Class X Physics, ICSE Chapter 1 - Force, Important question and Notes.

Physics Chapter 1 - Force

ICSE Chapter 1 - Force, Important question and Notes. 

Force when applied on a rigid body will cause only motion in it while force when applied on a non-rigid body will cause change in size, shape, and motion of the body.

Force applied on a body is defined as the rate of change of momentum of the body. It is a vector quantity.

The S.I. unit of force is Newton (N). Force is also represented in kgf (Kilogram Force). 1 kgf = g N. The average value of g (acceleration due to gravity) is 9.8 m/s2 on the surface of the Earth.

When a rigid body is acted upon by a force, it can have two kinds of motion.

1. Linear or Translatory motion: The motion of a body in a straight line is called linear or translatory motion.
   
   
2. Rotational motion: The motion of a pivoted body around its pivot is called rotational motion.

The moment of a body is the turning effect of the pivoted body.

There are two factors affecting the moment of a body:

1. The magnitude of force applied on the pivoted body
   
   
2. The perpendicular distance between the point of action of force and the axis of rotation

The moment of force of a pivoted body is the product of the magnitude of force applied on the body and the perpendicular distance between the line of action of force from the pivot. It is also known as torque.

For producing maximum momentum by a given force, the force should be applied at a point where the line of action has the maximum perpendicular distance from the pivot.

S.I. and C.G.S. units of moment of force are N m and dyne cm respectively.

1 N m       = 107 dyne cm

1 kgf m     = 9.8 N m

1 gf cm     = 980 dyne cm

Moment of force (expressed in N m) is a vector quantity and Work or Energy (also expressed in N m) is a scalar quantity. Hence, moment of force is not written as J (Joules).

If moment is anti-clockwise, moment of force is positive. If moment is clockwise, moment of force is negative.

The direction of rotation of the pivoted body depends upon two factors:

1. The point of application of force
   
   
2. The direction of application of force

Examples of moment of force:

1. To open a door, the handle is provided at the far opposite end to the hinge. As the perpendicular distance from the hinge increases, moment of force increases and the door can be opened with a very small force.
   
   
2. In a hand flour grinder, the force is applied at the top of the handle to grind. So, the perpendicular distance from the iron pivot is increased and the stone can be rotated with the small force.
   
   
3. For turning a steering wheel, a force is applied anywhere on the wheel. For turning the wheel again in a different direction, the same force can be applied in a different point on the wheel.
   
   
4. In a bicycle, for rotating the rear wheel, force is applied on a bigger toothed wheel so that the perpendicular distance between the point of application of force and the axle of rear wheel increases. Thus, small force can be applied to rotate the rear wheel. The rear wheel and the toothed wheel are connected by a chain.
   
   
5. In a spanner, the handle is provided long so that the perpendicular distance between the point of application of force and the axle of the nut increases. Thus, the nut can be tightened or loosened easily.
   
   
6. A jack screw which is used to lift heavy load is provided with a long handle so that a small force can be applied to lift the load.

A single force on a pivoted body does not cause rotation. Rotation is caused by two forces, one applied externally (force of action) and the other force (force of reaction) at the pivot. They are equal and opposite to each other. The moment of force of reaction at the pivot is 0 because the distance is 0. These two forces are together and are called a couple. Two equal and opposite parallel forces not acting along the same line form a couple.

For larger turning effect, two external forces in the opposite directions can be applied on the pivoted body so that the direction of rotation is the same.

Examples of couple:

Opening a water tap, opening a bottle, turning a steering wheel, and turning a key in a lock.

The distance between the lines of the two forces in a couple is called the couple arm. The two forces can only produce turning effect in a body.

When a number of forces acting on a body produce no change in its state of rest or of linear or rotational motion, the body is said to be in equilibrium.

When a body remains in the state of rest under the influence of several forces, the body is in static equilibrium.

When a body remains in the same state of motion (translational or rotational), under the influence of several forces, the body is said to be in dynamic equilibrium.

Conditions for equilibrium

1. The resultant of all the forces acting on the body should be zero.
   
   
2. In a rigid body, the sum of all forces on it should be zero. In a pivoted body, the sum of moments of all the forces on it should be zero.

Principle of moments:

According to the principle of moments, in equilibrium, sum of anticlockwise moments = sum of clockwise moments.

The centre of gravity of a body is the point about which the algebric sum of moments of weights of all the particles constituting the body is zero. The entire weight of the body can be considered to act at this point, howsoever the body is placed.

The position of centre of gravity in a body depends upon the distribution of mass of the body.

The centre of gravity need not be in the material part of the body.

A body can be considered a point particle because the weight acts on its centre of gravity.

When a body is freely suspended, it comes to rest in such a position that the centre of gravity is directly below the vertical line below the point of suspension. This method is used to find the centre of gravity of an irregular lamina. The lamina is suspended from 3-4 points near its edge. The vertical lines thus drawn intersect at the centre of gravity of the lamina.

A body covering equal distances in equal intervals of time travelling in a circular path has uniform speed but has non-uniform velocity. The motion is said to be accelerated eventhough the speed remains same because the velocity changes.

In a uniform linear motion, the speed and velocity are the same and acceleration is 0. While in a uniform circular motion, the speed is constant and the velocity continuously changes. So, the motion is said to be accelerated.

Motion of a body in a circular path is possible only under the influence of a centripetal force. The centripetal force is the force acting on a body moving in a circular path, in a direction towards the centre of the circular path. The word centripetal means centre seeking. A body moving in a circular path with a constant speed is in dynamic equilibrium.

The force assumed to be acting on the body in a direction away from the centre of circular path, is called centrifugal force. It acts in the opposite direction to the centripetal force. Though it is same in magnitude as the centripetal force, it is not the force of reaction of the centripetal force because forces of action and reaction do not act on the same body.

The centrifugal force is a fictious force. It is seen acting only relative to the position of the person.

A force which does not exist, but is considered to describe a certain motion, is called a fictious or virtual force.