Circular motion
Objects move in a
straight tine at a constant speed unless a force acts on them. This is Newton’s
First Law. However, many things move in curved paths, especially circles, and
so there must be a force acting on them to pull them out of their straight line
paths and make them turn corners.
The tighter the curve that the object is made to move
in, the bigger the change of direction and so the bigger the force.
Examples of objects moving in curves are:
- The hammer swung by a hammer thrower
- Clothes being dried in a spin drier
- Chemicals being separated in a centrifuge
- Cornering in a car or on a bike
- A stone being whirled round on a string
- A plane looping the loop
- A DVD, CD or record spinning on its turntable
- Satellites moving in orbits around the Earth
- Many fairground rides
We call the force
that makes objects move in a circle the CENTRIPETAL FORCE
(the
name comes from Latin and means centre-seeking)
The centripetal force always acts towards the centre
of the circle to pull the object out of its straight-line path. Although an
object may travel round the circle at a constant speed its direction of motion
is always changing and so its velocity must be changing. Since a change of
velocity is an acceleration there must be a force acting on the object - the
centripetal force.
What produces the centripetal force?
The actual way the force is produced depends on the
particular example:
In a spin drier it is the wall of the drum pressing on
the clothes. When a car, motorbike or bicycle corners it is the friction
between the wheels and the road. (You know how difficult it is to corner on ice
where there is hardly any friction.)
When the Earth orbits the Sun it is the pull of
gravity.
When a railway train corners it is the force of the
rails on the flanged wheels.
When a stone is whirled round on a string it is the
tension in the string.
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