Question 1

Question 61:
In case of negative work, the angle between the force and displacement is :

(a) 0°
(b) 45°
(c) 90°
(d) 180°

Accepted Answer

Answer:
Work done is negative when the direction of force is opposite to the direction of motion. So, the angle between force and displacement in the case of negative work is  .
Therefore, the answer is, (d)  180°

Question 2

Question 62:
An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass 3.5 kg. Both the spheres are dropped simultaneously from a tower. When they are 10 m above the ground, they have the same :

(a) acceleration
(b) momentum
(c) potential energy
(d) kinetic energy

Accepted Answer

Answer:
Momentum, potential energy and kinetic energy depend on the mass of the body. So, the answer is acceleration as it is independent of mass.
Therefore, the answer is,
(a) Acceleration

Question 3

Question 64:
The work done on an object does not depend on the :

(a) displacement
(b) angle between force and displacement
(c) force applied
(d) initial velocity of the object

Accepted Answer

Answer:
The work done on an object does not depend upon the initial velocity of the moving object.
Therefore, the answer is,
(d) Initial velocity of the object.

Question 4

Question 65:
Water stored in a dam possesses :

(a) no energy
(b) electrical energy
(c) kinetic energy
(d) potential energy

Accepted Answer

Answer:
Water stored in a tank possesses gravitational potential energy because of its position above the ground.
Therefore, the answer is,
(d) Potential energy.

Question 5

Question 67:
Each of the following statement describes a force acting. Which force is causing work to be done?

(a) the weight of a book at rest on a table
(b) the pull of a moving railway engine on its coaches
(b) the tension in an elastic band wrapped around a parcel
(d) the push of a person's feet when standing on the floor

Accepted Answer

Answer:
We will justify all statements one by one as:
(a) The weight of a book at rest on a table cannot do any work as the book is at rest. So, the work done by this force is zero.
(b) The pulling force applied by the railway engine on its coaches is doing work, as the force applied by the engine results in the motion of the coaches.
(c) The tension force in an elastic band wrapped around a parcel cannot do any work, as there is no motion of the parcel because of the tension force of the rubber band.
(d) The push of a person’s feet on a floor when he is standing cannot do any work because there is no motion involved due to the applied force.

Therefore, the answer is -
(b) The pull of a moving railway engine on its coaches.

Question 6

Question 69:
Which of the following does not posses the ability to do work not because of motion?

(a) a sparrow flying in the sky
(b) a sparrow moving slowly on the ground
(c) a sparrow in the nest on a tree
(d) a squirrel going up a tree

Accepted Answer

Answer:
We will justify all statements one by one as,
(a) A sparrow flying in the sky possesses kinetic energy, which can be transformed into work.
(b) A sparrow moving slowly on the ground possesses kinetic energy, which can be transformed into work.
(c) A sparrow in the nest does not possess any kinetic energy as it is at rest. It does not possess the ability to do work in terms of motion of the body.
(d) A squirrel going up a tree possesses kinetic energy, which can be transformed into work.

Therefore, the answer is:
(c) A sparrow in the nest of a tree.

Question 7

Question 70:
A stone is thrown upwards as shown in the diagram. When it reaches P, which of the following has the greatest value of the stone?

Figure

(a) its acceleration
(b) its kinetic energy
(c) its potential energy
(d) its weight

Accepted Answer

Answer:
When a stone is thrown vertically upwards, the following conclusions can be done for point P,
(a) The acceleration for the entire journey of the stone is constant, as acceleration due to gravity near the surface of the earth is constant.
(b) The velocity of the stone at P will be the least. Hence, kinetic energy at this point will be the least.
(c) The stone is at a maximum height at point P. Hence, the potential energy is the maximum at P.
(d) Weight of the stone will remain nearly constant, as acceleration due to gravity near the surface of the earth is constant.

Therefore, the answer is:
(c) Its potential energy.

Question 8

Question 71:
A boy tries to push a truck parked on the roadside. The truck does not move at all. Another boy pushes a bicycle. The bicycle moves through a certain distance. In which case was the work done more : on the trick or on the bicycle? Give a reason to support your answer.

Accepted Answer

Answer:
Work done is more when the boy pushes a bicycle because the force applied by him on the bicycle results in the motion of the bicycle. On the other hand, the boy trying to push a heavy truck parked on the roadside results in zero work, as there is no motion in the truck.

Question 9

Question 73:
What should be the angle between the direction of force and the direction of motion of a body so that the work done is zero?

Accepted Answer

Answer:
The relation between work, force, distance and angle between the two is:
W = (F) (S) cos θ
Where,
(W) - Work done
(F) - Force
(S) - Displacement
(θ) - Angle between force and displacement
When the force is perpendicular to the direction of motion,   and hence, the work done is also zero.
Example - The work done by the earth on a satellite moving in a circular orbit around it is zero.

Question 10

Question 74:
In which of the following case the work done by a force will be maximum : when the angle between the direction of force and direction of motion is 0° or 90°?

Accepted Answer

Answer:
The relation between work, force, distance and angle between the two is:
W = (F) (S) cos θ
Where,
(W) - Work done
(F) - Force
(S)- Displacement
(θ)- Angle between force and displacement
When the force is perpendicular to the direction of motion, and hence, the work done is also zero.
When the direction of force is similar the direction of motion, and hence, the work done is maximum.

Question 11

Question 75:
How much work is done by the gravitational force of earth acting on a satellite moving around it in a circular path? Give reason for your answer.

Accepted Answer

Answer:
The relation between work, force, distance and angle between the two is:
W = (F) (S) cos θ
Where,
(W) - Work done
 (F)- Force
 (S)- Displacement
 (θ) - Angle between force and displacement
The work done by the earth on a satellite moving in a circular orbit around it is zero, as gravitational force is perpendicular to the direction of motion at every instant, and hence, the work done is zero.

Question 12

Question 77:
When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero?

Accepted Answer

Answer:
When a ball is thrown vertically upwards, its velocity goes on decreasing and hence, its kinetic energy also keeps on decreasing. As the ball is thrown vertically upwards, its height continuously increases to a certain point and hence, its potential energy increases continuously and reaches its maximum value when the ball reaches the maximum height.

Question 13

Question 78:
A max X goes to the top of a building by a vertical spiral staircase. Another man Y of the same mass goes to the top of the same building by a slanting ladder. Which of the two does more work against gravity and why?

Accepted Answer

Answer:
'X’ and ‘Y’ are two persons doing the same amount of work against gravity in climbing to the top of the building. Work done against gravity depends on the vertical distance through which the body is lifted and is independent of the path travelled to achieve that height. Both men travel the same distance vertically, so the work done by them against gravity is equal.

Question 14

Question 79:
When a ball is thrown inside a moving bus, does its kinetic energy depend on the speed of the bus? Explain.

Accepted Answer

Answer:
The kinetic energy of a ball thrown inside a moving bus depends on the speed of the bus because the kinetic energy is directly proportional to the square of the speed of the moving ball. The speed of the bus adds up to the speed with which the ball is thrown inside the bus. Hence, more the speed of bus, higher will be the ball’s kinetic energy.

Question 15

Question 1:
Name the commercial unit of measurement of energy.

Accepted Answer

Answer:
Kilowatt- hour (kWh) is the commercial unit of measurement of energy.

Chapter 4 : Work and Energy