FORCES
V. Ryan © 2002  2010
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The diagram below clearly shows a state of
equilibrium. The cars on either side of the seesaw are exactly the
same in weight and height, in fact they are the same model. As a result, the
seesaw stays level. 

The cars are in a 'state of equilibrium' because the weight on either side is exactly the same 

If an extra
car is added to the right hand side (see diagram below), then the seesaw
will turn in a clockwise direction  called a clockwise moment.
Alternatively, if more cars are added to the left hand side, the seesaw will turn in an anticlockwise direction  called an anticlockwise moment. 



A clockwise moment as an extra car is added to the right side  
If the seesaw is to be in equilibrium, the clockwise moments must be equal to the anticlockwise moments. The seesaw is back in ‘equilibrium’ because a second car has been added to the left hand side, as well.  
If the seesaw is to be in equilibrium then the clockwise moments must be equal to the anticlockwise moments. 

A state of equilibrium is also seen below. Each of the cars weighs the same (1 Tonne). Despite the fact that there is only one car on the lefthand side, the moments balance because, the car on the lefthand side, is twice the distance from the fulcrum, compared to the cars on the righthand side. (see the calculation below).  
CLOCKWISE
MOMENTS = ANTICLOCKWISE MOMENTS 1 TONNE X 12m = 2 TONNE x 6m 12 = 12 STATE OF EQUILIBRIUM 

A state of equilibrium exists below. The single car on the left, balances the three cars on the righthand side. This is because, the single car is three times the distance from the fulcrum, compared to the three cars on the righthand side. Both sides of the fulcrum balance.  
QUESTIONS 

1. The diagram below shows a lever where an effort of 200 N balances a load of 600 N. The effort force is 6 metres from the fulcrum. The load force is two metres from the fulcrum. 

Clockwise moment = 600 x 2 Nm Anticlockwise moment = 200 x 6 Nm In a state of equilibrium, clockwise moments = anticlockwise moments 600 X 2 Nm = 200 x 6 Nm 1200 = 1200


2. In the diagram below a crowbar is used to move a 400n load. What effort is required to move the load?


Clockwise moments = 400 N x 0.6 m Anticlockwise moments = effort x 1.5m In equilibrium; clockwise moments = anticlockwise moments 400 x 0.6 = effort x 1.5 effort = 400 x 0.6 1.5
effort = 240 1.5
= 160 N


