Orbits
Earth and Space | Forces and Motion

Going around in circles

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Demonstrating the need for a centripetal force.

This activity provides a memorable demonstration of motion in a circle and demonstrates the need for an inward centripetal force for circular motion.

What to Prepare

  • a long length of cord with a large rubber bung on the end
  • a bucket half filled with water
  • a length of rope

Safety note: The teacher should try these demonstrations in advance. Check that the bucket and the rope will exert the force required. Make sure pupils stay out of the firing line.

What Happens During this Activity

This is a demonstration for the school yard or playing field. Line the class up, shoulder to shoulder, facing you in a line. Take the rubber bung and start swinging it around in a circular path on the end of the cord (cowboy style above your head). Draw attention to the way in which this particular satellite is being pulled along its circular path by the tension in the cord.

Now warn the class that you are going to release the cord so that the satellite can escape from its orbit! Tell them not to worry. You'll release the satellite as it passes directly in front of the class.

Some pupils may become apprehensive. But, of course, if released at this point the satellite continues along its tangential path, parallel to the line of pupils.

The demonstration can be extended by offering to use a bucket of water as a satellite. This does not introduce any new ideas, but the class will be fascinated by the possibility of you becoming soaked!

With the bucket half full of water, start swinging it around on a short length of rope and gradually pay out the rope to increase the radius of orbit of the satellite.

What force is acting to keep the water in the bucket moving along a circular path?

The question is certainly worth asking, but only after you have got the satellite back to Earth. (Answer: the force is provided by the sides and bottom of the bucket acting on the water.)

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