distance from the pivot, he will need to press with twice the force to balance her weight. A see-saw is an example of a beam, a long, rigid object that is pivoted at a point. The girl’s weight is making the beam tip one way. The father’s push is making it tip the other way. If the beam is to be balanced, the moments of the two forces must cancel each other out.
Equilibrium When a beam is balanced, we say that it is in equilibrium. If an object is in equilibrium: ◆ the forces on it must be balanced (no resultant force)
Activity 4.1 Balancing Skills AO3.1 Using techniques, apparatus and materials AO3.3 Observing, measuring and recording AO3.4 Interpreting and evaluating observations and data
Can you make a beam balance?
40
cm
◆
the turning effects of the forces on it must also be balanced (no resultant turning effect). If a resultant force acts on an object, it will start to move off in the direction of the resultant force. If there is a resultant turning effect, it will start to rotate.
Studyy tip p In science and in other subjects, you will often hear about things that are ‘in equilibrium’. This always means that two or more things are balanced.
3 Try different combinations of weights. For example, place 2 N at 20 cm from the pivot. Where must you place a 1 N weight to balance this? Copy the table shown and record your results in it. Can you see a pattern? Weight on left / N
Distance from pivot / cm
Weight on right / N
Distance from pivot / cm
50
30 20 10 0
1 Practise balancing the beam on the pivot. It should balance at its midpoint, as shown. 2 Check that the beam will still balance when you place single weights on each side at equal distances from the pivot.
40
cm
4 Can you balance the beam with a single weight? You will have to move the pivot from the midpoint. Can you work out how to use this method to measure the mass of the beam?
50
30 20 10 0
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Cambridge IGCSE Physics
Original material © Cambridge University Press 2014