8/14/2019 Work - definition

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Summary - Work

A person pulls a box upwards

The box and earth are the system

A person catches a ball

The ball is the system

A person carries a brick whilewalking at a constant pace

The brick and the earth are the

system

earth

person on boxF

boxd

balld

person on ballF

brickd

person on brickF

earth

Because the force exerted by theperson on the box is in the same

direction as the displacement of the

box:

Because the force exerted by theperson on the ball is in the opposite

direction as the displacement of the

ball:

Because the force exerted by theperson on the brick isperpendicular

to the displacement of the brick,

person on box( )( )W F d= person on ball( )( )W F d= 0W =

Example:

A man slowly lifts a 2kg book fromthe floor to a bookshelf 1.5m high.

The force that the man exerts on the

book is about equal to the force of

the earth on the book:

man on book earth on book

book (2kg)(10 N/kg) 20N

F F

m g

=

= = =

If the book and earth are selected as

the system, this is the work exertedby the man on the system:

man on book( )( )

(20N)(1.5m)

30 Nm

W F d=

=

=

Example:

A girl catches a medicine ball ofmass 8kg. She exerts an average

force of 360N and the ball stops in

0.2m.

If the ball is selected as the system,this is the work exerted by the girl

on the ball:

girl on ball( )( )

(360N)(0.2m)

72 Nm

W F d=

=

=

Example:

A woman carries a 9kg briefcase for22m along the sidewalk.

The force of the woman on the

briefcase is upwards, but the

displacement of the briefcase ishorizontal.

In any system that contains the

briefcase, but not the woman, the

external force on the briefcase is

perpendicular to the displacement ofthe briefcase, so no work is done.

0W =

Measurement of workThe unit in which work is calculated is Newton meters (Nm) also called a Joule.

Tips about work:

Always identify a system for energy processes. It is especially important with gravitational potential energy: there

can be no gravitational potential energy in the system unless the Earth is a part of the system.

Since we consider the force of the Earth on objects to be part of the system, the earth does not do work on the

system. If the Earth is not part of the system, then the force that it exerts on system objects does work but the systemdoes not possess gravitational potential energy.

When there are two surfaces rubbing against each other, and both surfaces are in the system, then the internal energy

of the system increases. There is no work done by friction because the surfaces are both inside the system.

Alternatively, you could exclude the supporting surface and include the work done by friction that the surface doeson an object moving across the surface.