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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 centre of the seesaw is called the ‘fulcrum’, seen here as a triangle and this is where the beam, that the cars rest on, tilts backwards and forwards. However, because of the state of equilibrium, they remain completely still.

The weight of the cars is called the effort.

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 left-hand side, the moments balance because, the car on the left-hand side, is twice the distance from the fulcrum, compared to the cars on the right-hand side. (see the calculation below).
CLOCKWISE MOMENTS = ANTI-CLOCKWISE 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 right-hand side. This is because, the single car is three times the distance from the fulcrum, compared to the three cars on the right-hand 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

Anti-clockwise moment = 200 x 6 Nm

In a state of equilibrium,

clockwise moments = anti-clockwise moments

600 X 2 Nm = 200 x 6 Nm

1200 = 1200

 

   

2. In the diagram below a crow-bar 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 = anti-clockwise moments

400 x 0.6 = effort x 1.5

effort = 400 x 0.6

           1.5

 

effort = 240

          1.5

 

= 160 N

 

 
 

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