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Std. 9th Subject - English

Topic - Picnicking in the Hills

WEL COME

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Std. 9th Subject-Science & Technology

Topic - Laws of motion

SUBJECT - SCIENCE

AND TECHNOLOG

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LESSON - 11THE LAWS OF MOTION

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11. The Laws of Motion

Introduction:1. Force and motion

2. Newton’s first law of motion

3. Newton’s second law of motion

4. Newton’s third law of motion

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Force and motion When there is a change in the position of an object with respective to its surroundings, it is said to its motion.

It is required to put a stationary body in motion or to stop a moving body.Force also can be used to change the velocity of an object or direction of motion.

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Force is an interaction between two objects .force is not visible but we can see the effect of force .

E.g. – when a book kept on table , the table exerts an upward force on it but it is not visible.

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• Force is an interaction between two objects .force is not visible but we can see the effect of force .

• E.g. – when a book kept on table , the table exerts an upward force on it but it is not visible.

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When a ball is hit on the ground, it bounces in an upward direction.It is the visible effect of force .

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• When two forces acting on a body are equal in magnitude and opposite in direction, we get the net forces acting on a body equal to zero.

• The two forces are balanced so the body remains at rest.

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• An unbalanced force is required to accelerate the motion of an object.

• So long as the unbalanced force is applied to a body, there is a continuous change in its speed.

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• A body resists change in its state of rest or of uniform motion.

• e.g. When we travel by a bus and a bus starts moving suddenly from a rest position, we get a jerk.

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• When the bus is at rest, our body is also at rest inside the bus. As the bus is put in motion, the portion of our body which is in contact with the bus acquires velocity. But the upper part of the body tries to remain at rest, so we fall back.

• The exact opposite situation occurs when a moving vehicle suddenly stops by application of brakes.

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• Inertia - the tendency of a body to resist change in a state of rest or state of motion is called ‘inertia’ .

• When no external force is acting on an object at rest; it will continue to remain at rest while a moving object continuous to be in motion with the same velocity.

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Inertia at rest Inertia at motion

Inertia of direction

Types of Inertia

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• Inertia at rest :The inherent property of

body by virtue of which it can not change its position of rest is called Inertia of rest.

e.g. When a bus starts suddenly, the passengers receive a backward jerk.

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• Inertia at motion : The inherent property

of body by virtue of which it can not change its state of motion is called Inertia at motion.

e.g. 1) When a rotating fan is switched off, it continues to rotate.

2) A person falls when he jumps from a moving bus.

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• Inertia of direction : The inherent property

of body by virtue of which it can not change its direction of motion is called Inertia of direction.

e.g. 1) While sharpening a knife, sparks fly off tangentially from the grinding stone.

2) When a vehicle moves, the mud sticking to its wheels fly off tangentially.

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Newton’s First Law Of Motion :• Every inanimate body continues in its state of

rest or of uniform motion in a straight line unless an eternal unbalanced force acts on it.

• Newton’s First Law explains inertia i.e. inability of a body to change the state of motion by itself.

• All the examples of inertia are examples of Newton’s first law of motion.

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Newton’s Second Law Of Motion:• Mass and velocity alone are not responsible to

produce the effect of force. There appear to exist some property that combines the mass and velocity of the object.

• Newton introduce this property as momentum.

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Momentum : • The total quantity of motion contained in the

same body is called momentum.• Mathematically it is measured as the product

of mass of body and its velocity P = mv

• Momentum has both magnitude and direction.

• Its S.I unit is kg m/sand CGS unit is gm cm/s.

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Newton’s second law explains about change in momentum.

The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the force acts.

Let a body of mass m , moving with velocity u be subjected to force F acting in the direction of motion

Suppose v is velocity of body after time t. The total initial momentum of a body is mu and final momentum is mv after time t.

