What keeps bodies moving in a circle?

Centripetal acceleration

You should remember the difference between speed and velocity. Velocity is a vector so it has direction.

An object accelerates if its velocity changes. Remember acceleration = change in velocity / time.

What if the direction of an objects velocity changes but not its speed?  Is it accelerating? The answer is yes.

If an object is moving in a circle the direction of its velocity is constantly changing even if its speed is constant. Imagine that it is being pulled towards the middle of the circle. Its acceleration is centripetal (centre seeking).

Centripetal force

Newton tells us that an object will not accelerate unless there is a resultant (unbalanced) force acting on it and that its acceleration will be in the direction of the resultant force.  An object moving in a circle must therefore have a resultant force on it which is also centripetal.

Remember - If an object is moving in a circle

a) it is accelerating centripetally    b) the resultant force acting on it is centripetal

You should be able to identify what is providing the centripetal force needed to keep various objects moving in a circle. Here are some examples.

Note that if the centripetal force disappeared suddenly then the object would continue in a straight line, i.e. fly off at a tangent to the circle, just like when the hammer thrower lets go.

How much force is needed?

Imagine a hammer thrower.        What would happen to the tension in the rope if the following changes were made?

1.    The mass on the end was heavier?

2.    She spins it round faster?

3.    The rope is shorter (with the mass at the same speed)

For each of these the force needed to keep the mass moving in a circle is bigger. The tension in the rope would be bigger.

The force that pulls it round is friction between the tyres and the road. The car is a lot more likely to skid if;

a)    It is heavier, e.g. full of passengers

b)    It goes round very fast

c)    The radius of the roundabout is smaller

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Revision Questions

 1. An object is moving in a circle. What is the direction of a) its velocity?   b) its acceleration?   c) the resultant force acting on it?

2. Sketch 3 other examples of objects moving in a circle. For each say what force acting on it provides the centripetal force.

3. The centripetal force acting on an object is bigger if it has a larger ............, it is moving .................... or if the ..................... of the circle is smaller.

4. Why do racing cars have big fat tyres?

5. Why should you take corners slowly on a wet or icy road?