7.4 Work Done by a Gravitational Force
To calculate the work done by the gravitational force 
is given by:
where q is the angle between the gravitational
force
,
and the displacement 
of the object.
We can see that we have two situations:
If the object is falling vertically , the work (done by gravity force) is positive (q = 0°)and energy is transferred to the object and its kinetic energy increases.
Ball (m = 0.1 kg) falling down from rest
Ball (m = 0.1 kg) projected vertically up with initial speed 14 m/s
| Script by Dr. Mohamed S. Kariapper using Physlets from |
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If the object is thrown vertically , the work (done by gravity force) is negative (q = 180°)and energy is transferred from the object and its kinetic energy decreases.
Work Done in Lifting and Lowering an Object
In the case where the object is lifted (upward displacement) by a person without acceleration, the work energy theorem tells us that
However, in this case q
= 180° and the work done by the person ( 
)
is positive on the object.
In the case where the object is lowered
(downward displacement) by a person without acceleration, the applied force
does negative work on the object ( 
).
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