# Class 9 Work and Energy Important Questions

Important Questions Notes**Work and Energy Important Questions**

**Define 1 watt of power.****What is the commercial unit of energy. Convert it to the S.I. unit.****Derive the formula for kinetic energy.**- .
**State law of conservation of energy.****Show that the total mechanical energy for a freely falling body remain constant/conserved.**

**Calculate the total electricity bill amount for the month of April, if the following devices are used as follows:-****3 bulbs of 40 w for 6 hours****4 tube-light of 50 w for 8 hours****A TV of 120 w for 6 hours.****Given the rate of electricity is ₹ 2.50 per unit.**

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**Where does an oscillating pendulum have maximum P.E. and K.E.?****When a pendulum bob oscillates, why does the bob eventually come to rest? What happens to its energy?**

**What are the differences between gravitational potential energy and elastic potential energy?****When a ball is thrown inside a moving bus, does its kinetic energy depend on the speed of the bus?****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**?**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.****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/s**^{2})**Mathematically prove that the total mechanical energy for a freely falling body remains conserved.****What will happen to the potential energy of a body when its height is doubled?****Derive the relationship between kinetic energy and momentum.****Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h.**

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