Question

You are standing on a cart moving at constant speed of 20 $m{s}^{-1}$ towards the left of a frictionless track. Suppose the mass of the cart along with your mass is 100 kg. There are 4 balls each of mass 2 kg in the cart that is now moving at a constant speed. If you throw a ball straight up with a velocity 20 $m{s}^{-1}$, how will the speed of the cart change?

• A. The speed of the cart will be greater than 20 $m{s}^{-1}$
• B. The speed of the cart will be less than 20 $m{s}^{-1}$
• C. The speed of the cart will not change
• D. The speed of cart will become zero

Answer 1

The correct option is C The speed of the cart will not change

The principle of conservation of linear momentum states that if in any assigned direction the net external force on a system is zero, then the linear momentum of the system will remain constant in that direction.
Since the ball is thrown perpendicular to the motion of cart, `throwing’ has no effect on the cart. While in air, the ball will maintain its motion due to inertia in the left-hand direction. Therefore there exists no external force in the horizontal direction that can change the speed of the cart.