Torques and Moments

What causes rotations?

What causes objects to rotate? The answer is the same forces that cause objects to change their inertial motions. Forces cause “push or pulls” altering object inertial motion, but depending on where and how forces are applied, they can also cause “twists and turns.” Not every force does this. So if a force is applied, you might hear the question, does it have any moments? This is another way of asking whether a force produces any “twists or turns.”

What is a torque or a moment?

A moment is another way of asking if a force has a rotational effect. This effect also goes by the more familiar name of a torque. The torque of a force is a measure of the amount of “twist or turn” generated.

How much is dependent on where the force is

applied relative to where the turning would occur. Torques are often associated with wrenches or screwdrivers which turn bolts or screws respectively.

In either case, the further from the bolt or the nut that you are applying the force, the greater the turning power. In this case, applying the force at location A will provide substantially less turning power than at point B.

How are torques calculated?

Torques are computed from the force being applied, but since we know the turning force increases the further from the turning point you are,  a torque is calculated by multiplying the force times the distance from the turning point. In the example above, if B is three times the distance from the turning point as is A, the turning force is then three times as much.

Torque = Force * Distance

Torques and Classical Mechanics

Forces cause objects to accelerate and change their inertial states. Torques are different in that they cause objects to rotate. Torques are produced by forces, so they can be thought of as a side effect of where forces are applied to a body.

Newton’s First Law says that an object will continue its inertial motion if all the forces balance out. For many problems in mechanics, that condition is simply one of adding up the forces in the x and y direction and showing their sum is zero. This works for most problems such as blocks sliding along a surface where there is no pivot point to rotate about.

However, if an object does have a pivot point, rotational effects need to be accounted for. In some cases such as pedaling, the question to be answered is not just how much pedal force is being applied but how much turning force is generated at the crank. In other cases, the rotational effects are not desirable and must be counter balanced so that equilibrium implies not only are the forces balanced but so are the torques.. Building structures must be such that all moments about support points must balance out.

Counterbalancing Cycling Rotational Effects

For cyclists, this equilibrium condition is important when torques are such they are pulling laterally, that is side to side, on the CyclistCycle. This occurs whenever the cyclist is seeking to change the CyclistCycle motion from its current straightline to another.

In the process of doing this, the cycle exerts a pull which can be represented by an artificial force called Centrifugal which acts as if something is wanting to force the cycle to rotate to its left or right. If the cycle is to remain stable, a counterbalancing force is required and this comes from leaning into the curve.


Next Topic:   Physical Work

Cycling as seen through the eyes of elite cyclists.