If two children who weigh different amounts sit on a see-saw, they can still balance the beam in a horizontal position (see Figure 2). The heavier person must move closer to the centre pivot point, the fulcrum. Remember that work, the downward pressure, is force multiplied by distance. So if the weight of one child is heavier than the other, the distance needs to be reduced.
It is important that you understand this principle, as it enables us to determine whether the output force is bigger than the input force. A crowbar is another example of a first-class lever (see Figure 3). The effort of the force on one end of the crowbar magnifies the lifting force on the nail at the other. The nail itself is a downward force which has to be overcome by the lifting force, in order to extract it. The greater the distance of the effort from the fulcrum, the greater the force will be.
Figure 2 A see-saw is a first-class lever.
Figure 3 A crowbar
Figure 4 Scissors, pliers and secateurs New words
Compound first-class levers Scissors, pliers and secateurs are compound first-class levers. They are two levers working together. The effort from your hand enables the scissors to apply a strong cutting force to the paper or card. The load is the resistance of the paper or cardboard to the cutting action. The effort on the plier’s handles enables them to grip a wire or metal bar tightly.
secateurs: a gardeners’ cutting tool used to trim off dead branches and flowers
compound: combining two or more items to increase the effectiveness of a system
Unit 1 Revise levers and gears 101
TERM 3
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