P3aL3 Circular Motion

Key Words

Accelerate - change velocity

Centripetal acceleration - acceleration towards the centre of a circle.

Centripetal force - a force acting towards the centre of the circle.

Circular motion - going round and round in circles.

Friction - a force that opposes movement.

Resultant force - overall force resulting from all other forces.

Tension - a pulling force.

Velocity - speed with a direction.

Grade E

The picture shows a toy aeroplane that is tethered to a pole with a light string.  It is flying at a constant speed.  However it is flying in a circular path, so its velocity is changing all the time.

This is because the the direction is changing all the time.

If there is a change in velocity the Laws of Physics state that there must also be an acceleration.  The acceleration is towards the centre of the circle; it is called centripetal acceleration.

And if there is acceleration, there must a force causing the acceleration.

The force is called centripetal force and its direction is towards the centre of the circle.  Centripetal force always acts at 90^o to the velocity.

If the string were to break, there would no longer be any centripetal force; the aeroplane would fly off tangentially in a straight line. 

This is because there is zero resultant force.

Grade C

Centripetal force is increased when:

• the mass is increased;

• the linear speed is increased;

• the radius is decreased

The first two would make sense.  However the last statement is not immediately obvious.  However if you think about a high speed railway line, the curves all have a very large radius.  This reduces the centripetal force.  On the other hand if you drive a car fast round a sharp bend, the car will skid.

Centripetal force explains all sorts of things that happen in physics from the toy plane flying around its pole, to why you can ride a bicycle (complicated), to the orbits of planets.  Fairground rides are where you pay £1.50 (€2) a go to feel the effects of centripetal force.

In the case of the aeroplane, the tension in the string provides the centripetal force.

If a car goes round a bend, the friction between the tyres and the road provides the centripetal force.

  There is no such thing as centrifugal force.

Grade A

Centrifuges are machines used a lot in scientific research.  Particles in suspension in a liquid are spun at high speed in glass tubes.

In the picture above, there are two particles, one big, one small.  

• The centripetal force acting on each one will depend on the mass. 

• Both will attempt to fly off in a straight line.

• The viscosity of the liquid will provide the tension.

• The small mass will move to a spot where the viscous force and the tension will balance out.

• The big mass will try to fly off in a straight line but will be stopped by the glass tube.