Discuss whether or not work is done in the following cases:
(i) When we twist a wire.
(ii) When we press a football.
(iii) When we push a table.
(iv) When a person carries a handbag and walks on a level road.
(v) When a person holds a book in his hand and keeps it stationary.

A body of mass 25 kg is raised to the top of a building 10 m high and then dropped freely under gravity.
(a) Calculate the work done in raising the body to the top of the building.
(b) What is the value of the gravitational potential energy at the top of the building?
(c) By what factor will the gravitational potential energy of the same body increase if it is raised to the top of a multistorey building 30 m high?
(d) When will the kinetic energy of the body be maximum?

A 100 W electric bulb is lighted for 2 hours every day and five 40W tubes are lighted for 4 hours everyday. Calculate
(i) the energy consumed for 60 days and the
(ii) cost of electricity consumed at the rate of Rs.3 per kWh.

Define the term work.State the conditions that need to be satisfied for work to be done.Give some situations in which work is done.

When is the work done by a force said to be (a) positive and (b) negative?Give examples.

What kind of energy transformation takes place in the following devices?
(i) Electric heater
(ii) electric motor
(iii) dynamo
(iv) electric cell
(v) headphone
(vi) hydroelectric power station
(vii) steam engine 
(viii) solar battery
(ix) photoelectric cell

Write the various energy transformations that occur in the following cases:
(a) Moving a car
(b) Going up in an escalator
(c) Rotating of an electric motor
(d) Release of an arrow from a bow
(e) Lighting of an electric bulb
(f) Tuning of an FM radio receiver
(g) Gas stove burner.

(i) A battery lights a bulb.Describe the energy changes involved in the process.
(ii) Calculate the amount of work needed to stop a car of 500 kg moving at a speed of 36 kmh^-1.