ISE 5-14 Curriculum Support Materials                                                           Overview advice

Group 31 exemplar Energy & Forces - Forces & Gravity (Word)

1. When two spring balances are linked together and pulled in opposite directions, you will notice that;-

a) when pulled equally in both directions, the amount of force shown on the spring balances will be the same and

b) if one person pulls harder, the balances move towards that person

In (a) we have balanced forces - they are equal and opposite and result in a steady state. In (b) we have unbalanced forces and there will be a change of state.

a) When both people are the same weight and the same distance from the middle, the seesaw will balance i.e. the forces are balanced.

You can make a very simple seesaw from a ruler and a pencil. Cubes or pennies could be used as weights. If you like you could introduce the term ‘fulcrum’ - this is the name given to the pivot.

3. When objects are weighed in air, the forces acting on them are

a) the force of gravity pulling down and

b) an equal and opposite force from the balance pulling up.

If either of these forces was missing the weight would not stay in place but would fall or rise up.

If the weight is now suspended from the spring balance in water (see diagram below) then we now have

a) gravity pulling down and

b) the balance pulling up but we also have

c) the water pushing up.

The weight will appear to be less. The forces are still balanced because the weight is not moving but the upwards force from the balance is less.