A child of mass stands on a light skateboard initially at rest on a horizontal
PHYSICS · Multi-part · CBSE · Physics · Laws of Motion · Laws of Motion
Problem
A child of mass 35kg stands on a light skateboard initially at rest on a horizontal, frictionless surface. He throws a 2kg ball horizontally forward.
Answer the following questions concerning the forces involved.
(i) State Newton’s Third Law of Motion.
(ii) At the instant the child exerts a force on the ball, identify the force exerted by the child on the ball and the corresponding force on the child. Show that these two forces satisfy the conditions required for an action–reaction pair.
(iii) Draw a clearly labelled free-body diagram for the child–skateboard system immediately after the ball leaves his hands, indicating all external forces acting on that system.
(iv) Explain qualitatively why the skateboard begins to roll backward even though the forces between the child and the ball act only within the child–ball system.