 # FORCE AND LAWS OF MOTION / THE LAWS OF MOTION – SUMMARY

Summary

• If the resultant of all the forces acting on a body is zero, the forces are called balanced forces.
• If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. An unbalanced force acting on a body gives it an acceleration.
• Force is an external effort in the form of pushing, pulling, stretching, compressing, etc.
• First law of motion – An object continues to be in a state of rest or of uniform motion along a straight line unless acted upon by an unbalanced force.
• The natural tendency of objects to resist a change in their state of rest or of uniform motion is called inertia.
• The mass of an object is a measure of its inertia. Its SI unit is kilogram (kg).
• There are three types of inertia –
(i) inertia at rest,
(ii) inertia of motion,
(iii) inertia of direction.
• All inertia examples are examples of Newton’s first law of motion.
• Force of friction always opposes motion of objects.
• Second law of motion – The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Force is a physical quantity that changes or tends to change the state of rest or of uniform motion of a body in a straight line. It is a vector quantity that has magnitude and direction. The SI unit of force is kg m s-². This is also known as newton (N). A force of 1newton produces an acceleration of 1 m s-² on an object of 1 kg mass.
• The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its SI unit is kg m s-1.
• Newton’s first and second laws deal with force and its effects on motion of objects.
• Third law of motionTo every action, there is an equal and opposite reaction and they act on two different bodies. Hence, they do not cancel each other.
• Newton’s third law deals with relationship between forces on two interacting bodies. Force never occurs singly in nature.
• The motion of a rocket is based on Newton’s third law of motion.
• In an isolated system ( where there is no external force), the total momentum remains conserved. In a collision momentum is conserved.

Latest CBSE syllabus topics –
Force and Newton’s Laws – Force and Motion, Newton’s laws of motion, Inertia of a body, Inertia
and mass, Momentum, Force and acceleration, Elementary idea of conservation of momentum,
action and reaction forces.

Latest Maharashtra SSC Board syllabus topics
The Laws of Motion – Force and Motion, Newton’s first law of motion, Newton’s second law of motion, Newton’s third law of motion, Inertia of a body, Inertia and mass, Momentum, Force and acceleration,
Elementary idea of conservation of momentum, action and reaction forces.

INTRODUCTION

We have seen effects of forces in our daily life like lift, pulling passengers up; train pulling carriages etc. A push or pull on a body is called force. Some examples are pulling the drawer of the table, stretching a rubber band, twisting a wet cloth to squeeze out water, pushing of leaves by wind etc.

Effects of force

A force is not visible but it can be judged by the effects which it can produce in various bodies. The effects of force are –

• A force can move a stationary body.
• A force can stop a moving body.
• A force can change the speed and velocity of a moving body.
• As it can change the velocity, therefore, the force can change the direction of a moving body.
• A force can change the shape and size of a body.

Force can change the shape and size of a body. For example, when we take a spring and pull it at both ends with our hands, then the shape and size of the spring changes.The shape of dough (kneaded flour) changes on pressing it with a roller to make bread.The shape of kneaded wet clay changes when a potter converts it into pots of different shapes. This happens because the potter applies force on the kneaded wet clay. When a balloon is pressed, it changes its shape due to effect of force.
Force has magnitude and direction, therefore it is a vector quantity. Force changes the state of an object from rest to motion in a straight line or from motion to rest .

Next –

Balanced and Unbalanced Force, Inertia