# Physics Problem Please?

Two boys of mass 45 kg and 60 kg are sitting on 15 kg wagons, facing each other and holding a rope between them. The lighter boy pulls on the rope and acquires a velocity of 2.0 m/s. What is the velocity of the other boy?

### 3 Answers

- PinkgreenLv 73 weeks agoBest answer
Consider the conservation of the linear momentum of the system:

2(45+15)-(60+15)v=0

=>75v=2(60)

=>v=1.6 m/s

The speed of the heavier boy

=1.6 m/s.

- electron1Lv 73 weeks ago
Momentum is always conserved. Since both boys are sitting on 15 kg wagons, we need to add 15 kg to their mass.

Mass 1 = 45 + 15 = 60 kg

Mass 2 = 60 + 15 = 75 kg

Since both boys are not moving, the initial momentum is 0 kg * m/s.

Initial momentum = 60 * 2 + 75 * v

120 * v + 75 = 0

v = -0.625 m/s

The negative sign means the 60 kg boy is moving in the opposite direction of the 45 kg boy. I hope this is helpful for you.

- Dayton JC4PMLv 73 weeks ago
It's conservation of momentum - do you know how to work out the momentum of the lighter boy and his wagon? The bigger boy and his wagon will have the same sized momentum as the lighter boy and his wagon, at some velocity.

Momentum has a direction, the momentums will be the negative of each other, making the total momentum zero, which it was at the start