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Force
A force is an effort that changes the state of an object at rest or at motion.
A push or pull on a body is called force.
Forces are used in our everyday actions like pushing, lifting, pulling, stretching,
twisting and pressing.
A force cannot be seen.
A force can be judged only by the effects which it can produce in several bodies
(or objects) around us.
Characteristics of force:
Force has both magnitude and direction, making it a vector quantity.
It is measured in the SI unit of newton (N)
It is represented by the symbol F.
Effects of Force:
It can move a stationary body.
It can stop a moving body.
It can change the speed of a body.
It can change the direction of a moving body.
It can change the shape of a body.
Balanced and Unbalanced Forces
(i) Balanced Forces:
If the resultant of applied forces is equal to zero, it is called balanced force.
For example: In the tug of war game when the force applied by both teams is equal in
magnitude then the rope does not move in either side. This is due to the balanced forces
in which resultant of applied forces comes out to be zero.
Characteristics:
Balanced forces do not cause any change of state of an object.
Balanced forces are equal in magnitude and opposite in direction.
Balanced forces can change the shape and size of an object. For example: When
we press a balloon from opposite sides, the size and shape of balloon is changed.
(ii)Unbalanced Forces:
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If the resultant of applied forces are greater than zero, the forces are called
unbalanced forces.
To move an object, unbalanced forces are to be applied from the opposite
directions.
Characteristics:
Change the speed and position of an object.
Change the shape and size of an object.
In case of unbalanced forces acting on a body, the body moves in the direction of
the greater force.
Some Common Forces
Muscular Force: The force exerted by the human body muscles is called muscular
force.
Gravitational Force: The attractive force applied by earth on an object in
downward direction is called gravitational force.
Frictional Force: The force that opposes relative motion is called friction. It arises
between the surfaces in contact.
Example: When we try to push a table and it does not move is because it is
balanced by the frictional force.
Air Resistance: Force which is exerted on the objects while flying in air is named
as air resistance. It acts in a direction opposite to the velocity of the object.
Galileo’s Observation
He observed the motion of objects on an inclined plane.
When a marble is rolled down an inclined plane its velocity increases.
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We need an unbalanced force to change the motion of the marble but no force is
required when the marble is moving uniformly. In other words, objects move at a
constant speed if no force acts upon them.
Based on Galileo’s ideas Newton presented the three Laws of Motion
First law of motion or The Law of Inertia
Newton's first law states that every object will remain at rest or in uniform motion in a
straight line unless compelled to change its state by the action of an external force.
Inertia
The tendency of objects to remain in the state of rest or to keep moving uniformly
is called Inertia.
Inertia depends on the mass of a body. Mass of an object is the measure of its
inertia.
More the mass more inertia and vice versa.
Types of Inertia
(i) Inertia of Rest : It is the tendency of an object (or body) to continue its state
of rest unless it is acted upon by an external force.
EXAMPLES:
We fall back when a vehicle starts moving in the forward direction because our body
is in the rest state and it opposes the motion of the vehicle.
Dust particles on a carpet fall if we beat the carpet with a stick.
Fall of coin kept on a cardboard into the glass.
When you shake a branch, the leaves got detached.
With a quick pull, a table cloth can be removed from a dining table without
disturbing dishes on it.
(ii) Inertia of motion: A body in motion in a specific direction continues to be
in motion in that direction unless acted upon by an external force to change
the direction.
EXAMPLES
Satellites move in space due to inertia of motion.
A car continues to move little forward even its engine has stopped.
In a sports field, an athlete often runs before taking a long jump.
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Passenger in a running bus tends to lean forward when the bus stops.
(iii) Inertia of direction: It is defined as the inability of a body or object to
change its direction of motion by itself. That is external force is required to
change its direction of motion.
A stone thrown vertical up do not change its direction.
When a knife is sharpened on a rotating grinding wheel, the fiery particles move
tangent to the wheel.
When an object is placed on top of a running bus and the bus takes sudden turn
then the object placed on the top of the bus will be thrown out in opposite in
direction of turning bus.
Mud through rotating wheels.
Tangential movement of untied stone.
Momentum
The product of mass and velocity is called Momentum.
The impact produced by a moving object depends upon its mass and velocity.
For Example, a small bullet fired at a high velocity can kill a person.
It is a vector quantity. Its direction is same as that of the object’s velocity.
Denoted by ‘ pp = mv,
where m is the mass of the object,
v is the velocity of the object
SI unit kg m/s
The momentum of a stationary object
Let the mass of a stationary object be ‘m’,
Let the velocity of a stationary object be ‘v’,
The stationary object has no velocity, so v = 0,
Therefore, p = mv = m×0 = 0
So, the momentum of a stationary object is zero.
Note:
We know that the velocity of an object can be changed by applying an unbalanced
force on to it. Similarly, the momentum of an object can be changed by applying an
unbalanced force.
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Second law of motion
The rate of change of momentum of an object is directly proportional to the
applied unbalanced force on the object in the direction of the force.
Examples
1. A cricketer when catches a ball pulls his hands in the backward direction to give
some time to decrease the velocity of the ball. As the acceleration of the ball decreases
the force exerted on catching the moving ball also decreases. If the cricketer would try
to stop a moving ball suddenly he would have to apply larger force.
2.In high jump if surface is hard, athlete’s body changes velocity in very less time.
Large force acts on his body due to which he may get injured. If the surfaces are soft,
athlete’s body changes velocity in more time. And less force acts on his body which is
safe.
Mathematical Formulation of the Second Law of Motion
Consider a body of mass m having an initial velocity u.
The initial momentum of this body = p
1
= mu
Suppose a force ‘F’ acts on this body for time ‘t’ and causes the final velocity to
become ‘v’
The final momentum of this body = p
2
= mv
According to the second law of motion,
The rate of change of momentum of an object is directly proportional to the applied
unbalanced force on the object in the direction of the force.
Change in momentum α p
2
- p
1
α mv - mu
α m (v – u )
Rate of change of momentum α m (v – u ) / t
Rate of change of momentum α m a
Force α m a acceleration a = (v-u) / t
Force = k m a Rate of change of momentum = force
Where k is a constant. (k = 1)
F = ma S.I unit of force is kgm/s
2
or newton(N)
From the formula stated above, we can see that the force is directly proportional to
acceleration. So the acceleration of an object can change depending upon the change in
force applied.
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newton:
A newton is that force which when acting on a body of mass 1 kg produces an
acceleration of 1 m/s
2
Action and Reaction Forces
Two forces acting from opposite directions are called Action and Reaction Forces.
For Example, a ball when hits the ground (action) bounces back with a certain force
(reaction).
The Third Law of Motion
When an object exerts a force on another object, the second object instantly
exerts a force back onto the first object.
These forces are always equal in magnitude but opposite in direction. These
forces always act on two different objects.
Every action has an equal and opposite reaction.
NOTE
The magnitudes of forces acting upon the objects are same but the acceleration
produced in them may or may not be the same because the objects can differ in
masses.
For Example, when a bullet is fired from a gun, the gun only moves a little
backwards (recoils) while the bullet can travel a large distance. This is because of
the difference in the mass of the bullet and the gun.
Application of Newton’s 3
rd
Law:
1. Gun applies force on bullet due to which it moves ahead. By Newton’s 3
rd
Law,
bullet will also apply same force on gun in backward direction. Due to this force,
gun moves back. This is called recoil of gun. Gun moves back only by small
amount due to its heavy mass.
2. Man pushes the boat backwards and by newton’s 3
rd
law, boat pushes man
forward.
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