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Conditions for translational and rotational equilibrium
While zero net force is sufficient to ensure translational equilibrium, zero net torque is also required to ensure rotational equilibrium. When bothtranslational and rotational equilibrium is achieved, the object is said to be in a state of static equilibrium.
In order to achieve static equilibrium, the sum of the forces acting on the object mustbe zero and the sum of the torques about all axes must also be zero.
Choosing an axis of rotation
If an object is not already rotating, it may not be obvious how to determine the axis aboutwhich the object is likely to rotate. You need to ensure that the sum of the torques about that axis are zero.
As it turns out, you can choose just about any axis you please when doing the calculations to confirm that the sum of the torques are zero. You touched onthis in an earlier module involving the parallel axis theorem. It can be shown that if the net force acting on an object is zero and the net torque about oneaxis is zero, the net torque about every other parallel axis will also be zero.
The worst case scenario
In the worst case, therefore, you can ensure rotational equilibrium by ensuring that the net torque about three orthogonal axes are zero. In somecases, it will be obvious that you only need to worry about one axis. In some cases, you would need to worry about two or three axes.
A ladder
Before climbing a ladder, for example, you need to ensure rotational equilibrium about
The lack of rotational equilibrium about any of these three axes can result in a nasty fall from a ladder.
Ensuring rotational equilibrium for a ladder
Ladder manufacturers install special "feet" with a high coefficient of friction on ladders to avoid accidents from the first item in the above list.These feet are designed to prevent the ladder from sliding away from the wall that it is leaning on.
They also install wide outriggers on the base of very tall ladders to avoid accidents from the second item in the list. The outriggers are designed toprevent top of the ladder from sliding sideways on the wall.
Tree trimmers that use ladders to trim trees take special precautions to prevent accidents from the third item in the list in those cases where bothsides of the top of the ladder are not firmly supported.
Simplification through the choice of an axis
A judicious choice of axis can often greatly simplify the solution of an equilibrium problem by causing one or more forces to go through the axis. Thiseliminates the effect of those forces from the computation of rotational equilibrium.
I will present several example scenarios involving rotational equilibrium in this section. In several cases, I will ask you to construct a replica of thescenario on your graph board.
All scenarios assume that counter clockwise rotation is the positive direction of rotation. All scenarios also assume that the positive horizontal axis is to the right and the positive vertical axis is up.
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