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Gallery 7

Cross Hockey

A cross shaped black and yellow air hockey table with a white surface.
A cross shaped black and yellow air hockey table with a white surface. There are 3 white plastic hitting items on the table top.

The angle at which something rebounds depends on friction, movement and force.

How it works

One to four visitors develop their own game of air hockey using the cross-shaped air hockey table, laws of physics and their motor co-ordination skills. A spinning square shape in the centre of the table adds an extra element of difficulty, and visitors need to take into consideration the rotating speed of the square and calculate their attack when they slide the puck either against, or past the square.

Things to try or ask around the exhibit

  • Can you predict where the puck is going to slide after it hits the rotating square?
  • Can you design a four-person game? What rules will you use?
  • Does it make much difference whether the square is rotating, or would it be more difficult if the square was stationary?
  • Would the game work better without the air?
  • Does it make much difference if you push the puck more softly, with less force?

Background

Air hockey tables use air as a lubricant, to reduce friction between the playing surface and the puck. So, you could say that pucks slide along on cushions of air. Friction is a force that slows down the movement of two surfaces that are passing along each other. Lumps and bumps on the surfaces get caught on each other as they travel past, so it follows that the rougher the surfaces, the greater the friction between them. Through friction, the moving (kinetic) energy of the two surfaces in contact is transformed into heat energy by the rubbing and catching of the surfaces. To reduce this, a fluid is needed as a lubricator between the surfaces.

This exhibit is rich in demonstrations of Newton’s Laws of Motion. You push on the puck (apply force), the puck slides across the table (object in motion/straight line), and it will change direction or possibly even stop when it either hits the square or the wall or another player’s baton, or it eventually stops due to friction (apply force to change direction or motion). Any pushing force you use over and above the opposing frictional force of the floor is called ‘net force’. The air hockey table has greatly reduced frictional forces, so it’s relatively easy to give the puck a light push and have it slide across the air hockey table much farther than when the air is switched off and the force of friction is stronger in acting on the puck to slow it down to a stop.

If the central square was not rotating, the angle at which the puck hits the square (the angle of incidence) would be the same as the angle at which the puck reflects or ‘bounces off’ the square (the angle of reflection). Whilst this is true for light on a mirrored surface, when describing the movement of, say, billiard balls or hockey pucks, it only works in a perfect system. As the puck or ball hits the edge of the table, or another ball, its angle of reflection will be slightly different due to the friction lost in that (albeit short) contact between surfaces, and potential spin that occurs within the puck or ball. So a billiard ball or a puck should bounce off the side of a table at the almost same angle at which you sent it into the wall.

Finding the science in your world

Friction is very important in our everyday life. Without the force of friction we wouldn’t be able to do normal things like walk without slipping over. Nor would we be able to make fire without friction! Because friction is the force that converts moving (kinetic) energy into heat energy, as you strike a match, the friction between the two materials causes a spark.

Air being piped through an air hockey table does not completely remove friction, but it does reduce the force of friction greatly. Similarly, we use oils in car engines, to reduce as much friction as possible between moving engine parts, so the engine lasts longer and operates more efficiently.

To slide something, such as a bookcase across the floor, you need to push with enough force (in Newtons) to overcome the opposing force of friction (also in Newtons) between the bottom of the bookcase and the floor. The net force does the work in sliding the bookcase along.