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Conceptually force may be described as a pull or push. But to define the force, the effects force on bodies need to be

studied.

Effects of force on bodies

If a force is applied on a body it may change its velocity or shape.

1. Change in velocity

When a force is applied on a body

a). If the body is at rest it may move.

b). If the body is in uniform motion, then it may speed up, slow down or change the direction of motion

**If a force of constant magnitude applied always perpendicular to a

motion.

2. Change in shape or deformation

When a force is applied on a body, it may change its shape. For instance

compressed spring or a stretched rubber band. Even the rigid bodies

metal blocks or wood changes its shape under the application of force, but

the effect may not be noticeable.

**On the removal of the force, the body regains its shape to differing

degrees. Those which completely regains the shape are known as perfectly

elastic. Those bodies which does not regain the shape at all are

plastic bodies

Definition of force.

Force can be defined as the cause of the change in velocity or shape of a body.

1. Contact & Non-contact forces

1

FORCE

Conceptually force may be described as a pull or push. But to define the force, the effects force on bodies need to be

change its velocity or shape.

b). If the body is in uniform motion, then it may speed up, slow down or change the direction of motion

**If a force of constant magnitude applied always perpendicular to a body at constant speed, it follows uniform circular

When a force is applied on a body, it may change its shape. For instance, a

compressed spring or a stretched rubber band. Even the rigid bodies like

ks or wood changes its shape under the application of force, but

**On the removal of the force, the body regains its shape to differing

degrees. Those which completely regains the shape are known as perfectly

e bodies which does not regain the shape at all are perfectly

Force can be defined as the cause of the change in velocity or shape of a body.

Conceptually force may be described as a pull or push. But to define the force, the effects force on bodies need to be

b). If the body is in uniform motion, then it may speed up, slow down or change the direction of motion.

body at constant speed, it follows uniform circular

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Forces which act on bodies which are in physical contact are called contact forces

Important Examples of contact forces

Friction

Force friction is the opposing force on two bodies in contact

when there is a relative velocity between them. Friction acts

tangential to the contact surface and oppo

direction of the relative motion.

Normal reaction force

A body exert a force equal to its weight downward on the

surface it is placed. Then the supporting surface exerts an

equal force upward on the body normal (perpendicular) to

the contact surface. This is Normal reaction.

Tension

When two bodies are connected by a flexible string and one

of the bodies is pulled, the string exerts forces on both the

bodies in opposite directions. These forces exerted by

connecting strings on bodies are called tension.

Force of collision

When bodies collide force exerted by one body on the other is equal to

the force exerted by the

second body on first.

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physical contact are called contact forces

Force friction is the opposing force on two bodies in contact

when there is a relative velocity between them. Friction acts

opposite to the

A body exert a force equal to its weight downward on the

surface it is placed. Then the supporting surface exerts an

equal force upward on the body normal (perpendicular) to

surface. This is Normal reaction.

When two bodies are connected by a flexible string and one

of the bodies is pulled, the string exerts forces on both the

bodies in opposite directions. These forces exerted by

ed tension.

When bodies collide force exerted by one body on the other is equal to

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Force exerted by springs

When a spring is stretched or compressed, it exerts an equal force

direction called, restoring force. Restoring force is proportional to the change in

length of the spring.

F = k x, here F is the restoring force, x is the change length and k is called spring

constant(unit N/m).

***From these discussions, we get that forces always come in equal opposite

pairs called action and reaction.

2. Non- Contact forces

Forces between bodies even if they are not in physical contact.

1) gravitational force

Every body in the universe attract every other body with a

inversely proportional to the square of the distance between, known as gravitational force.

Effect of gravitational forces.

Every object dropped falls down. Bodies roll down the slopes. Planets move

celestial bodies are evolved by the effect of gravitational force.

Fg =�����

�� Here, G-universal gravitational

of the two bodies and r is the separation between them

Properties

(i). Always attractive. (ii). Independent of the medium between.

Electrostatic force

Force between two charges

Fe =k �� ��

�� Here k = 9 x 10

9 Nm

2/C

2, q1 and q2 are

Properties

(i). Similar charges repel, and unlike charges attract (ii). Depends on the medium between (iii). Inverse square law

Magnetic force

Force between magnetic poles

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When a spring is stretched or compressed, it exerts an equal force in the opposite

direction called, restoring force. Restoring force is proportional to the change in

, here F is the restoring force, x is the change length and k is called spring

, we get that forces always come in equal opposite

Forces between bodies even if they are not in physical contact.

Every body in the universe attract every other body with a force which is directly proportional to their mases and

inversely proportional to the square of the distance between, known as gravitational force.

Every object dropped falls down. Bodies roll down the slopes. Planets move around the sun. Stars palnets and all other

celestial bodies are evolved by the effect of gravitational force.

universal gravitational constant (6.67259 x 10 -11

N m2/kg

2), m1 and m2 are the masses

separation between them

Independent of the medium between. (iii). Follows inverse square law.

, q1 and q2 are charges and r is the distance between them.

(i). Similar charges repel, and unlike charges attract (ii). Depends on the medium between (iii). Inverse square law

force which is directly proportional to their mases and

around the sun. Stars palnets and all other

, m1 and m2 are the masses

Follows inverse square law.( F α �

)

charges and r is the distance between them.

(i). Similar charges repel, and unlike charges attract (ii). Depends on the medium between (iii). Inverse square law

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Fm = Km �� ��

km = 10-7 N/A

2, m1 and m2 are pole strengths and r is the separation between poles

Properties

(i), Like poles repel and unlike poles attreact (ii) depends on the medium, (iii). Inverse square law.

Newton’s laws of motion

1. Newton’s First law (law of inertia)

A body continues in its state of rest or uniform motion unless an external force is applied on it.

Inertia:- tendency of a body to resist the change of its state rest or state uniform motion.

2. Newton’s Second law

The rate of change of momentum of a body is directl

Momentum (linear momentum) Momentum is the product of mass and velocity

Linear momentum P = mv its unit is kg m/s and it is a vector quantity towards the direction of velocity.

By Newton’s second law F α �

� Or F =�

∆�

∆� where Δp is change in momentum and Δt is the change in time. Here

proportionality constant k=1, therefore according to Newton’s second law

Here Δp =Δ(mv) in case of speeds far less than the velocity of light in vacuum, there is no change in mass

Therefore Δ(mv) = mΔv, where Δv is the change in velocity. That is

F = m ��

�� =

���������

������� that is F = ma (Non

When the speed is comparable with the speed of light, m also

Then F = ∆����

∆� (relativistic equation)

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m1 and m2 are pole strengths and r is the separation between poles

(i), Like poles repel and unlike poles attreact (ii) depends on the medium, (iii). Inverse square law.

tinues in its state of rest or uniform motion unless an external force is applied on it.

the change of its state rest or state uniform motion.

of a body is directly proportional to the net external force applied on that body.

Momentum (linear momentum) Momentum is the product of mass and velocity

Linear momentum P = mv its unit is kg m/s and it is a vector quantity towards the direction of velocity.

where Δp is change in momentum and Δt is the change in time. Here

proportionality constant k=1, therefore according to Newton’s second law F = ∆�

∆� =

������ �� ������ �

!�"��� ��

Δp =Δ(mv) in case of speeds far less than the velocity of light in vacuum, there is no change in mass

Therefore Δ(mv) = mΔv, where Δv is the change in velocity. That is

F = ma (Non- relativistic equation) Low speeds

When the speed is comparable with the speed of light, m also changes

(relativistic equation) Very high speeds

m1 and m2 are pole strengths and r is the separation between poles

(i), Like poles repel and unlike poles attreact (ii) depends on the medium, (iii). Inverse square law.

tinues in its state of rest or uniform motion unless an external force is applied on it.

y proportional to the net external force applied on that body.

Linear momentum P = mv its unit is kg m/s and it is a vector quantity towards the direction of velocity.

where Δp is change in momentum and Δt is the change in time. Here

������ �

�� ����

Δp =Δ(mv) in case of speeds far less than the velocity of light in vacuum, there is no change in mass

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Newton’s third law

To every action there is an equal and opposite reaction.

If A exerts a force F21 on B, then b exerts an equal and opposite force F21 on A.

F21 = - F12

Units of Force

1. SI unit of force in Newton (N)= kg m/S2

Definition of 1 N

One Newton is the force to be applied on a body of mass 1 kg to accelerate it at 1 m/S2

2. CGS unit of force is Dyne = g cm/s2

I Newton = 10 5 Dyne

Gravitational unit of force in MKS system is Kilogram force (kgf)

2. One kgf is the force with which the earth pulls a body of mass 1 kg

1 kgf = 9.8 N

Gravitational unit of force in CGS system is g f. 1 gf = 980 Dayne.

Impulse

Impulse is the product of force and time or it is the change in momentum

Impulse I = Fx Δt = mv2 – mv1 . Unit of Impulse is kg m/S( same as momentum)

For the same change in momentum, F is inversely proportional to time of action of force.

Examples of Impulse

To hammer a nail, large force is applied for a short time.

To catch a cricket ball, the player draws his hands back to increase the impact time to reduce the force.

Equations of motion

V = u + at

S = ut + �

�#$�

%� = '� + 2#*