**Sir Issac Newton**made systematic study of motion and gave three laws of motion which made the study of motion easy.

**It states that**

__Newton's First law:__**This law is also called as law of inertia.**

*"If a body is at rest ,it will remain in rest and if it is moving with uniform velocity, it will continue to move with uniform velocity unless an external force is applied on it*."**Inertia**is the property of a body to resist the change in its state of rest and state of uniform motion along a straight line.

**Mass is the measure of inertia of a body**.More the mass of the body,more its inertia.

**Inertia of Rest:**

*"**A body at rest continues in its state of rest."*

Examples: (i) If we place a coin on a playing card covering a glass tumbler and give a sudden jerk to the card and card flies off and the coin drops in to the tumbler because coin tends to remain at rest due to inertia of rest.

(ii) when a bus suddenly starts moving, passengers falls backwards because lower part of passenger comes into the motion while upper part tends to remain at rest due to inertia of rest.

**Inertia of motion:**

*"**A body resists its state of uniform motion."*

Examples:(i)when a bus suddenly stops, passengers falls forward because lower part of passenger's body comes to rest while upper part of his body tends to keep moving due to inertia of motion.

(ii)A ball thrown upwards in a moving train comes back into the thrower's hands because the ball aquires the horizontal velocity of the train and maintains it during its upward and downward journey due to inertia of motion so the ball covers same horizontal distance as covered by train and ball comes back to the thrower's hands.

**Inertia of direction: "**

*A body moving with a uniform speed can not change its direction of motion itself."*

Examples: (i)When a car moves round a curve,the person sitting inside the car is thrown outward in order to maintain its direction due to its inertia of direction.

(ii) When a dog chases a hare, the hare runs along a zig - zag path because it becomes difficult for the dog to catch the hare as dog has more mass than hare so more inertia of direction.

__Newton's Second law:__

*"Rate of change of momentum of a body is directly proportional to applied force to the body"*
If a body of mass "m" is moving with velocity "v" , then its linear momentum is p = mv

F ∝ dp/dt

F ∝ d(mv)/dt

F = k m a

here k=1

**F = m a**

- Force is a vector quantity.
- S. I.unit of force is
**N (newton)**and C.G.S. unit of force is**dyne**.

**Impulse of a force:**Impulse is the total effect of a large force which acts for a short time to produce a finite change in momentum.

**It is defined as the product of force and time during which force acts on the body and it is equal to change in momentum of the body.**

**Impulse = Force x Time duration**

**= Total change in momentum**

Some practical Applications of concept of Impulse are:

(i) While catching a ball cricket player lowers his hands,by doing so he increases time period so force exerted on his hands decreases and it does not hurt him.

**(ii)****Automobiles are provided with shockers.**when a vehicle moves on an uneven road,it receives a jerk.the shocker increases the time of jerk and hence reduces its force.so,the ride becomes comfortable.bike with shocker |

**(iii****)**Buffers are provided between bogies of a train which increase the time of jerk hence decreases the force of impact between the bogies.
(iv) Chinawares are packed in straw paper because straw paper increases the time of jerk during transportation.hence,they strike against each other with a lesser force so, less damage occurs.

__Newton's Third law:__

__"To every action, there is equal and opposite reaction." i.e. Forces in nature always occur in pairs.__(i) A book is kept on a table,book exerts a force equal to its weight on the table and table also exerts an equal and upwards force on the table.

(iii) While walking,we press the ground (action) with our feet slightly slanted in backward direction and the ground exerts equal and opposite force on us.

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Now Test Your Understanding

1. Why do the passengers fall in backward direction when a bus suddenly starts moving from rest position ?

2. A man jumping out of a moving train falls with his head forward.Why ?

3. Why do the blades of an electric fan continue to rotate for some time after the current is switched off ?

4. We beat a blanket with stick to remove dust particles. why ?

5. Why an athlete runs some steps before taking a jump ?

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