An average force of 125 N is applied on a machine gun firing bullets each of mass 10 g at the speed of 250 m / s to keep it in position. The number of bullets fired per second by the machine gun is :

Solution

Physics sir · 3 steps

Each bullet carries momentum away, so the gun is pushed back; your force must cancel that every second.

1. Momentum per bullet. Convert the mass first: 10 g = 0.01 kg. Each bullet leaves with momentum mv = 0.01 × 250 = 2.5 kg m s⁻¹, and the gun gets the same momentum backwards.
2. Force as momentum rate. If n bullets are fired each second, the gun gains n × mv of backward momentum every second. To keep the gun still, the applied force must supply exactly this momentum per second.

   F = n mv ⇒ n = F/mv = 125/2.5

3. So. n = 50 bullets per second, which is option (b).

The idea to keep. For a steady stream of bullets, water or sand, force equals momentum carried per second: F = n mv. Always convert grams to kilograms before you multiply.

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