IGCSE
Physics Notes: Motion In a Circle
Newton's First Law states: A body continues in a state
of uniform motion unless acted on by an external force. At any point
on the circle below, at any point the moving body will seek to move
in a tangent. To keep it moving in a circle a force must act so that
the direction of motion is constantly changing
As shown on the right hand diagram, as the the particle
moves around the circle it must be making constant changes to it's
velocity, always towards the centre. This means that it is always
accelerating towards the centre, and implies from Newton's Second
Law, F=ma, that it is accelerating towards the centre constantly. The
force could be a tension, gravity, some reaction force, an electric
force or some other force.
For
a planet moving in a circle around a star the speed is always the
same. Generally planets move in ellipses. At any point on its orbit
the planet has two types of energy: kinetic energy and potential
energy.
When the planet is closest to the star most of it's
energy is kinetic energy because it is moving faster there. As it
moves on its orbit and becomes more distant from the star some of
this kinetic energy is changed into gravitational potential energy
and the star moves slower. When the planet is on the right in the
diagram above it is moving slowest, and it's energy is mostly
gravitational potential energy.
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