Triple Physics Topic 9 - Circular Motion

Motion in a Circle

We have looked at motion in a straight line.  In this topic we are going to explore some of the basic ideas of circular motion.

Think about a toy aeroplane tied to a pole:

The aeroplane is flying at a constant speed.  However:

Its velocity is changing all the time.

 Question 1 Why is the velocity changing when the linear speed is constant? ANSWER

If there is a change in velocity the Laws of Physics state that there must also be an 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.

 Question 2 What direction is the acceleration?  Why? ANSWER

Notice that:

• The centripetal force is always towards the centre of the circle.

• The force always acts at 90 degrees to the direction of the movement.

If the string were to snap, the aeroplane would immediately fly in a straight line because there would be no sideways force to keep it in a circular path.  It would fly off at a tangent as shown in the picture below:

Centripetal force is increased when:

• the mass is increased;

• the linear speed is increased;

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 £4.50 (€5) a go to feel the effects of centripetal force.

Centrifugal Force

Centrifugal force is a very common bear trap:

There is no such thing as centrifugal force.

This runs counter to what you might think, because you feel a force pushing you away from the centre of the circle.  In reality you are trying to fly off at a tangent.  If you do come unstuck, that is exactly what will happen.

Any reference to centrifugal force is bad physics and you will get no marks for it in the exam.

So don't.

Extension Only

The material in the extension, about the movement of stars, planets, and satellites is NOT on the syllabus, but it is an important application of centripetal force.    It's worth knowing about.  Click HERE to proceed.

 Summary When a force acts at 90 degrees to the motion, the object moves in a circle The linear speed is constant, but the velocity is changing, because the direction is changing. The change in velocity results in acceleration. Acceleration is the result of a force. The force is towards the centre of the circle and is called centripetal force. There is no such thing as centrifugal force.