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
From now on we will just think of the simple case of things moving in a circular path.
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.
object
original path
circular path
centripetal
force pulling
object out of a
straight line
path
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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What happens if we remove the centripetal force?
If you are whirling a stone round your head on a
piece of string and the string suddenly breaks,
which way does the stone fly off?
object
continues to
move in a
straight line
You could try this with a soft ball instead of a
stone and just let go. The ball will carry on in the
direction in which it was moving when you let go,
as you ought to expect from Newton’s First Law.
It DOES NOT go off along a radius of the circle.
centre
If the string wraps itself round your finger then
the stone will be moving in a circle with a smaller
radius and the force will be bigger.
If you whirl a bucket of water around in a
vertical circle you will not get wet if the speed of
the bucket is great enough. At every point in the
circle the water tries to fall vertically out of the
bucket due to the force of gravity but also tries
to move in a straight line due to its circular
motion.
If the velocity is large enough the water will not
drop out of the bucket far enough before it is
moved round the circle. However, if you spin it
slowly……!
string breaks
bucket of water
string
The coat hanger experiment
A very good demonstration of centripetal force is the coat
hanger experiment.
Bend a wire coat hanger into a square shape, file the end
flat and bend it so that the end points towards the opposite
corner of the square (see diagram).
finger
Put one finger in the top corner and then balance a penny
on the end of the hook. Now swing the hanger in a vertical
circle. The penny should stay in place. (It may need a bit of
practice).
The centripetal force acts towards the centre (the top
corner) and so keeps the penny balanced on the hook
while the hanger is swung in a vertical circle. No water and
no mess!
coat hanger
penny
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