Velocity is a vector quantity, it has both size (the
speed) and direction.
If either the speed or direction changes, you have a
change in velocity - you have an acceleration!
With this in mind, imagine whirling a conker around on the end of a piece of
string (right of diagram below).
What velocity are we dealing with? What force are we
dealing with?
Diagram illustrating circular motion - velocity and centripetal force
To keep a body moving in a circle there must be a force
directing it towards the centre.
This is called the centripetal force and produces the continuous change in direction
of circular motion.
Even though the speed may be constant, the object is
constantly
accelerating because the direction is constantly changing via
the circular path - i.e.
the velocity is constantly changing (purple arrows, on the diagram).
For an object to be accelerated, it
must be subjected to a force that can act on it - Newton's 1st law of
motion.
Here the resultant centripetal force
is acting towards the centre, so always directing the object to 'fall'
towards the centre of motion (blue arrows on the diagram).
But the object is already moving, so
the force causes it to change direction.
SO, the actual circular path of motion is determined by
the resultant centripetal force (black arrows and circle) and the
circling object keeps accelerating towards what it is orbiting.
The centripetal force stops the
object from going off at a tangent in a straight line.
When you swing something round on the end of a string, the
tension in the string is the centripetal force.
You yourself feel this force of tension as the 'pull' in
the string.
If you could use a fast action camera to monitor the
motion and the string broke, you would observe the object would fly off
at the precise tangent to the circular path and in a straight line of
constant velocity - the result resultant of Newton's 1st law!
Since gravity and air friction act on the object, you do
have to keep on 'inputting' kinetic energy to keep it swinging round.
The centripetal force will vary with the mass of the object,
the speed of the object and the radius of the path the object takes.
For more on motion and acceleration see
Acceleration, velocity-time graph interpretation and calculations,
problem solving
The same arguments on circular motion apply to the movements
of planets around a sun, a moon around a planet and a satellite orbiting a
planet. The orbits are usually elliptical, rarely a perfect circle, but the
physics is the same.
In these cases, it is the force of gravitational attraction
that provides the centripetal force and it acts at right angles to the
direction of motion.
You should also realise that they are moving through empty
space (vacuum), so there are no forces of friction to slow the object down.
This is why the planets keep going around
the Sun and the moon keeps going around the Earth.
When satellites are put into orbit they
are given just the right amount of horizontal velocity so that the resultant centripetal force of gravity keeps the satellite in its a
circular orbit.
You can vary this horizontal velocity to
position satellites at different distances above the Earth's surface.
For more on motion and acceleration see
Acceleration, velocity-time graph interpretation and calculations,
problem solving
INDEX physics notes FORCES
section 2 on mass, weight and gravity
Keywords, phrases and learning objectives for forces
Be able to explain how the forces of gravity keeps
objects orbiting around each other e.g. moons around a planet,
planets orbiting a star like our sun forming the solar system.
Know in these orbit system the speed is
constant but the velocity is constantly changing,
Know that the centripetal force determines the
circular motion of moons, satellites and planets.
