A force acts for 20 s on a body of mass 20 kg, starting from rest, after which the force ceases and then body describes 50 m in the next 10 s. The value of force will be:

Solution

Physics sir · 3 steps

Once the force stops, the body coasts at a steady speed, so the last 50 m tells you its final speed.

1. Speed after force. After the force ceases, no net force acts, so the body moves with constant velocity. It covers 50 m in 10 s, so v = 50/10 = 5 m s⁻¹.
2. Acceleration phase. This 5 m s⁻¹ was reached from rest in the 20 s the force acted. So a = (v - 0)/t = 5/20 = 0.25 m s⁻².

   F = ma = 20 × 0.25 = 5 N

3. So. The force is 5 N, which is option (c).

The idea to keep. When a force stops acting, the body keeps the velocity it had at that moment. Use the coasting part to find that velocity, then v = at and F = ma for the pushed part.

Answer from the NTA final answer key. Solution by PrepWiser.