Solve for unknown
2.7 m/s = vf

3. Ex. #2 Starting from rest, two skaters “push off” against each other on smooth level ice, where friction is negligible. The woman has a mass of 54 kg and the man has a mass of 88 kg. The woman moves away with a velocity of +2.5 m/s. Find the “recoil” velocity of the man.
m1 = 54 kg
m2 =88 kg
v1 = +2.5 m/s
v2 = ???
m1 = 54 kg
m2 =88 kg
They start together so . . .
vo = 0 m/s
Inelastic Collision
Substitute
Simplify
Solve for unknown
-1.5 m/s = v2

4. Ex. #3 Tyrrell throws his 0.20 kg football in the living room and knocks over his mom’s 0.80 kg antique vase. After the collision, the football bounces straight back with a speed of 3.9 m/s, while the vase is moving at 2.6 m/s in the opposite direction. How fast did Tyrrell throw the football?
v1 = ??? m/s 1
v1 = -3.9 m/s 1
v2 = +2.6 m/s 2
v2 = 0 m/s 2
m1 = 0.20 kg
m2 =0.80 kg
v1 = ??? m/s
v2 = 0 m/s
m1 = 0.2 kg
m2 =0.80 kg
v1 = -3.9 m/s
v2 = +2.6 m/s
Elastic Collision
Substitute
Simplify
Solve for unknown
v1 = 6.5 m/s