It is about three laws of motion in Physics.

1. LAWS OF MOTION
We have discussed velocity and acceleration in
previous chapter. In this chapter we are going to learn
about
 What does govern the motion of bodies?
 Is an external force is required to keep a body in
uniform motion?

2. FORCE :
 push/ pull
 Something that changes or tries to produce motion in a body at rest,
Tries to stop a moving body , changes or tries to change the direction of motion of the body.
 Contact force (pulling the table) and Non contact force (Gravitational force, Magnetic
force, Electrostatic force).
 External force and Internal force
 Balanced force and unbalanced force
external unbalanced force is required to produce motion in a body

3. ARISTOTLES FALLACY(Mistake)
Aristotelian law of motion
An external force is required to keep a body in motion.
Fallacy of Aristotelian law of motion
According to Aristotelian law, a force is required to keep a
body in uniform motion. But actually ,no force is required to
move a body in uniform motion.

4. THE LAW OF INERTIA
Galileo studied motion of object on
 Inclined plane
 Double inclined plane
Experiment on inclined pane
He observed that bodies moving down an inclined plane have
an acceleration(which is caused by a force of gravity)

5. Bodies moving up an inclined plane suffer retardation.
Bodies moving on a horizondal plane should be of constant velocity.

6. Experiment on double inclined plane.
A ball released from rest on one of the planes rolls down and
climbs up the other .The final height of the ball is nearly equal to
the initial height(if the planes are smooth).

7. If the slope of the second plane is decreased and the experiment
repeated, the ball will reach the same height. But travel a longer
distance.

8. If the slope of the second plane is zero,the ball travels an infinite
distance.

9. Conclusion of experiment
 The state of rest and the state of uniform linear motion are
equivalent (ie, there is no net force acting on the body)
 If the net external force is zero, a body at rest continuous to
remain at rest and a body in motion continuous to move with
uniform velocity (this property of the body is called inertia).

10. NEWTONS LAWS
 Newton built on Galileo s ideas and laid the foundation of mechanics in
terms of three laws: Newtons first law ,Newtons second law
,Newtons third law
NEWTONS FIRST LAW OF MOTION
 Everybody continuous in its state of rest or of uniform motion along a
straight line unless it is compelled by an external unbalanced force to
change that state:
Newton’s first law of motion brings the idea of inertia.

11. INERTIA
• The inability of a body to change by itself its state of rest or uniform motion
along a straight line is called inertia.
• Inertia of a body is measured by the mass of the body. Heavier the body
,greater is the force required to change its state and hence greater is its
inertia.

12. Examples of inertia
 When a fast moving bus is suddenly stopped, a standing passanger tends to fall in the
forward direction.
Explanation
The passanger has the same velocity as that of bus . when the bus stops suddenly the
lower part of his body is brought to rest suddenly because of the friction between his feet
and floor of the bus.But the upper part continuous to move because of its inertia.
 When a bus suddenly takes off , a standing passanger tends to fall in the backward
direction. This is because the lower part of the body gets a speed when the bus picks up
speed and upper part continuous to beat rest because of its inertia.

13.  Consider a person sitting inside a stationary train and tossing a coin. The coin
falls into his own hand. If he repeats the experiment when the train is moving
with uniform speed, then also the coin falls into his own hand.
 Cleaning a carpet by beating is in accordance with law of inertia.
 Rabbit chased by a dog runs in zigzag manner .This is to the advantage of the
large inertia of the massive dog.
 A person chased by an elephant runs in a zigzag manner or in a circle. This is
to take the advantage of the large inertia of the massiive elephant.

14. NEWTON’S SECOND LAW OF MOTION
LINEAR MOMENTUM
 Momentum of a body is defined as the product of its mass and
velocity.
 It can be produced or destroyed by the application of force on
it.
 It is measured by the force required to stop the body in unit
time.
 Momentum depends on Mass and velocity of the body.
 Momentum is vector quantity. Its unit is kgms.

15. NEWTON’S SECOND LAW OF MOTION
The rate of change of momentum of a body is direct proportional
to the applied force and takes place in the direction in which the
force acts.
 We can derive F = ma from newton’s second law.
 Unit of force is newton.
 Force in terms of the components : Newton’s second law agree
with Newton’s first law.

16. IMPULSIVE FORCE
The force in which act on bodies for short time
 Example
IMPULSE
 An impulse force does not remain constant, but changes from zero to maximum. The
impulsive force is not easy to measure ,bcz it changes with time. In such a case ,we measure
the total effect of the force called impulse.
 The impulse of force is the product of the average force and the time for which it acts.
 Relation between impulse and momentum :
change of momentum = impulse

17. Q : A bullet of mass 0.04 kg moving with speed of 90 ms -1 enters heavy wooden
block and is stopped after a distance of 60 cm .what is the average resistive force
exerted by the block on the bullet?
Q :A batsman hits back a ball straight in the direction of the bowler without
changing its initial speed of 2 ms- 1. If the mass of the ball is 0.5kg. Determine
the impulse imparted to the ball.( Assume linear motion of the ball).

18. NEWTONS THIRD LAW OF MOTION
For every action, there is always equal and opposite reaction .