Question

Bob who has a mass of 100 kg stands ona. Frictionless frozen lake with a 20 kg rock in his hands. He throws the rock forward with a velocity of +5 m/s. What is the velocity of bob after he throws the rock

A. -2 m/s
B. -1 m/s
C. 0 m/s
D. 1 m/s

Answer 1

Option B

Step-by-step explanation:

This involves the conservation of momentum:

m1v1 = m2v2

Now initially neither the rock nor Bob is moving. Then the rock moves forward with a velocity of 5 m/s, having a mass of 20 kg. But Bob moves back by the conservation of momentum, so at what velocity?

0 = 20(5) + 100(vf),

-100 = 100vf,

vf = - 1m/s