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Slide 2

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First, lets think about Velocity
Here is a picture I've
made of the earth's
orbit of the sun.
Although it moves in a
circle its velocity must
be drawn as a straight
line because velocity
is a speed in a fixed
This means that
something that is in
circular motion must
be constantly
changing velocity.
First thing to remember for circular motion: Speed=CONSTANT.
Velocity=CHANGING…read more

Slide 3

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Because the earth has a
changing velocity, we
must conclude that there
is a force acting upon it
to cause this change in
We call this either the
Centripetal force or the
Resultant force. Either
can be used in an exam,
so long as you use it in Centripetal/Resultant Force
the right context.
It is a pulling force from
the object towards the
centre of the circle.
Anything in Circular Motion has a force acting on it towards
the centre of the circle.…read more

Slide 4

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This is the Irn-Bru Revolution at
Blackpool Pleasure Beach, which
I've never been on (and never
want to!!).
I'm using it to demonstrate two
other forces we take into account
with circular motion: WEIGHT and
This arrow represents the reaction force,
which is the force that the track exerts on
the car. It always acts in the same direction
as the resultant force.
This arrow represents weight. It
always acts downwards, wherever
the car is in the loop.
This arrow represents the resultant
force. It is the SUM of the reactant force
and weight. It has the same value
wherever the car is in the loop and
always acts towards the centre.…read more

Slide 5

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When the car is coming to the
bottom of the loop, the same
forces are still in action but in a
slightly different way.
You'll notice that the forces this
time are acting in opposite
directions, and that this time the
reactant force is bigger than the
resultant force.
This is because of two laws
regarding circular motion:
-The resultant force must be the
same value all the way round.
-Weight always acts downwards.
Therefore the reactant force
changes so that the resultant
force is still the same value.…read more

Slide 6

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Resultant Force =
Reactant Force
PLUS Weight.…read more

Slide 7

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