**Work and Energy Worksheet-9**

- The power output of an engine is 3 kW. How much work does the engine do in 20 s ?

- An electric heater uses 600 kj of electrical energy in 5 minutes. Calculate its power rating.

- Five electric fans of 120 watts each are used for 4 hours. Calculate the electrical energy consumed in kilowatt-hours.

- Describe the energy changes which take place in a radio.

- Write the energy transformations which take place in an electric bulb (or electric lamp).

- Name five appliances or machines which use an electric motor.

- A bulb lights up when connected to a battery. State the energy change which takes place :

in the battery and in the bulb.

- The hanging bob of a simple pendulum is displaced to one extreme position B and then released. It swings towards centre position A and then to the other extreme position C. In which position does the bob have :

(i) maximum potential energy ?

(ii) maximum kinetic energy ?

- A car of weight 20000 N climbs up a hill at a steady speed of 8 m/s, gaining a height of 120 m in 100 s. Calculate :

work done by the car and power of engine of car.

- What do you understand by the term "transformation of energy" ? Explain with an example. Explain the transformation of energy in the following cases :

When a ball thrown upwards, and A stone dropped from the roof of a building.

- Explain how, the total energy a swinging pendulum at any instant of time remains conserved. Illustrate your answer with the help of a labelled diagram.

- How much electric energy in kWh is consumed by an electrical appliance of 1000 watts wheit is switched on for 60 minutes ?

- A boy weighing 40 kg carries a box weighing 20 kg to the top of a building 15 m high in 25

seconds. Calculate the power, (g = 10 m/s^{2})

- The following data was obtained for a ball of mass 1 kg dropped from a height of 5 meters:

Distance above ground – Velocity

5 m 0 m/s

3.2 m 6 m/s

0 m 10 m/s

Show by calculations that the above data verifies the law of conservation of energy (Neglect air resistance). (g = 10 m/s^{2})

- In loading a tanker, a man lifts boxes of 100 N each through a height of 1.5 m.

(a) How much work does he do in lifting one box ?

(b) How much energy is transferred when one box is lifted ?

(c) If the man lifts 4 boxes per minute, at what power is he working ? (g = 10 ms^{–2})

- Name the energy transfers which occur when :

an electric bell rings

- Name the energy transfers which occur when :

someone speaks into a microphone

- Name the energy transfers which occur when :

there is a picture on a television screen

- Name the energy transfers which occur when :

a torch is on

**Answer:**

- 60 kJ

- 2 kW

- 2.4 kWh

- –

- Electrical energy to light energy

- Fan ; Flour grinder, Washing machine, vacuum cleaner ; Water pump ; Hair dryer

- (a) Chemical energy to Electrical energy (b) Electrical energy to Light energy and Heat energy

- (i) Maximum potential energy in position C ; Because at position C, the bob is at the maximum height

(ii) Maximum kinetic energy in position A ; Because at position A, the bob has maximum speed (or velocity)

- 2400 kj & 24 kW

- –

- –

- 1 kWh

- 360 W

- Total energy at 5 m above ground = P.E. 50 J + K.E. 0 J = 50 J ;

Total energy at 3.2 m above ground = P.E. 32 J + K.E. 18 J = 50 J;

Total energy at 0 m above ground = P.E. 0 J + K.E. 50 J = 50 J;

Since the total energy (potential energy + kinetic energy) of the freely falling body at all the three positions during its fall is same , therefore, the given data verifies the law of conservation of energy

- Work = force × displacement

Work = 100 × 1.5 = 150 J

(b) 150 J

(c) 10 W

- Electrical energy to Sound energy

- Sound energy to Electrical energy

- Electrical energy to Light energy (and Heat energy)

- Chemical energy to Electrical energy to Light energy (and Heat energy)