Force can be defined as the cause due to which we can change the magnitude
of velocity of an object or change its direction of motion. Force can also
change the shape and size of object.
Balanced and unbalanced force
Force when applied on an object does not change the state of rest or state
of motion of that object it is called as a balanced force. If a wooden
block is pulled from both the sides with equal force such that it didn't
move, then it is an example of Balanced force.
When two forces are applied on an object in such a way that the force
applied from one side is more than the force applied form other side. Then
the object tends to move in the direction of force applied that is more in
magnitude. This is the conditions of an unbalanced force.
A box when pushed or pulled by a small force, the box does not move, this
is due to the friction between the box surface and the floor, the
frictional force acting in the direction opposite to the direction of the
push or pull.
First law of motion
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 external
force.
The tendency of an undisturbed object to stay at rest or to keep moving
with the same velocity is called inertia. This is the reason sometimes the
first law of motion is also known as the law of inertia.
From the first law we can deduce that all object resist change in their
state of motion. When an unbalanced force is applied by the steering of a
car to change the direction of the car then we tend to slide towards outer
side of the seat this is due to the inertia of our body. The fact thus says
that a body will remain at rest unless acted upon by an unbalanced force.
Inertia and mass
The resistance offered by an object to change its state of motion or its
state of rest, is due to the inertia of that particular object. When we
apply same amount of force on a ball and a stone then we see movement in
the stone is much lesser than that of the ball. So, from this we can say
that heavier the object larger is the inertia. Thus, inertia of an object
is measured by its mass and vice versa.
Second law of motion
According to the second law of motion the rate of change of momentum of
an object is proportional to the applied unbalanced force in the
direction of force.
The momentum of an object is defined as the product of its mass m and velocity v. Momentum has both
direction and magnitude in the same direction of the velocity. SI unit of
momentum is kilogram metre per second. Since application of an unbalanced
force brings a change in the velocity of an object, it also produces a
change of momentum.
The term Momentum was introduced by Newton. From the equation of momentum,
we can say 1 unit of force can be defined as the amount that produces an
acceleration of 1 metre per second square in an object of mass 1 kg.
SI unit of force is kg metre per second square or commonly called as
Newton. Therefore, the second law of motion gives us the formula of force
acting on an object as a product of its mass and acceleration.
Formulation of second law of motion
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 constant force F throughout the time t. The initial and
final momentum of the object will be P1 = mu and P2 = mv. And the change in
momentum will be given by the difference of P2 from P1.
Third law of motion
To every action there is always an equal and opposite reaction
.
The third law states that when one object exerts a force on another object
the second object instantaneously exerts a force back on the first. The two
forces are always equal in magnitude and opposite in direction. The
opposite forces are known as action and reaction forces.
Even though the action and reaction forces are always equal in magnitude,
these forces may not produce accelerations of equal magnitudes. This is
because each force acts on a different object that may have a different
mass.
When a Gun is fired it exerts a forward force on the bullet. The bullet
exerts an equal and opposite reaction force on the gun this results in the
recoil of the gun. But since the gun have a greater mass than the bullet,
acceleration of the gun is much less than the acceleration of the bullet.
Conservation of momentum
The sum of momenta of two objects before collision is equal to the sum of
momenta after the collision, provided there is no external unbalanced force
acting on them. This is known as the law of conservation of momentum.
In an ideal collision experiment, when two balls of mass mA and
mB rolling in the same direction along a straight line at
different initial velocity uA and uB, such that the
velocity of the ball 1 less than the velocity of ball 2 then after a
certain point of time the balls collides. Then some force is applied by the
ball 2 to ball 1. Now both the ball continuously moving in the same
direction with a momentum which is equal to the sum of the momentum of the
two balls before collision. From the law of conservation of momentum, we
can tell that the total momentum of 2 objects is unchanged or conserved by
the collision.
