## Note on Work

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Work is done when a force acts on an object to displace it. The SI unit of work is Joule (J). The relationship between work, force and displacement are:

Work = force x distance by which the body moves

That is, W = F x D

When a body moves under the influence of a force, work is said to be done. But if there is no motion produced in the body, the work done is zero. Thus, work is done only when the force applied to the body makes it move.

One joule is the amount of work done when a force of one newton acting on a body moves it through a distance of one meter in the direction of the force.

There are two types of work. They are:

1. Work done against the friction
2. Work done against the gravity

#### Work done against the friction

When force is applied to an object then the equal force exerted in the opposite direction. When an object is dragged then equal and the opposite direction is exerted which is called friction. So, when you pull or drag then friction is produced.

Activity 1

1. Take a strong spring balance and a heavy wooden block.
2. Tie the wooden block with the spring.
3. Apply the force on the spring balance to pull the wooden block.

Let the mass of the wooden block be 16 Kg. And you have applied the force to move it through a distance of 40 m. How much work is done on the wooden block?
Here, force is applied to move the body. The force of a body is the product of mass and acceleration due to gravity acting on it. Hence, the weight of the wood is:
F = 16 Kg x 10 m/s2
= 160 N
Distance(D)= 40 m
W = F x D = 160 x 40
=6400 Joules

#### Work done against the Gravity

Activity 2

1. Take a wooden block and tie it with a string.
2. Attach a spring balance to the block and lift it slowly.
3. Note down the reading shown by the pointer of the spring balance.
4. Measure the height through which it is raised.

When the block is lifted, the force of gravity acts on the block constantly and pulls it downward. If the force applied is greater than the force of gravity, the block is raised upwards.

• Work is done when a force acts on an object to displace it. The SI unit of work is newton (N) or joule (J). The relationship between work, force and displacement is : Work = force x distance (W = F x D)
• If there is no motion produced in the body, the work becomes zero.
• We can do work against gravity or against friction.
• When we lift something then it is work done against gravity and when we slide something then it is work done against friction.

.

### Very Short Questions

 s.n Work Energy power 1 Work is a transformation of energy. Energy is the capacity of a body to do work. Power is work or energy per unit time. 2 W = F x d E = W = F x d P = Work done/ time taken 3 Work involves force and distance. Energy is measured by the amount of work a body does. Power involves force and velocity.

Work is a transformation of energy. The unit of work is newton-meter (Nm).

 S.n Force Work 1 Force is the product of mass and acceleration. Work is a transformation of energy. 2 The unit of force is (N) i.e Newton. The unit of work is newton meter (Nm). 3 F= m x a W = F x d

A person gets tired when he stands for a long time though he is not working because the chemical energy stored in the body slowly finishes after converting into muscular and heat energy. Finally, he lacks energy in his body to overcome the force of gravity and he gets tired.

One joule of work is said to be done if a force of 1N displaces a body of 1m in the direction of a force.

Work is a scalar quantity because it has only magnitude and direction.

Any two sources of sound are:

• Ringing bell
• Horn of vehicles

A man guarding a house for an hour isn’t considered as work because he doesn’t apply any force and covers the certain distance.

They are:
Work done by fuel is chemical energy and work done by a moving object is kinetic energy.

Solution,

f = 5 N

d = 20cm

= 0.2m

W =?

We know,

W = F x d

= 5 x 0.2

= 1.0 Nm = 1 J

Thus, work done by this force is 1 J.

Solutions,

F = 5 x 9.8 N

= 49.0 N

d = 10 m

Now,

W = F x d

= 49.0 x 10

= 490 J

Therefore, work done when a load is lifted to a height of 10 m from the ground is 490 J.

\begin{align*} F &= mg \:\:\: [g = 9.8\: m/s^2]\\ F &= 60 Kg \times 9.8 m/s^2 \\ &= 588 N \\ \text {Distance}(D)&= 3 m \\ &= F \times D \\ &= 588 \times 3
&=1746 \: \text {Joules} \end{align*}

0%

Pascal

kg

Joule

Dyne

• ### The relationship between work, force and distance is _______.

Work = force + distance

Work = force x distance

Work = force / distance

Work = force - distance

• ### Force is a _______.

vector quantity

fundamental
>quantity

scalar

• ### Though a man is standing with a heavy load on his head but the work done is zero because ______.

he does not move any distance

the force is not enough to produce any work

force acting is downward on the body

• ### Whenever any work is done, it is done against ________.

gravity

both friction and gravity

none

friction

• ### The capacity of a body to do the work is called _______.

energy

power

all of the above

work

600 J
7200 J
6000 J
50 J
• ### If there is no motion produced in the body, then the work done is _______.

force applied

1

pressure on the body

zero

-980

+980

+490

-490

• ## You scored /9

Forum Time Replies Report
##### Ujjwal Reezal

Good to read from it but better to put grade 7 all books

##### Sam-zzu

what is the difference between work done by fuel and work done by moving object?

WHAT IS VECTOR?