CHAPTER 4 WORK AND ENERGY

 

4.1      Multiple choice questions.

1.      When the applied force and covered displacement are parallel to each other than work done of the body is;
(a) Zero	(b) Negative
(c) Maximum	(d) Minimum
2.      A man pulling a bag with force of 15 N at angle 60° with horizontal plane. If Bag covers 10 m, then work done by the man is
(a) 50 J	(b) 75 J
(c) 100 J	(d) 150 J
3.      The area under a curved in a force and displacement graph shows that
(a) Work under a constant force	(b) Work under a variable force
(c) Work under a maximum force	(d) Work under a minimum force
4.      If the velocity is doubled then the K.E of the body will be
(a) Remain same	(b) Double
(c) Three times	(d) Four times
5.      A bullet of mass 20 g is fired with velocity of 2000 ms-1, the K.E of the Bullet is;
(a) 2000 J	(b) 4000 J
(c) 20000	(d) 40000J
6.      A body of mass 100 g is raised vertical from surface of earth in a gravitational field. The P.E of the body at height 100 m is.
(a) 0.98 J	(b) 9.8 J
(c) 98 J	(d) 980 J
7.      What is the power of an electric motor when it consumes energy of 9 √103J in 3 sec.
(a) 1 hp	(b) 2 hp
(c) 3 hp	(d) 4 hp
8.      Absolute potential energy of a boy at the surface of the earth is
(a) Gm/R	(b) Gm/R2
(c) GmM/R	(d) GmM/R2
9.      What is the value of potential energy of a body at the height where the value of ‘g’ is zero?
(a) Zero	(b) Negative
(c) Maximum	(d) mgh
10.  When velocity of body of mass 10 Kg is increased from 2 ms-1 to 8 ms-1 then work done of the body is
(a) 100 J	(b) 200 J
(c) 300 J	(d) 400 J
11.  Which force is a non-conservative?
(a) Gravitational force	(b) Friction force
(c) Electrostatic force	(d) Magnetic force
12.  Which one source of energy is not a fossil fuel?
(a) Coal	(b) Uranium
(c) Oil	(d) Gas
13.  Conventional method of energy extracted is
(a) Hydro power	(b) Wind power
(c) Tidal power	(d) Biomass power
14.  One megawatt hour is equal to
(a) 3.6 x 107 J	(b) 3.6 x 109J
(c) 3.6 x 1012J	(d) 3.6 x 1018J
15.  If the speed of an object is tripled, its kinetic energy is increased by time.
(a) 1/9 times	(b) 1/3 times
(c) 6 times	(d) 9 times
Answer

1. (c) Maximum
2. (b) 75 J
3. (a) Work under a constant force
4. (d) Four times
5. (d) 40000J
6. (c) 98 J
7. (d) 4 hp
8. (c) GmM/R
9. (a) Zero
10. (c) 300 J
11. (b) Friction force
12. (b) Uranium
13. (a) Hydro power
14. (b) 3.6 x 109J
15. (d) 9 times

4.2      Shorts question

1. Using work formula, at what angle work will be negative? Give example of Negative work.

ans.    When the displacement and force are anti-parallel to each other (θ = 180° ) then the work done in this case will be negative.

2. Calculate the work of a body when it is moving with uniform velocity.

5. How can you prove mathematically that power is a scalar quantity?

Since the dot product of two vectors give a scalar quantity and power is the dot product of F and v

6. How can Absolute potential energy be achieved?

4.3      Numerical problems

1. A man pulls a bag along the ground with a force of 80 N at an angle of 30° From the ground. How much work done the man do in pulling the bag 10 m? 

3. A pump lifts water from a well to a tank 30 m above the well. If there are 100 m³ water stored in tank, then how much work against the gravity does by the pump. Density of water is 1000 kg m³.

6. A car of mass 1500 kg is accelerated from rest to a speed of 30 ms^-1 in a time of 10 sec. What is the power of car when friction is neglected?

9. A body of mass ‘m’ is dropped from a tower 100 m above the ground. What will be the height from ground to the point at which the velocity of body becomes 30 ms^-1. Air resistance is neglected.