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So , rate of change of momentum = change in momentum time = mv -mu

t = m (v-u)

t =ma

Hence by Newton’s second law of motion, Rate of change of momentum ά F

ma ά F . F =kma (where k =1 ,a is constant )In SI system Newton is unit of force - 1 N = 1 kg x 1 cm/s2 In CGS system dyne is unit of force . 1 dyne =1 g x 1cm/s2

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• in high jump athletes event,the athletes are made to fall on a sand bed. This is to increase the time of the athlete’s fall after making the jump. Why ?

• Consider two bodies of different masses initially at rest .suppose a fixed force is applied for certain interval of time on two bodies. The lighter body picks up greater speed than the heavier object.But at the end of the time interval ,each body acquires the same momentum. Therefore the same force for the same time causes the same change in momentum for different bodies. It shows that force is directly proportional to change in momentum at fixed time for different bodies having different masses.

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• NEWTON’S THIRD LAW OF MOTION : • STATEMENT - To every action there is an equal, opposite and instantaneous

reaction .1)Action and reaction are the terms explaining force. 2)These forses exist in pairs. An isolated force does not exist. 3)These forces act at the same instant .4)Action and reaction forces act on different bodies. They don’t act on the same body hence they don’t cancel each other

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• Examples to illustrate Newton’s 3rd law of motion.

1. When a bat strikes a ball, the ball also exerts an equal and opposite force of reaction on the bat. The force acting on the ball projects it with high velocity, while the reaction force acting on the bat slows down its forward motion.

2. When a bullet is fired from a gun, a force is exerted by the gun on the bullet and therefore the bullet acquires high velocity. At the same time the bullet also exerts an equal and opposite force on the gun and the gun moves in the opposite direction with small velocity.

3. The principle used for the motion of a rocket is based on Newton’s 3rd law of motion. When the fuel in the rocket is ignited; it burns as a result of chemical reaction. The exhaust gases escape with force through a small opening at the tail end of rocket. They exert an equal and opposite reaction force on the rocket. It is this reaction force which makes the rocket move in forward direction.

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Principle of conservation of momentum• Consider two objects A and B having masses m1 and m2 moving

with initial velocities u1 And u 2 respectively.

The values of initial momentum are m1 u1 and m 2 u 2 respectively. When they collide with each other, a force F1 due to body B acts on body A; the body A accelerates. Its velocity after collision becomes v1 hence the momentum changes to m1 v1 .

According to Newton’s third law of motion, body A also exerts equal and opposite force on body B and it also suffers a change in momentum. Suppose its velocity becomes v 2 and momentum changes to m2v 2.

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• If F2 is the force acting on body B,F2 = - F1

m2a2 = - m1a1

= -

m2(v2 - u2) = - m1 (v1 - u1)

m2v2 - m2u2 = - m1v1 + m1u1

m1v1 + m2v2 = m1u1 + m2u2

Final momentum = initial momentum.Hence the initial momentum is equal to total final momentum if no external force is acting on the two bodies.If there is no net force acting on two interacting bodies then their total momentum is conserved.

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• If two freely moving bodies collide, their total momentum after collision is the same as their total momentum before collision.

• e.g . A bullet fired from a gun.As mass of the gun is much greater than mass of the

bullet, velocity of the gun is very small as compared to velocity of the bullet. So momentum of gun and bullet are equal in magnitude but opposite in direction.

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Solved examples

• Calculate the acceleration produced when a force of 350 N acts on a body of mass 500kg.Given : F = 350 N, m = 500kgAcceleration = ?

F = maa = F/ma = 350 / 500a = 0.7 m/s2

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• Calculate momentum of a field gun of mass 500 kg when it recoils with velocity of 0.25 m/s after firing a shell from it.

Given : Mass of the field gun = 500 kg Recoil velocity = 0.25 m/s. Momentum = ?

Momentum = m x v = 500 x 0.25 = 125 kg m/s.

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PRESENTED BY –

MS. JADHAV. P. L

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THANK YOU

c c , , , std. 9th subject - english topic - picnicking in the hills wel come

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