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.

Light - Reflection and Refraction

Compare Reflection of light and Refraction of light

FORCE ICSE CLASS 10 PHYSICS EFFECTS OF FORCE

ICSE PHYSICS CLASS 10 - CHAPTER 1 - FORCE - EFFECTS OF FORCE - Notes

    Effects of Force :

i)        Force can change the state of rest or the state of motion.

a)      Force can move a rigid stationary body which is free to move in the direction of force applied.

b)      A moving body can be stopped by applying a force in the opposite direction of its motion.

c)      The speed of a moving body increases when a force is applied in the direction of its motion.

d)      The direction of a body can be changed by a force applied perpendicular to the direction of the motion.

e)      If a force is applied in any other direction then both the direction and speed of the body will change.

ii)      If a force is applied on a non-rigid body which is not free to move, it changes the shape or size of the body.

    Units of Force:

C.G.S unit        1 dyne = gm cm s^-2

dyne: One dyne is that force which when acts on a body of mass 1gm, produces  an acceleration of 1cm/s² in it.

S.I unit             I newton = kg m s^-2

newton: One newton is that force which when acts on a body of mass 1kg, produces an acceleration of kg/s²_.

1N = 10⁵ dynes

Gravitational unit of force F = mg

M.K.S unit        9.8N = 1kgf

C.G.S unit        980 dynes = 1gf

Contact and Non-contact Forces, Effect of A Force and Units - PHYSICS ICSE

Contact and Non-contact Forces, Effect of A Force and Units - Notes

  FORCE

1A   Contact and Non-contact Forces, Effect of A Force and Units

    Force: A physical cause which changes or tends to change either the size and shape of the body or the               state   of rest or the state of motion of the body.

    Forces are categorized into two kinds            :           1. Contact Forces

                                                                                    2. Non – contact forces

   1. Contact Force: The force acting between two bodies when the bodies are in physical contact. It is also

    called Direct Force. The following forces are considered contact forces.

a) Frictional Force: When a body slides or rolls over the surface of another body, a force resists the motion

     of the body and acts in the opposite direction. This is called Frictional force (Force of Friction).

    

     b) Normal Reaction Force: When a body is placed on a surface the body exerts a force which is equal to

      its weight and the surface also exerts an equal and opposite force normal to the surface.

     c) Tension Force as applied through a string: When a body is suspended by a string, the body exerts a

      force on the string and pulls it vertically downwards which balances the weight of the body. This force

      is called the Tension Force(T).

     d) Force exerted by a spring: Restoring Force: The force exerted by a compressed or stretched spring

      on an object attached to the spring.

      This force is directly proportional to its elongation or compression (the displacement of the other end of

      the spring).

      The elongation of a spring is directly proportional to the weight suspended from it – Spring balance

      works on this principle.

      e) Force exerted during collision: When two bodies collide, equal and opposite forces act each body

      and they push each other.

When a body A moves towards another body B and collides with it, the force exerted by A on B is called

     Force of action and B exerts an equal and opposite force on A i.e. called Force of Reaction.

2. Non-contact Forces: The Force experienced by the bodies when they are not in physical contact (Forces

     at distance).

a) Gravitational Force: The force of attraction existing between any two particles in the universe due to

     their masses.

     The force on a body due to Earth’s attraction is called Pull of Gravity or Force of Gravity. Due to this

     force an object released from a height falls down.

     b)  Electrostatic Force : The force acting between two charges when they are at a distance .

     example: A comb rubbed on a dry hair, attracts bits of paper due to electrostatic force

     c) Magnetic Force: The force existing between two magnetic poles even when they are at a distance.