Class 7 practice questions based on energy

 Q1. A man keeps A ball of mass 1/2 kg at a height of 10 m.if ball falls on the ground then, what would be the velocity of the ball just before hitting on the ground?

Ans. Mass= 1/2 kg

          Height= 10 m

         g = 10 ms-²

PE = mgh

⇒PE= 0.5 kg ×10m × 10ms-²

⇒PE = 50 kgms-² × m

⇒PE = 50 N× m

⇒PE= 50 J

From law of conservation of mechanical energy, we know that:

Some total of kinetic energy and potential energy is equal to mechanical energy. So at any given point mechanical energy will remain constant provided there is no resistive force.

So, here resumes that loss of energy due to friction is a negligible. Then we can write

KE = PE

But KE = 1/2 mv²

 1/2 mv² = 50 J

⇒mv² = 2 × 50 J

⇒v² = 100 J / 0.5 kg

⇒ v = √ (100 ×2 J kg-¹ )

⇒v = 10√2 ms-¹