Couple and Moment of Couple and example

 ICSE PHYSICS Couple and Moment of Couple 

Couple

When a force is applied in a pivoted body it causes that body to rotate about the axis passing through the pivoted point.

Actually the rotation of the body is not only due to the force applied but also due to the equal and opposite force of reaction at the pivoted point. This pair of forces formed by the external force and the force of reaction is called couple.

When you open or shut a door by applying force at the handle an equal and opposite force of reaction is produced at the pivoted point.

If you need a large turning effect to turn an object pivoted at a point then you have to apply two forces of equal magnitude but opposite to each other which turns the body in the same direction.

To turn a water tap we have to apply two forces of equal magnitude but in opposite direction.

Other examples:

1. Tightening the cap of a bottle.

2. Turning a key in a lock.

3. Turning a steering wheel.

 

Moment of couple

 Let us take a bar of length 'd' and it is pivoted at a point ‘O’.

Two equal forces are applied at points A and B at the same time but in opposite direction and the bar turns in clockwise direction.

The moment of force at ‘O’ due the force applied at A = FxOA

The moment of force at ‘O’ due the force applied at B = FxOB

The total moment of both forces = FxOA + FxOB = Fx(OA + OB) = FxAB = Fxd

hence the moment of couple = F x d

Applications of Moment of Force

Common applications (examples) of moment of force - Physics Notes ICSE

1. To open or shut a door, a force is applied normal to the door at its handle which is provided at the maximum distance from the hinges.

2. To turn a steering wheel we apply a force tangentially on the rim of the wheel.

3. In a bicycle the toothed wheel of bigger size can be turned by applying a force on the foot pedal and this toothed wheel is joined to the rear wheel of smaller size by a chain passing over their teeth.

4. A spanner is provided with a long handle which is used to tighten or loosen a nut by applying a small force normally at the end of the handle.

5. A jack screw a long arm so that it can be rotated by applying a small force to raise or lower the load table.

Important: In all these cases larger the perpendicular distance less is the force needed to produce the same moment of force.

The moment of force not only depends on the magnitude of the Force but also on the perpendicular distance of the line of the applied action from the axis of rotation.

Clockwise and anticlockwise moments

Clockwise and anticlockwise moments of Force - Notes

Conventionally,

If the turning effect on the body turns it anticlockwise and the moment of force is called anticlockwise moment and taken positive. The direction of anticlockwise moment is along the axis of rotation outwards i.e moment of force is a vector quantity.

If the turning effect on the body turns it clockwise and the moment of force is called clockwise moment and taken negative. The direction of clockwise moment is along the axis of rotation inwards.

Light - Reflection and Refraction

Compare Reflection of light and Refraction of light

What is Moment of Force - Torque ?

Moment of a force - Torque - ICSE - Notes

A.     Translational Motion : The motion of a body in a straight path is called translational motion.

B.      Rotational Motion : The motion of a body which rotates about an axis passing through a point at which that body pivoted.

Moment (Turning effect) of  a force (or) Torque:

The turning effect of force acting on a body about an axis is called the moment of force or torque.

Factors affecting the turning of a body:

i)     The magnitude of the force applied on the body.

ii)   The perpendicular distance of line of action of force from the axis of rotation.

The moment of force or Torque is equal to the product of the force applied and the perpendicular distance of the line of action of force from the axis of rotation.

Torque = F x OP

Maximum turning of a body is produced when the perpendicular distance of line of action of force from

the fixed point is maximum for a given force.

1.      If the force applied on a body causes clockwise rotation the

moment of force is called clockwise moment.

2.      If the force causes anticlockwise rotation then the moment of force is called anticlockwise moment.

Units of Moment of force

S.I unit                   newton-meter (N-m)

C.G.S unit              dyne-cm

1 N-m = 10⁷ dyne-cm