Test: Work And Energy

QUESTION: 1

When a body falls freely under gravity, then the work done by the gravity is

A.
Positive
B.
Negative
C.
Zero
D.
Infinity

Solution:

If a force acting on a body has a component in the direction of displacement, then the work done by the force is positive. Hence when a body falls freely under the influence of gravity the work done by the gravity is positive.

QUESTION: 2

There are two bodies X and Y with equal kinetic energy but different masses m and 4m respectively. The ratio of their linear momentum is-

A.
1:2
B.
4:1
C.
1:√2
D.
1:4

Solution:

If X and Y have equal kinetic energy but their masses are m and 4m respectively. Thus we get the ratio of their speed is 2:1. Hence the ratio of their linear momentum is 1:2.

QUESTION: 3

Which of the following is not an example of zero work?

A.
The work done by the tension in the string, when a body tied with the string move along a circular path
B.
When a horse pulls a cart on a level road.
C.
The work done by a coolie when he travels on a horizontal platform, with a load on his head.
D.
When a person holds some amount of load but, does not move from his position.

Solution:

Work done, W = Force x Δdisplacement

So, when a person holds some amount of load but does not move from his position, the displacement is 0. Hence, W done = 0.

QUESTION: 4

A truck moves at a steady speed on a horizontal road. What is the work done by the road on the truck?

A.
Not defined as the truck is non- living object.
B.
Negative work is done on the truck
C.
No work is done on the truck
D.
Positive work is done on the truck

Solution:

The work is zero because while the truck does have a displacement over time, the force of the road (the normal force) is perpendicular and so the dot product of the two is zero.

QUESTION: 5

Which of the following is not a unit of energy?

A.
newton meter
B.
electron volt
C.
joule/meter
D.
kilowatt hour

Solution:

Energy The joule ( symbol: J) is a derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 newton metreor N⋅m).

QUESTION: 6

Which of the following statements is false:

A.
kinetic energy is positive
B.
potential energy is positive
C.
kinetic energy is negative
D.
potential energy is negative

Solution:

We know that KE = ½ mv²
And as nighter the mass nor v² can be negative.
We get KE cant be negative.

QUESTION: 7

Raindrops fall on the earth with certain kinetic energy on the earth. From where do they get this energy?

A.
Chemical Energy
B.
Gravitational Potential Energy
C.
Energy between molecules of water
D.
Heat Energy

Solution:

Every object that tends to fall or does fall on earth's surface does have a gravitational acceleration which is possessed by them due to their gravitational potential energy.

QUESTION: 8

Time rate at which work is done by a force is

A.
Power
B.
Torque
C.
Centrifugal Force
D.
Acceleration

Solution:

We know that Work done, W = F.s where F is force and s is displacement due to that force
Thus rate of work done = dW/dt = d(F.s)/dt
So as F is constant we get dW/dt = F.d(s)/dt
= F.v = P

QUESTION: 9

Output of a truck is 4500 J and its efficiency is 50%, the input energy provided to the truck is

A.
5000 J
B.
900 J
C.
9000 J
D.
500 J

Solution:

QUESTION: 10

When a body slides against a rough horizontal surface, the work done by friction is

A.
Positive
B.
Zero
C.
Negative
D.
Constant

Solution:

If a force acting on a body has a component in the opposite direction of displacement, the work done is negative, when a body slides against a rough horizontal surface, its displacement is opposite to that of the force of friction. He work done by the friction is negative.