Working and filling stores

What the Activity is for

This experiment is to get a feeling for just how small a joule is – how little mechanical working needs to be done over just how short a duration.

The second part is to see that when gravity does this working, the energy shifted between the stores allows us to predict where an object will end up.

What to Prepare

• a 10 gram mass hanging over a pulley, so that it can be lifted by exactly one metre
• three or more 100 millilitre clear beakers
• a litre of orange liquid (use yellow food colouring)
• a ball on a flexible track
• some retort stands, bosses and clamps to hold the track
• some white-tack to hold the ball-bearing in place whilst you discuss this snapshot
• some cardboard velocity arrows of different lengths
• some labels for the kinetic and gravity stores (optional: add a thermal store) - see below

What Happens During this Activity

In the first part, introduce the hanging mass over the pulley, perhaps mounted semi-permanently in some kind of frame and left at the side of the laboratory, for students to try as and when they have time to do so. This allows students to get a feel for what's involved in shifting a joule of energy.

Now for a fresh start. As gravity is working, so energy is being shifted. We suggest allowing the ball-bearing to run from one side of a u-shaped track to another. Discuss the energy shifted by the mechanical working.

Then we suggest a change of focus by having the orange liquid model of the energy shifted in the foreground, and the u-shaped track in the background. After sharing the energy out, ask where the ball might be on the track, and what it's velocity might be, as a result of this particular sharing of the energy. Use the arrows and tack to position and label the ball. (Take care: as the track is u-shaped, there will usually be two positions, and also we can't tell which way the ball is travelling.)

You might add to the complexity by considering situations where the frictional forces are significant, but for many classes this may be a step too far.

You might also show the interactive from the Physics Narrative to complement the physical model and real-world that you've shown.