1. Momentum and Collisions
Test Review

2. Momentum is a vector. Its direction is the direction of the velocity.
Momentum is mass x velocity and
its units are Kgm/s

3. Momentum is always conserved if there
is no net force (like friction or gravity).
If I shoot a rifle, the bullet has some
momentum forward, and the rifle recoils
backwards with an equal and opposite
momentum.
Momentum is equal, but kinetic energy
is not, as KE has the velocity squared

4. Change in momentum, Dp, is the final p –
the initial p.
If I kick a 1 kg ball against a wall at 10 m/s,
and it bounces back at 7 m/s, what is the
change in momentum?
7 kgm/s – (- 10 kgm/s) = 17 kgm/s

5. FDt = mDv If I drop my calculator off a 10 story building
onto the sidewalk, it will break. If I drop it
onto a mattress, it might survive. Why?
FDt = mDv
FDt = mDv
FDt = mDv

6. Types of collisions
perfectly inelastic
two things stick together
conserved: momentum
elastic
two things hit and bounce off
conserved: momentum and KE

7. A ball is going 5 m/s. A second identical ball
is going 15 m/s. How is the second ball’s
momentum different? How is its KE different?
p = mDv
KE = ½mv2

8. An 8 g. steel ball rolls across the floor at 5 m/s.
It comes to rest after 23.5 s. What was the force
of friction?
mass = .008 kg
vi = 5 m/s
Dt = 23.5 s
F = ?
FDt = mDv
F = mDv/Dt
F = (.008 kg)(5 m/s)/23.5 s
F = 1.7 x 10-3 N

9. Think of a pool table, if the white ball hits the
black ball, describe the motion of each

10. I am standing on a skateboard and throw a
.7 kg ball forward at 20 m/s. If I have a mass
of 90 kg, what would be my velocity and
direction?
m1v1 = m2v2
v2 = m1v1/m2
v2 = (.7 kg)(20 m/s)/90 kg
v2 = .16 m/s backwards

11. I throw a 2 kg lump of clay at a student at 24 m/s
left. The student simultaneously throws a 2.4 kg
lump at me. The lumps hit and stick to each
other, moving at 4 m/s right. What was the velocity
of the students lump?
m1v1 + m2v2 = (m1 + m2)vf
m1 = 2 kg
m2 = 2.4 kg
v1 = -24 m/s
vf = 4 m/s
v2 = ?
v2 = [(m1 + m2)vf - m1v1]/m2
v2 = [(2 kg kg) 4 m/s –
(2 kg)(-24 m/s)]/ 2.4 kg
v2 = 27.3 m/s

12. 0 m/s
24 cm
1.75 kg kg
The yellow ball smashes into the white ball, sticks, and
both travel up the ramp to a vertical height of 24 cm.
What was the initial velocity of the yellow ball?
mgh = ½mvf2
m1 = 3.15 kg
m2 = 1.75 kg
v2 = 0 m/s
h = .24 m
v1 = ?
vf = 2.17 m/s
m1v1 + m2v2 = (m1 + m2)vf
v1 = (m1 + m2)vf /m1
v1 = (3.15 kg kg)(2.17 m/s) / 3.15 kg
= 3.38 m/s