How stable is that?

What the Activity is for

Toppling an animal: you can use this activity to get pupils to relate moments to the stability of objects.

What to Prepare

• a collection of model animals
• a set of one-litre squash bottles with screw tops, marked so that they can easily be filled to 1/4 full, 1/2 full, 3/4 full
• one litre plastic measuring jugs, filled with water
• boards with protractors marked on every 15 degrees, for one quadrant only
• blocks or bench clamps to hold the boards vertical
• a coloured perspex or wooden rod, about 15 centimetre long
• the interactive (see below)

What Happens During this Activity

Animals will topple as soon as the line of action of the force of gravity exertedon them falls outside of their feet. So as you push them sideways, they are turning around the outside of the foot that touches the ground. Pupils need to understand this to see the connection with levers. You might use the coloured rod with a large animal to locate the axis about which the animal turns. You might even discuss the two forces and the lengths to the pivot. But keep it simple: This is not necessary! Ask which way the force of gravity is turning the animal. An end on diagram will be useful – you could use the interactive provided.

Ask the class to put the animals in order of stability – from the most stable (can be tilted farthest and still fall back on its feet) to the least stable.

Introduce the bottles as models of road tankers, which can be anything from empty to full. Show how they can be made to topple, again using the coloured rod to show the axis about which they topple. Ask when they would be most stable – half full? Challenge the pupils to produce a graph to show how the stability changes with the fullness of the tanker. See if they can explain their results.