Force And Laws Of Motion

Force

A force is a push or pull acting on a body which changes or tends to change the
state of rest or of uniform motion. It's a vector quantity

SI/CGS unit of force

Newton/dyne

ONE NEWTON

One newton is a force which when acted upon a body of mass 1 kg produces in it an acceleration of 1 m/s^2 .

1 N = ? dyne

10^5 dynes

BALANCED FORCE

When a number of forces acting simultaneously on a body do not bring about any change in its state of rest or of uniform motion in a straight line, is said to be balanced force.

UNBALANCED FORCE

When a number of forces acting simultaneously on a body bring about a change in its state of rest or of uniform motion in a straight line, are said to be unbalanced forces.

NEWTON'S FIRST LAW OF MOTION

Every body continues to be in its state of rest or of uniform motion in a straight line, unless it is compelled by some external applied force to change its state.

THE FIRST LAW OF MOTION IS ALSO CALLED AS

Law of inertia

Inertia

The property of a body by virtue of which it resists any change in its state of rest or of uniform motion in a straight line on its own is called Inertia.

What is the measure of inertia of a body

MASS

INERTIA OF REST

A body at rest remains at rest and cannot start on its own due to inertia of rest.
Eg: A passenger in a bus tends to fall backward when the bus starts suddenly.

INERTIA OF MOTION

A body in uniform motion can neither get accelerated or decelerated nor can come to rest on its own due to inertia of motion
Eg: A passenger in a moving bus tends to lean or fall forward when the bus stops suddenly.

INERTIA OF DIRECTION

The resistance offered by a body to change its direction of motion.
Eg: A passenger in a bus falls to the left as the bus take a turn to the right and vice versa.

NEWTON'S SECOND LAW OF MOTION

The rate of change of momentum of a body is directly proportional to the applied unbalanced force in the direction of force.

MOMENTUM

The linear momentum of a body of mass 'm travelling with a velocity v is defined as the product of its mass and velocity.
P = MV; It's a vector quantity

SI unit of momentum

kg m/s

v^2 - u^2 = 2as is the same as

v^2 - u^2 = 2gh. a = g; s = h.

Frictional force is always

negative

Applications of Newton's second law

*An athlete taking a high jump falls onto a cushion bed than on to a rough surface.
*Karate martial breaks a pile of tiles or a slab of ice in a single blow
*Car passengers are advised to wear seat belts.

Impulsive forces

Forces which act on the bodies for a short time

Impulse =

force x time (SI unit = Ns)

NEWTON'S III LAW

To every action, there is an equal and opposite reaction where action and reaction forces act on different bodies.

Examples of third law of motion

*A gun recoils when a bullet is fired from it
*Swimming, walking, rowing etc.,
*We feel pain in our hand when we hammer a nail into a wooden plank.

LAW OF CONSERVATION OF MOMENTUM

According to this law, in the absence of an external fore, the total momentum of a system remains constant ( or)
Total initial momentum = Total final momentum

Recoil velocity

V = -mv/M where V = recoil velocity, M = mass of gun, m = mass of bullet and v = velocity of bullet

Newton's III law is applicable only if

action and reaction forces are acting on two different bodies