WORK

Definition

Mathematical Form

Unit

Cases of Work

Q. What is work?

A. Work is said to be done when a force acts on a

body and moves it through a certain

displacement.

WORK

dWork done by the Force F

m mF

Mathematical Form:

WORK

dWork done by the Force F

m mF

If force is applied on a body and it moves the

body through a displacement ‘d ’, then the work 'W' is

defined by the relation

W = F. d

We have already studied scalars and vectors.

Q. What is the nature of work?

A. Work is a scalar quantity.

WORK

Unit of work:

The SI unit of work is joule (J).

One Joule

When a force of one Newton moves a body

through a distance of one meter in the direction of

force, then the work done is equal to one joule.

WORK

Positive Work

CASES OF WORK

When force and displacement are in the same

direction

Then = 0°

W = F d cos 0°

= F d x 1

= F d

Example

Work done by the Force

m m

F

CASES OF WORK

d

Zero Work

When force is perpendicular to the

displacement

Then = 90°

W = F S cos 90°

= F x 0

= 0

In this case work done is zero. 

CASES OF WORK

Example

CASES OF WORK

When force and displacement are in the opposite direction

Then = 180°

W = F d cos 180°

= F d x (-1) cos 180° = -1

W = - F d

CASES OF WORK

Negative Work

Condition for Negative work

d

      W

CASES OF WORK

Q. Give an example of positive work?

A. Pushing something horizontally is an example of

positive work.

Q. Give an example of negative work?

A. Lifting something vertically upwards is an example

of negative work.

WORK

Why work is Zero ?

POWER

• Power is measured by the amount of Work done in One Second.

• If Work W is done in ‘t’ seconds, then Power

Thus, • Smaller the time in which Work is done, the greater is the

Power.• Power is a Scalar Quantity.• Unit of Power is Watt.• Watt is equal to Joule/second.• Watt is equal to Kgm2/s3 .• Dimension of power is M1 L2 T-3.

P=W/T

Energy

• Energy is the Capacity of a body to do Work.

• Energy represents the total amount of Work that a Body can do.

• Unit of Energy is Joule.• Joule = Kgm2/s2.

Numerical

The two springs for reversing the motion of a heald shaft each have to be stretched 15cm to put them in position with the heald shaft down. If the stiffness of each spring is 1.5N/cm find the work done in putting the springs in position.

Stiffness∞ force

Solution :

Total force required to stretch each spring = 1.5×15 = 22.5N

Total force = 22.5×2

= 45N

Work done = f.d

=45×15 Ncm

=675Ncm

Solution:

W = 675÷100 Nm

W = 6.75Nm

W = 6.75 J

Numerical on Power

A ringframe traveller, moving in a circle of 5cm in diameter at 9000rev/min, offers a resistance to movement of 0.15N. If the frame has 240 spindles, calculate the power expended in moving the travellers.

Solution:

Distance moved by traveller in one revolution = 5¶

= 5(3.14)

= 15.70cm

Distance moved per second = 15.70×9000÷60

= 23.55m

Solution:

W = f.d

Work done per second on each traveller = 23.55×0.15N

W = 3.53J

Total work done = 3.53 on each of 240 spindles per second

Power expended = 847.2W

References:http://www.school-for-champions.com/science/work.htm

http://library.thinkquest.org/2745/data/ke.htm

http://www.regentsprep.org/Regents/physics/phys02/rolcoast/default.htm

http://www.discoveryeducation.com/teachers/free-lesson-plans/elements-of-physics-energy-and-work.cfm

http://www.sparknotes.com/physics/workenergypower/workpower/section2.rhtml

http://everythingscience.co.za/grade-12/08-work-energy-and-power/08-work-energy-and-power-03.cnxmlplus

http://hyperphysics.phy-astr.gsu.edu/hbase/work.html

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