Work and Energy Worksheet-9

Work and Energy Worksheet-9

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

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

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

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

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

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

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

in the battery and in the bulb.

8. 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 ?

9. 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.

10. 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.

11. 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.

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

13. 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/s²)

14. 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/s²)

15. 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)

16. Name the energy transfers which occur when :

an electric bell rings

17. Name the energy transfers which occur when :

 someone speaks into a microphone

18. Name the energy transfers which occur when :

 there is a picture on a television screen

19. Name the energy transfers which occur when :

 a torch is on

Answer:

1. 60 kJ

2. 2 kW

3. 2.4 kWh

4. –

5. Electrical energy to light energy

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

7. (a) Chemical energy to Electrical energy (b) Electrical energy to Light energy and Heat energy

8. (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)

9. 2400 kj  &  24 kW

10. –

11. –

12. 1 kWh

13. 360 W

14. 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 

15. Work = force × displacement

Work =  100 × 1.5 = 150 J

(b) 150 J

(c) 10 W

16. Electrical energy to Sound energy

17. Sound energy to Electrical energy

18. Electrical energy to Light energy (and Heat energy)

19. Chemical energy to Electrical energy to Light energy (and Heat energy)