Forces and Motion

Guiding with forces

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Students guide a ball along a path by exerting forces on it using puffs of air through a straw. The key idea is that the puffs can be re-described as forces acting on the ball, and that these forces (puffs) change the existing motion by adding to it. The discussion needs to bring this out.

This is a tabletop version: you could choose a larger version of the experiment, depending on the space and apparatus available. Any large ball and mallets or nylon-headed hammers could be substituted for the small balls and straws.

There are variants of this that are effectively computer simulations. We'd suggest preferring the physical, for a more immediate connection to the lived-in world. The point is that forces change motions, they don't set motions.

What to Prepare

  • a small ball (About 1 centimetre in diameter.)
  • a straw to blow down (Perhaps with a cardboard arrow added, to show the direction in which the force is exerted by the stream of air.)
  • a drawn path (To propel the ball along: A3 paper is suitable.)

Safety note: Students should not share straws.

What Happens During this Activity

Students take turns to guide the ball along the path, using only short puffs through the straw.

The summary discussion might very usefully bring out the rather obvious link between the puffs and the changes in motion, and that the puffs alter the existing motion, rather than setting a new motion. Somewhat lightheartedly, you might write:

new motion = old motion + change due to puff

You can vary the difficulty of the paths and the mass of the ball: both introduce new things to discuss, particularly if some paths contain sharp angles, rather than smooth curves, and some balls are steel, rather than glass or plastic.

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