Teaching Modern Physics - QuarkNet - Fermilab

Sample Problems:
Conservation of Momentum
KEY
m1v1i+m2 v2i = m1v1f + m2v2f
Adrian and Iqbal have a combined mass of 200 kg and are zooming along in a 100-kg
amusement park bumper car at 10 m/s. They bump Ethan's car, which is sitting still. Ethan has a
mass of 50 kg. After the elastic collision, Adrian and Iqbal continue ahead with a speed of 4 m/s.
How fast is Ethan's car bumped across the floor?
(300 kg)(10 m/s) + (150 kg)(0 m/s) = (300 kg)(4 m/s) + (150 kg) v
v = 12 m/s
If an 800-kg sports car slows to 13 m/s to check out an accident scene and the 1200-kg pick-up
truck behind him continues traveling at 25 m/s, with what velocity will the two move if they lock
bumpers after a rear-end collision?
(800 kg)(13 m/s) + (1200 kg)(25 m/s) = (800 kg + 1200 kg) v
v = 20.0 m/s
Ruth, a 65-kg skin diver, shoots a 2-kg spear with a speed of 15 m/s at a fish who darts quickly
away without getting hit. How fast does Ruth move backwards when the spear is shot?
Homework:
0 = (65 kg) v + (2 kg)(15 m/s)
v = -0.46 m/s
1. A 100-g marble strikes a 25-g marble that is at rest on a smooth horizontal surface. In the
impact, the speed of the larger marble is reduced from 100 cm/s to 60 cm/s. What is the
speed of the smaller marble immediately after impact?
(100 g)(100 cm/s) + (25 g)(0 cm/s) = (100g)(60 cm/s) + (25 g) v
v = 160 cm/s
2. Running at 2 m/s, Chris, the 50-kg running back, collides with Yoon, the 90-kg defensive
lineman, who is traveling at 7 m/s in the other direction. Upon collision, Yoon continues to
travel forward at 1 m/s. How fast is Chris knocked backward?
(50 kg)(2 m/s) + (90 kg)(-7 m/s) = (50 kg) v +(90 kg)(- 1 m/s)
v = -8.8 m/s
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