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Static Equilibrium
The concept of torque is useful in determining whether or not
a given object will be in static equilibrium. Consider two
children, each of mass 30 kg, sitting on a teetertotter
as below.
Figure 5.4:
Static equilibrium for a teetertotter

There are two general conditions for the teetertotter to
be stationary. The first is that the net force on it must
vanish, since by Newton's second law (Force = mass x acceleration),
a net force will cause an object to accelerate. In this situation
this condition can be satisfied by requiring that total (downward)
weight of the two children equals
the (upward) force that
the fulcrum exerts on the teetertotter at O. However, this condition
does not guarantee that the teetertotter remain motionless, as
it still might rotate about the fulcrum. To prevent this, one
also requires that the net torque about the fulcrum must vanish.
In this case there are two torques acting on the teetertotter,
one from each child, which act in opposite directions. Since
the torque is the product of the force and distance from O,
the force in this case being the weight of a child, and since
the children have the same weight, we see that for the teetertotter
not to rotate that the children must sit at equal distance from O.
We can also infer that if one child, for example, was twice as
heavy as the other, that the heavier child would have to sit closer to
O by a factor of 2 relative to the distance the lighter child is away
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Up: Torque
Previous: Torque
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19990929