X Physics - Chapter 5 - Force and Motion

DYNAMICS
"It is the branch of Physics which deals with causes of motion and their effects"

LAW OF MOTIONS
Newton formulated three laws of motion in his book.




NEWTON FIRST LAW OF MOTIONS
Newton's first law of motion is also known as the Law of Inertia.

STATEMENT

"Every body continues its state of rest or uniform motion in a straight path until it is acted upon by an external, or unbalance force to change its state of rest or uniform motion".


EXPLANATION

This law consists of a two parts

(a) When body is at rest

(b) When body is moving with uniform velocity


When Body is At Rest

Newton's Law states that when a body is at rest, it continues its rest unless we apply a force on it. When we apply a force, it changes its state of rest and starts moving along a straight line.

When Body is in Motion

Newton's Law states that when a body is moving, it moves in a straight line with uniform velocity, but when we apply an opposite force, it changes its state of motion and come to rest.


Examples

A body riding a push-bike along a leveled road does not come to rest immediately when we apply a force, it changes its state of rest and starts moving along a straight line.
If a bus suddenly starts moving, the passengers standing in the bus will fall in the backward direction. It is due to the reason that the lower part of the passengers which is in contract with the floor of the bus is carried forward by the motion of the bus, but the upper part of the body remains at rest due to inertia and so the passengers fall in backward direction.



SECOND LAW OF MOTIONS
STATEMENT

"When a force acts on an object it produces an acceleration which is directly proportion to the amount of the force and inversely proportional to the product of mass"

EXPLANATION

It is well known fact that if we push a body with greater force then its velocity increases and change of velocity takes place in the direction of the force. If we apply a certain force F on a mass m, then it moves with certain velocity in the direction of the force. If the force becomes twice then its velocity will also increase two times. In this way if we go on increasing the fore there will be increase in velocity, which will increase the acceleration.

DERIVATION

According to the Newton`s Second law of motion when a force acts on an object it produces an acceleration which is directly proportion to the amount of the force.

a < a =" K" a =" F/m" f =" ma" f =" ma" m =" F/a" w =" mg" p =" mV" collision =" m1U1" collision =" m1V1" collision =" Total" collision =" m1U1" m2u2 =" m1V1" f =" UR"> Fs = uR ........... (i)

u = F(s)/R

u is the constant of proportionality, which depends upon the nature of the surfaces of the two surfaces in contact. It is known as the coefficient of friction. It is only a number without any unit. We know that the normal reaction is directly proportional to the weight of the block, therefore,

R = W = mg

Substituting the value of R in equation (i)

=> Fs = umg




Rolling Friction
If we set a heavy spherical ball rolling, it experiences an opposing force called rolling friction. When a body rolls over a surface, the force of friction is called rolling friction. Rolling friction is much less than the sliding friction. This is because the surfaces in contact are very much less.