Question

1.Certain force acting on a 20 kg mass changes its velocity from
5 m s–1 to 2 m s–1. Calculate the work done by the force.
2. 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.
3. The potential energy of a freely falling object decreases
progressively. Does this violate the law of conservation of
energy? Why?
4. What are the various energy transformations that occur when
you are riding a bicycle?
5. Does the transfer of energy take place when you push a
huge rock with all your might and fail to move it? Where is
the energy you spend going?
6. A certain household has consumed 250 units of energy during
a month. How much energy is this in joules?
​

Answer 1

Answer:

1. Kinetic energy is given by the expression, (E k) v = 1/2 mv^2

Where,

E k= Kinetic energy of the object moving with a velocity, v

(i) Kinetic energy when the object was moving with a velocity 5 m s-1

(E k)5 = 1/2 — 20 — (5)2 = 250 J

Kinetic energy when the object was moving with a velocity 2 m s-1

(E k)2 = (1 )/2x 20 — (2)2 = 40 J.

2. Work done by gravity depends only on the vertical displacement of the body. It does not depend upon the path of the body. Therefore, work done by gravity is given by the expression,

W= m g h

Where,

Vertical displacement, h = 0

ˆ´ W= mg — 0 = 0

Hence, the work done by gravity on the body is zero.

3. No. The process does not violate the law of conservation of energy. This is because when the body falls from a height, then its potential energy changes into kinetic energy progressively. A decrease in the potential energy is equal to an increase in the kinetic energy of the body. During the process, total mechanical energy of the body remains conserved. Therefore, the law of conservation of energy is not violated.

4. The muscular energy of the cyclist is converted into kinetic energy (rotational) of the pedals of the bicycle which is transferred to its wheels. The kinetic energy of the rotation of the wheels is converted into the kinetic energy of the bicycle and the cyclist.  

5. Yes, the transfer of energy takes place when we push a huge rock and fail to move it. The energy spent by us gets stored in the rock as potential energy of configuration due to deformation of rock. The deformation in rock is so small that it cannot be observed by us. Some of the energy is also used up in stretching the muscles of our arms while pushing the rock and rapid displacement of blood to the stretching muscles.

6. 1 unit of energy is equal to 1 kilowatt hour (kWh).

1 unit = 1 kWh

1 kWh = 3.6 x 106 J

Therefore, 250 units of energy = 250 — 3.6 — 106 = 9 — 108 J.

Explanation: