Question

A 40.0 kg physics student at rest on a frictionless rink throws a 3.0 kg box, giving the box a velocity of 8.0 m/s. Which statement describes the initial velocity of the student after the box is thrown?
a. 0.6 m/s in the same direction as the box.
b. 0.6 m/s in the opposite direction of the box.
c. 1.7 m/s in the same direction as the box.
d. 1.7 m/s in the opposite direction of the box

Answer 1

Answer: (b)

Explanation:

Given

mass of Student M=40 kg

mass of box m=3 kg

velocity of box v=8 m/s

suppose u is the velocity of the student after the throw

As there is no external force is applied so momentum is conserved

Initial momentum=final momentum=0

0=Mu+mv

Mu=-mv

$u=-\frac{3}{40}\times 8=-\frac{3}{5}\\u=-0.6\ m/s$

Here negative sign indicates the velocity is opposite in direction i.e. opposite to the direction of the box