## Class 9 - Physics

**Question:**
How is the power related to the speed at which a body can be lifted ? How many kilograms will a man working at the power of 100 W,be able to lift at constant speed of 1 m/s vertically ?(g = 10$m/s2$)

**Answer:**

Let a body of mass m be lifted to a height h is time t.

Work done in time t,i.e., W = mgh

Thus,power = $\frac{\mathrm{w}}{\mathrm{t}}$=$\frac{\mathrm{mgh}}{\mathrm{t}}$=mg (h/t) = mg x speed

where h/t is the speed at which the body is being lifted.

Aliter : P = Fv = mg x speed.

Since power = mg x speed,

m = $\frac{\mathrm{power}}{\mathrm{gxspeed}}$=$\frac{\mathrm{100\; W}}{\mathrm{(10$ m/s2)(1\; m/s)$}}$=10 kg (as power=100 W and speed = 1 m/s)

## Questions and Answers

** Work and Energy** - Questions

1. Calculate potential energyof a person having 60 kg mass on the summit of Mt. Everest. Height of Mt. Everest is 8848 m from sea level. (g = 9.8 m/s

^{2})

Answer

3. Fill in the blanks :

Kinetic Energy = ½ × _ × _

Answer

4.

What is law of conservation of energy?

Answer

5. Can there be displacement of an object in the absence of any force acting on it?

Answer

6. A body of mass 120 g is taken vertically upwards to reach the height of 5m. Calculate work done. (Take g = 10 m/s

^{2})

Answer

7. Calculate potential energy of a person having 60 kg mass on the summit of Mt. Everest. Height of Mt. Everest is 8848 m from sea level. (g = 9.8 m/s

^{2})

Answer

8. Fill in the blanks :

SI Unit of work is _______.

Answer

9. An object of mass m is raised from the the earth to an height of equal to the radius of the earth (R). Calculate the gain in potential energy by the object. Assume gravity of the earth is g.

Answer

10. Write an expression for the kinetic energy of an object in terms (i) velocity (

*v*) of an object (ii) in terms of momentum (

*p*) of an object.

Answer

11. What is a conservative forces? Give an example.

Answer

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

Answer

13. Name the type of energy possessed by the following? (i) Stretched slinky (ii) Speeding car

Answer

14. A body of mass 15 kg undergoes downward displacement of 40m under the effect of gravitational force. How much work is done? (take g = 10 m/s

^{2})

Answer

15. Fill in the blanks :

Ability to do work is called _______.

Answer

16. Fill in the blanks :

Work done is defined as the product of _____ and ______ in the direction of force.

Answer

17. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Answer

18. Fill in the blanks :

The sum of P.E. and K.E. is called __________.

Answer

19. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Answer

20. The magnitude of momentum of a moving object is equal to its kinetic energy. What will be the velocity of the object considering SI units for the system?

Answer

21.

A person throws a stone of mass 3kg. It crosses the top of a wall of 2m high at a speed of 4m/s. What is the total mechanical energy possessed by the stone when it crosses the wall?(Take g = 10 m/s)

Answer

22. If the momentum of a body is increased

*n* times, its kinetic energy increases (a) n times (b) 2n times (c) √n times (d) n

^{2} times

Answer

23.

Mathematically show that law of conservation of energy holds true for a free falling object. Consider negligible air resistance.

Answer

24. A motor pump does same amount of water by lifting underground water in shorter time than a hand pump. What do you infer from this statement?

Answer

25. State the relation between commercial unit of energy and joules.

Answer

26. How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?

Answer

27. When do we say that work is done?

Answer

28. Calculate the amount of work done in drawing a bucket of water weighing 10kg from a well of depth 20m.

Answer

29. Write an expression for the kinetic energy of an object.

Answer

30. Fill in the blanks :

Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.

Answer

31.

Is work a scalar or vector quantity?

Answer

32.

A battery lights a bulb. Describe the energy changes involved in the process.

Answer

33.

Assuming the earth revolves around the sun in a perfect circular orbit due to centripetal force exerted by the Sun. Is the work done by the Sun positive or negative.

Answer

34. What is the relationship between kinetic energy and work?

Answer

35. Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.

Answer

36.

A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Answer

37.

What are other forms of energies apart from mechanical energy? Does Law of Conservation of Energy apply to all of them?

Answer

38. A boy of mass 20 kg climbs up a ladder in 20 seconds. If the height of a ladder is 10 metres, calculate amount of power used.

(Take g = 10 m/s

^{2})

Answer

39. Is Work a scalar or vector quantity?

Answer

40. A stone of mass 2Kg is tied with a string and is whirled with a centripetal force of 10N in a circular path. How much work is done for making one revolution?

Answer

41. A force of 10 N moves a body with a constant speed of 2 m/s. Calculate the power of the body.

Answer

42. When a player hits a football it moves along the curved path and then falls to the ground. Calculate the work done by the force of gravity on the football.

Answer

43. Fill in the blanks :

A force acting on a body shows a displacement in the direction of the force. It is said _______ is done on the body.

Answer

44. Fill in the blanks :

An object moving in a circular orbit. The work done by the centrepetal force acting on the object is ______ (zero/non-zero).

Answer

45. Fill in the blanks :

If the mass is doubled while its speed remains the same, the kinetic energy will be _____ (be same/double/four times) of the initial kinetic energy.

Answer

46.

A roller coaster is shown in figure, at points i.e. ➊ and ➋, comparatively at what point (i) potential energy is maximum (ii) kinetic energy is maximum

Answer

47.

A spring is stretched, will the work done by the stretching force positive or negative or zero.

Answer

48. A heavy stone is lowered to the ground with the help of rope. Is the work done by the applied force positive or negative?

Answer

49. Can kinetic energy of an object be negative?

Answer

50. What is mechanical energy? What are its types?

Answer

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