Lesson 5, Topic 1
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Couple

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A couple is a system of two equal and parallel but opposite forces not acting in a straight line. This system of forces in parallel can cause a body to rotate and not move in a straight line (linearly).

The moment of a couple is the product of one of the forces and the perpendicular distance between the lines of action of the two forces.

From figure 1 above;

Moment of a couple = Force x perpendicular distance

Moment = F x d

Moment = F x 2r

where r is the radius of the wheel and d is its diameter.

In figure 2 above;

Moment of a couple = $$\scriptsize F \: \times \: x$$

The perpendicular distance between the lines of action of two equal and opposite forces is known as the arm of the couple. In fig. 1 the arm of the couple is (d), while in fig. 2 the arm of the couple is (x).

The magnitude of the couple (i.e. moment of a couple) is the product of one of the forces and the arm of the couple.

The moment of a couple is also called a torque.

Applications of Torque:

Some of the applications of torque or moment of couple include;

1. The easiness to turn a tap on or off by applying two equal and parallel forces compared to applying a single force.
2. It is easier to turn a steering wheel of a vehicle by applying a couple using two hands instead of a single force with one arm.
3. The wind vane.
4. Spinning top.

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