CLASS -9 CHAPTER - FORCE AND LAWS OF MOTION

Force:- Force is defined as a push or a pull that changes or tend to change the state of rest or uniform motion of an object or changes the direction or shape of an object. 

Balanced Force:- Balanced forces are those two forces acting in opposite direction on an object and they are equal in magnitude.Balanced force doesn't change state of object.

Unbalanced Force:-Forces that cause a change in the motion of an object are unbalanced forces.Unbalanced forces are not equal in magnitude and opposite in direction.

Laws of MoTion

There are three types of Laws of motion which is also known as Newton's Laws of Motion.

First Law of Motion:-The first law of

motion is stated as:

An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.In other words, all objects resist a change in their state of motion.  the first law of motion is also known as the law of inertia.

Second Law of Motion:-The second law of motion states that the rate of change of momentum of an object is

proportional to the applied force in the direction of force.

Suppose an object of mass, m is moving along a straight line with an initial velocity, u. It is uniformly accelerated to velocity, v in time, t

by the application of a constant force, F throughout the time, t.

Momentum(p)= mass x velocity

Initial momentum(p1)= mass x initial

                                                          velocity(u)

Final momentum(p2)= mass x final velocity

                                                                   (v)

The change in momentum ∝ p2 – p1

                                               ∝ mv – mu

                                               ∝ m × (v – u)

The rate of change of momentum ∝  m x (v - u)/t

   F  ∝ m x (v-u)/t

   F = k x m x (v - u)/t

we know that,

      a = (v-u)/t

so, 

   F = k m a

where k is a constant of proportionality, m is mass and 'a' is acceleration. 

S.I. unit = kg meter per second square

Third Law of motion:- The Third law of motion states that every action has equal and opposite reaction.

Example : If a ball is thrown to the wall and that ball return back with same speed.

Conservation of Momentum:-

The Conservation of Momentum states that:-

For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2. 

Suppose two objects (two balls A and B, say)of masses mA and mB are travelling in the same direction along a straight line at different velocities uA and uB, respectively . And there are no other external unbalanced forces acting on them. Let uA > uB and the

two balls collide with each other as shown in Fig. During collision which lasts for a time t, the ball A exerts a force FAB on ball B and the ball B exerts a force FBA on ball A. Suppose vA and vB are the velocities of the

two balls A and B after the collision,

respectively. The momenta (plural of

momentum) of ball A before and after the collision are mAuA and mAvA, respectively. The rate of change of its momentum (or FAB, action) during the collision will be mA  x (vA - uA)/t.

Similarly, the rate of change of momentum of ball B (= FBA or reaction) during the collision

will be mB  x (vB - uB)/t.

According to the third law of motion, the force FAB exerted by ball A on ball B (action) and the force FBA exerted by the ball B on ball

A (reaction) must be equal and opposite to each other. Therefore,

               FAB = – FBA

or  mA  x (vA - uA)/t  =   -mB  x (vB - uB)/t

This gives,

        mAuA + mBuB = mAvA + mBvB

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