Cambridge IGCSE Maths Skills for Science
maths skills practice How is direction important in physics problems? In many situations the direction of a force or the direction of motion is important to the outcome. Here are just two examples: • If you are pushing on a door to open it and someone else also pushes it, the direction of the push from the other person can make it harder or easier for you to open the door.
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• When you are driving a bumper car and are hit by another one, the direction in which you are each initially moving affects the impact.
Maths skill 1: Calculating moments
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The moment of a force is a measure of the turning effect of a force. The direction of the force is important. When an object is balanced, that is, not turning:
• sum of the anticlockwise moments = sum of the clockwise moments (the principle of moments) Also, as in any physics problem involving equilibrium:
• sum of the upwards forces = sum of the downwards forces. TIP
anticlockwise
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11 12 1
10 9
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10 9
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8
4
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Make sure you know the difference between anticlockwise and clockwise.
clockwise
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Worked example 10
Three children are playing on a see-saw which is pivoted in the middle. Where must Jessie be sitting if the see saw is balanced (i.e. in equilibrium)? 3.0 m
TIP
It is easier to apply the principle of moments if you calculate the anticlockwise moment first. Then keep this on the left-hand side of the equation.
Where is Jessie 700 N
Mira 500 N
0.5 m
Akmal 800 N
0.5 m
keY QueStIon to aSk YourSelF:
• Which forces are tending to turn the see-saw anticlockwise about the pivot, and which are tending to turn it clockwise? Jessie’s weight causes an anticlockwise moment, while Mira and Akmal contribute to a clockwise moment.
Original material © Cambridge University Press 2018