An object in motion will continue with the same velocity unless acted on by an external force.

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Possible Incorporated Topics:

  • Centripetal motion and forces
  • Newton's Laws
  • Circular Motion


Newton’s first law of motion says that a moving object will continue moving in a straight line unless it is acted on by a force. For example, if you threw a football, it would keep flying forever if the force of gravity were not pulling it down to Earth; a curling rock would slide forever if the ice it was sliding on was frictionless.

In this demonstration, a steel ball is given a push and rolls around on the inside of a plastic cylinder. If an object moves in a circle like this, it is obviously not moving in a straight line, and therefore it must have a force acting on it. The force present is the normal force from the ball's contact with the inside of the cylinder (see Fig.1).

Centripetal Force Demo 1

Figure 1: Normal force acting on the ball

If the ball loses contact with the cylinder (like when it encounters the hole in the cylinder’s side), it no longer has the normal force acting on it. It will then continue to roll in a straight line tangential to the direction it was traveling before encountering the hole since the only force acting on it is the force of the push that was initially given to it (see Fig. 2).

Centripetal Force Demo 2
Figure 2: Motion of ball in notched cylinder 


  • Small steel ball
  • Plastic cylinder with a notch cut out of the circumference of one of the cylinder's faces


  • Place the cylinder on a flat surface with the notched side up
  • Place the steel ball against the inside of the cylinder and push it, observing the ball's motion around the cylinder's circumference
  • Flip the cylinder so that the notched side is against the surface
  • Push the ball as in Step 2 and observe its motion when it encounters the hole in the cylinder


  • Push the ball at a safe speed and make sure the demonstrator is ready to catch it when it reaches the notched part of the cylinder.


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