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Definition: The work done by an agent exerting a constant force
( )
and causing a displacement ( ) equals the magnitude of the
displacement,
s, times the component of along the direction of . In
Figure 5.1, the work done by is:
W = s Fcos .


 
Figure 5.1:
Work

Note:
 If
= 0 W = 0. (ie: no work is done when holding a
heavy box, or pushing against a wall).
 W = 0 if
(ie: no work is done by carrying a
bucket of water horizontally).
 The sign of W depends on the direction of relative to :
W > 0 when component of along is in the same direction as
, and W < 0 when it is in the opposite direction. This sign is given
automatically if we write as the angle between and
and write
W = Fscos .

If acts along the direction of then W = Fs , since
cos = cos 0 = 1.
 Work is a scalar.
 The SI units of work are Joules (J) (1 Joule = 1
Newton meter). In
cgs units: 1 erg = 1 dyne cm.
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10/9/1997