# Class 9 Work and Energy Important Questions

Work and Energy Important Questions

1. Define 1 watt of power.
2. What is the commercial unit of energy. Convert it to the S.I. unit.
3. Derive the formula for kinetic energy.
4. .
1. State law of conservation of energy.
2. Show that the total mechanical energy for a freely falling body remain constant/conserved.
5. Calculate the total electricity bill amount for the month of April, if the following devices are used as follows:-
1. 3 bulbs of 40 w for 6 hours
2. 4 tube-light of 50 w for 8 hours
3. A TV of 120 w for 6 hours.
Given the rate of electricity is ₹ 2.50 per unit.
6. .
1. Where does an oscillating pendulum have maximum P.E. and K.E.?
2. When a pendulum bob oscillates, why does the bob eventually come to rest? What happens to its energy?
7. What are the differences between gravitational potential energy and elastic potential energy?
8. When a ball is thrown inside a moving bus, does its kinetic energy depend on the speed of the bus?
9. The work done by a force acting obliquely is given by the formula: W = F S Cos θ. What will happen to the work done if angle θ between direction of force and motion of the body is increased gradually? Will it increase, decrease or remain constant?
10. A bullet of mass 15 kg has a speed of 400 m/s. What is its kinetic energy? If bullet strikes a thick target and is brought to rest in 2 cm, calculate the average net force acting on the bullet. What happens to the kinetic energy originally in the bullet.
11. A ball is dropped from a height of 10 m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back? ( g = 10 m/s2 )
12. Mathematically prove that the total mechanical energy for a freely falling body remains conserved.
13. What will happen to the potential energy of a body when its height is doubled?
14. Derive the relationship between kinetic energy and momentum.
15. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h.