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Work and Energy : Practice Questions

1 Mark Questions

1. Define SI unit of work.

2. A student is writing a three hours science paper. How much work is done by the student? Give reasons to your answer.

3. List two essential conditions for work to be done.

4. Give one example when work done by a body negative.

5. A 2 m high person is holding a 25 kg trunk on his head and is standing at a roadways bus-terminus. How much work is done by the person?

6. Calculate the work done when a force of 15 N moves a body by 5 m in its direction.

7. Moon is experiencing a gravitational force due to earth and is revolving around the earth in a circular orbit. How much work is done by moon?

8. State law of conservation of energy.

9. Name the term used for the sum of kinetic energy and potential energy of a body.

10. If the heart works 60 joules in one minute, what is its power?

11. Define 1 KWh.

12. Define 1 W of power.

13. Name the type of energy possessed by a raised hammer.

14. In an oscillating pendulum, at what position the potential and kinetic energy are maximum?

15. A coolie is walking on a railway platform with a load of 30 kg on his head. How much work is done by coolie?

16. Identify the kind of energy possessed by a running athlete.

17. At what speed a body of mass 1 kg will have a kinetic energy of 1 J.?

18. If the speed of the body is halved, what is the change in its kinetic energy?

19. A horse of mass 210 kg and a dog of mass 25 kg are running at the same speed. Which of the two possesses more kinetic energy? How?

20. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

21. What will cause greater change in Kinetic energy of a body. Changing its mass or changing its velocity?

22. What is power? What is its SI unit ?

23. To what height should a body of mass 5 kg be raised so that its potential energy is 490 J? [g = 9.8 m/${s^2}$]

24. On pushing a mighty stone, a child is unable to move it. If work done is zero, where is the energy, he, lost?

25. State the SI unit of

(a) work,
(b) power.

26. Relate watt to joule.

27. What is one unit of electrical energy consumed equal to?

28. Relate one horsepower to the SI unit of power?

29. A ball of mass 5 kg is dropped from a tower d, height 2 m. What is its K.E. half way down?

30. Define one joule.

31. Define one Watt.

32. In what situation(s) is work done equal to zero?

33. When a body falls freely, its potential energy

(A) .........and simultaneously,
(B) its kinetic energy...........

34. A machine of 500 W works for 2 hours. Find the number of units of electricity consumed.

35. How is energy conversion occurring in a Hydel power station?

36. If a force of 50 N moves a body with constant speed of 10 m/s, how much power is spent?

37. State the CGS unit of energy.

38. Relate the CGS unit of energy to its SI unit.

39. Derive a relation between kinetic energy of a body and its momentum.

Solutions

1. SI unit of work is Joule. 1 Joule of work is done when a force of 1 N displaces a body by 1 m.

2. Work done = Force × displacement as displacement is zero. No work is done by the student.

3. (i) Force should be applied on body.

(ii) Body should move in the direction of force.

4. When a body is raised vertical upward, displacement and gravitation force are is in opposite direction. The work done by gravity is negative.

5. Zero. As displacement is zero.

6. Work done = 15 × 5 = 75 J.

7. No work is done by moon as the gravity force and displacement are perpendicular to each other.

8. Law of conservation of energy states that energy can only be converted from one form to another, it can neither be created nor destroyed.

9. Mechanical Energy.

10. Power = ${{60J} \over {60s}}$ = 1 W. [1 min = 60 s]

11. 1 KWh is the energy consumed when power of 1000 w is consumed for 1 hour.

12. When one joule of work is done in one second, the power is said to be 1 watt.

13. Potential Energy.

14. At the highest point of the bob, potential energy is maximum, while at the lowest point kinetic energy is maximum.

15. Since force due to load is downward, and the coolie is moving in horizontal direction, no work is done by coolie.

16. Kinetic energy.

17. K.E = ${1 \over 2}m{v^2}$

$V\, = \,\sqrt {{{2KE} \over m}} \, = \sqrt {{{2 \times 1} \over 1}\,} = \,\sqrt {2\,} \,m\,/s$

18. Since $K.E\,\alpha {v^2}$

If v is halved; KE = 1/4 of the original.

19. $K.E\,\alpha \,Mass$.

Since mass of horse is greater than that of dog, the horse will posses greater kinetic energy.

20. A freely falling body loses potential energy , but it gains equal amount of kinetic energy such that the sum total of energy remains constant. Hence , this does not violate the law of conservation of energy.

21. Changing velocity of a body will cause greater change in the kinetic energy of the body.

22. Power is the rate of doing work. Its unit is watt.

1 Watt = 1 Joule/s.

23. P.E = mg.h

$\,h\, = \,{{P.E} \over {mg\,}}\, = \,{{490} \over {5 \times \,9.8}}\, = \,10\,m.$

24. The energy is lost in the form of heat energy.

25. (a) SI unit of work is Joule.

(b) SI unit of power is Watt

26. 1 watt = 1 Joule / 1 sec

27. 1 unit of electrical energy = $3.6\, \times \,{10^6}\,$ Joule

28. 1 horse power = 746 Watts

29. A ball is dropped from 2 m. The halfway point is when it has moved through 1m.

By III equation of motion
${v^2}\, - \,{\mu ^{2\,}}\, = \,2 gh$
$\,{v^2}\, = \,2\, \times \,10\, \times \,1$
K.E = ${1 \over 2}m{v^2}\, = \,{1 \over 2}\, \times \,2\, \times 10\, \times \,1\, = \,1\,J.$

30. 1 Joule of work is said to be done when a body is displaced by 1m due to a force of 1N

31. 1 watt is the power of an object which does work at the rate of 1 Joule per second

32. Work is equal to zero when-

(i) Force is zero
(ii) Displacement is zero
(iii) Displacement is perpendicular to the direction of force.

33. (A) decreases, (B) increases

34. Work done 500 × 2 = 1000 Wh

= 1 KWh.
unit of electricity consumed = 1.

35. Potential Energy of water in dam $\to$ Kinetic energy of water passing through gates $\to$ Kinetic energy of turbines $\to$ Electrical Energy.

36. Power = ${{Work\,done} \over {Time}} = {{F \times d} \over t}$ = F × V.

= 50 × 10 = 500 W

37. CGS unit of energy is ergs

38. 1 Joule = ${10^7}$ ergs

39. Kinetic energy = ${1 \over 2}m{v^2}$

= ${1 \over 2}{{{m^2}{v^2}} \over m}\, = {{{p^2}} \over {2m}}$ , where p is the momentum.

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