Work and Energy Test

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Work and Energy Test - 30

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Question 1
1 / -0

A rigid body of mass $$m$$ kg is lifted uniformly by a man to a height of one metre in $$30$$ sec. Another man lifts the same mass to the same height in $$60$$ sec. The work done on the body against gravitation in both the cases are in ratio:

A
1 : 2

B
1 : 1

C
2 : 1

D
4 : 1

Solution

$$W=F. S$$
As work done is independent of time. So, both the man does same work.
Hence, the work done on the body against gravitation are in ratio $$1:1$$.
Question 2
1 / -0

What is work done in holding a body of mass $$20\ kg$$ at a height of $$2\ m$$ above the ground? $$(g = 10\ m/s^2)$$

A
$$40\ J$$

B
$$400\ J$$

C
$$10\ J$$

D
Zero

Solution

A body of mass $$20\ kg$$ is held at a height of $$2\ m$$ above the ground means there is no displacement because of which there is no change in its potential energy. Hence work done is zero.
Question 3
1 / -0

The law of conservation of energy implies that the:

A
Total mechanical energy is conserved

B
Total kinetic energy is conserved

C
Total potential energy is conserved

D
Sum of all kinds of energies is conserved

Solution

Law of conservation of energy implies that energy can neither be created nor destroyed, but can merely be transfered from one form to other. These forms are varied. Thus, when considering all forms of energy,(including workdone) the total sum never changes.
Question 4
1 / -0

In which one of the following cases is work done the maximum ?

A
Mohan pushes against a wall for $$30 \ minutes$$

B
Mohan pushes a bicycle which moves through a distance of $$1.5 \ m$$

C
Mohan pushes a truck which does not move at all

D
Mohan stands in the centre of his room

Solution

Work done is defined as $$W = Fs\cos\theta$$, where $$F$$ is the force applied, $$s$$ is the displacement and $$\theta$$ is the angle between force applied and displacement.

In cases A, C and D there is no displacement($$s$$) taking place.
So, $$W=Fs\cos\theta=0$$ (as $$s=0$$)

In case B: $$s=1.5m$$ so, Mohan does some work.

Hence, work done in case B is maximum.
So, option B is correct.
Question 5
1 / -0

How fast should a girl of 40 kg run so that her kinetic energy is 320J?

A
64 m/s

B
8 m/s

C
16 m/s

D
4 m/s

Solution

Given, mass $$m=40kg , K.E=320J , v=?$$
$$K.E=\dfrac{1}{2}mv^2$$

$$320=\dfrac{1}{2}\times 40v^2$$

$$v^2=\dfrac{640}{40}=16$$

$$v=4m/s$$
So, the girl of $$40$$ $$kg$$ should run at $$4m/s$$ so that her kinetic energy is $$320J.$$
Question 6
1 / -0

A vehicle travelling at $$30 \ m/s$$ stops at a distance of $$50 \ m$$ decelerating uniformly. A force of $$9000 \ N$$ is exerted on it by the brakes. What is the work done by the brakes?

A
$$4.5 \times 10^4 \ J$$

B
$$2.7 \times 10^5 \ J$$

C
$$4.5 \times 10^5 \ J$$

D
$$2.7 \times 10^4 \ J$$

Solution

The work done by the brakes is calculated using the relation ,
$$W=F \times s $$,
where $$F$$ is the force applied and $$s$$ is the displacement in the direction of force.

Hence, $$W=9000 \times 50=4.5\times 10^5 \ N$$
Question 7
1 / -0

Work done by static friction force when a person starts running is .......... .

A
equal to the weight of the person

B
zero

C
infinite

D
cannot be calculated

Solution

Static friction is the friction force which does not allow the object to move but here the person is running which means that static friction becomes zero. Hence the work done by the static frictional force is also zero.
Question 8
1 / -0

Which one of the following possesses kinetic energy?

A
A moving car

B
A compressed spring

C
Water stored in the reservoir of a dam

D
A car parked in the garage

Solution

Any moving object must have kinetic energy because of its velocity. So, a moving car possesses kinetic energy.
Question 9
1 / -0

Energy conservation law says that:

A
Energy can't be destroyed but can be produced.

B
Neither energy can be produced nor can be destroyed.

C
Energy can't be produced but can be destroyed.

D
Energy can be produced and can be destoyed

Solution

In physics, the law of conservation of energy states that the total energy of an isolated system remains constant—it is said to be conserved over time. Energy can neither be created nor destroyed; rather, it transforms from one form to another.
Question 10
1 / -0

A body at rest have:

A
Energy

B
Momentum

C
Speed

D
Velocity

Solution

$$\textbf{Step 1-Momentum of body}$$
Here, body is at rest. So, it is not in motion means its speed and velocity both will be zero. Similarly, momentum is product of mass and velocity, so its value will be also zero.

$$\textbf{Step 2-Energies of body}$$
Kinetic energy of body which depends on velocity is also zero but it will have some potential energy which is given by the formula,

$$U = m\ g\ h$$, where, m is mass, h is the height of body and g is the acceleration due to gravity.

So, its potential energy will not be zero.

And total energy is sum of both kinetic energy and potential energy. Which will be also non-zero.

So, a body at rest will have some energy.

Hence, option A is the correct answer.

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Work and Energy Test - 30

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Work and Energy Test - 30

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SHARING IS CARING

Question 1

1 : 2

1 : 1

2 : 1

4 : 1

Solution

Question 2

$$40\ J$$

$$400\ J$$

$$10\ J$$

Zero

Solution

Question 3

Total mechanical energy is conserved

Total kinetic energy is conserved

Total potential energy is conserved

Sum of all kinds of energies is conserved

Solution

Question 4

Mohan pushes against a wall for $$30 \ minutes$$

Mohan pushes a bicycle which moves through a distance of $$1.5 \ m$$

Mohan pushes a truck which does not move at all

Mohan stands in the centre of his room

Solution

Question 5

64 m/s

8 m/s

16 m/s

4 m/s

Solution

Question 6

$$4.5 \times 10^4 \ J$$

$$2.7 \times 10^5 \ J$$

$$4.5 \times 10^5 \ J$$

$$2.7 \times 10^4 \ J$$

Solution

Question 7

equal to the weight of the person

zero

infinite

cannot be calculated

Solution

Question 8

A moving car

A compressed spring

Water stored in the reservoir of a dam

A car parked in the garage

Solution

Question 9

Energy can't be destroyed but can be produced.

Neither energy can be produced nor can be destroyed.

Energy can't be produced but can be destroyed.

Energy can be produced and can be destoyed

Solution

Question 10

Energy

Momentum

Speed

Velocity

Solution

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