A man pulling an empty wagon causes it to accelerate at 1.4 m/s^2.

Question

A man pulling an empty wagon causes it to accelerate at 1.4 m/s^2.

What will the acceleration be if he pulls with the same force when the wagon contains a child whose mass is three times that of the wagon?

Answer 1

Let M be the mass of the wagon. Then the child has mass 3M, so the total mass of the wagon and child is 4M. When the wagon contains this child, the pulling force on the wagon is then 4Ma, where a is the acceleration of the wagon. On the other hand, when the wagon is empty, the force is 1.4M. Since the forces are equal by assumption, 4Ma = 1.4M, or a = 0.35 m/s².