std09th chapter 08 force and law of motion

1. FORCE AND LAW OF MOTION
Chapter Number: 08
ScienceByMohammadafakPatel
MO. 9974015703 Email. Pmo.afak313@gmail.com

2. Force and Law of motion
Force:
• Push or pull on an object is celled force.
• It is defined as, the push or pull on an object with mass that causes it to
change its velocity.
• It is a vector quantity
Effects of force on object:
• Change in the state of rest of a body/object
• Change in the position of a body/object.
• To changes the speed of the body/object.
• To change the direction of motion of a body/object.

3. FORCE
• A force is a push or pull acting on an object which changes the state of rest or of uniform
linear motion.
F=Ma
Where,
F means Force
M means mass
A means Acceleration
• SI unit of Force: N (Newton)
• There are Two type of forces
TYPES OF FORCES:
Balanced Force Unbalanced Force

4. Balanced Force:
• When a number of force acting simultaneously on an object do not produce any change in its
state of rest of linear motion, the force is said to be balanced force.
• Example of Balanced Forces
In tug of war games when both the teams start pulling the rope with equal and opposite forces, then
the rope remains in place as the forces acting on it are equal and opposite and their resultant
becomes zero.
Unbalanced Force:
• When two or more force acting simultanesouly on an object that changes the state of rest or of
Uniform linear motion of an object, the force is said to be unbalanced force.
• Example of Balanced Forces
If the block is pulled from both sides with the same force then the block remains stationary (i.e at its
position). The forces applied are unequal and opposite to each other. The resultant of the forces
acting on this block is now not zero as block will shift.

5. GALILEO CONCEPT OF MOTION:
• By observing the motion of objects on an inclined plane Galileo concluded that on objects
move with constant velocity when no external unbalanced force acts on it.
GALIEO EXPIREMENT:
• Galileo suggested that the speed of the ball moving on a horizontal plane remains constant when no
external force or force of friction acts on it.
• Galileo noticed that it is the natural tendency of all bodies to oppose any change in their state of rest or
motion.

6. NEWTON’S LAW OF MOTION
• Sir Isaac Newton further developed Galileo’s ideas on force and motion.
• Newton presented three fundamental laws of governing the motion of objects.
NEWTONS 1st LAW OF MOTION
• The first law states that, an object remains in the state of rest or of uniform motion in a straight
line unless or until an external unbalanced force is applied on it.
• The first law of motion give rise to the idea of Inertia. We can also define force from first law of
motion
• The first law of motion is also known as Law of Inertia.
INERTIA
• It is a inherent property of an object by virtue of which it cannot change the state of rest or of
uniform motion along a straight line.

7. INERTIA
• Inertia is a property or tendency of every object to resist any change in its state of rest or of
uniform motion along a straight line.
TYPES OF INERTIA
1.Inertia of rest: examples to this is a passenger standing in a bus leans backwards when brakes
are applied suddenly, fruits falling down from the tree when it is shaken, dust particles on a
carpet when it is beaten with a stick.
2. Inertia of motion: Example to this is man alighting from a moving train leans forward.
3. Inertia of direction: example to this water particles stuck to the cycle tyre and fly off
tangentially, when a drive takes a turn, the passenger feels the force away from the centre of
the curve.
INERTIA AND MASS
• The inertia of a body depends on the mass of the body.
• Inertia is directly proportional to mass of the object.

8. Application Of Newton's First Law Of Motion:
• When a straight moving bus suddenly stops down, the passengers sitting inside fall in
the direction. This is because the body of the passenger initially moving in a straight
line tends to move the same way even after the brakes are applied, making the
passenger fall in the forward direction.
• When we hit a carpet it loses inertia of rest and moves. But the dust in it retains
inertia of rest and is left behind. Thus dust and carpet are separated.
• When a tree is shaken, it moves to and fro. But fruit remains at rest due to its inertia
of rest. Due to this fruit breaks off the tree.

9. MOMENTUM (p)
• The momentum of a body is defined as the product of its mass and velocity.
Thus, momentum = mass × velocity
p = m x v
where, p = momentum
m = mass of the body
v = velocity of the body
• The SI unit of momentum : kilograms meter per second (kgm/s)

10. CHANGE IN MOMENTUM:
• It is defined as the difference between final momentum and initial momentum.
• Let u be the initial momentum of a body and v be its final momentum, then
• Change in momentum = mv – mu
RATE OF CHANGE OF MOMENTUM:
• The rate at which the momentum of an object is changing is known as the rate of change of
momentum.

11. NEWTON'S SECOND LAW OF MOTION
It states that the rate of change of momentum of a body is directly proportional to the applied force and
takes place in the direction in which the force acts.
Mathematical Formulation Of Newton’s Second Law Of Motion
• Let mass of an moving object be m.
• The initial velocity be u
• The final velocity be v.
We know that momentum (p) = Mass × velocity Therefore,
• The Initial momentum of object = mu
• The Final momentum of the object = mv
• therefore change in momentum = mv – mu

13. APPLICATIONS OF NEWTON’S 2ND LAW OF MOTION
• A fielder pulls his hand backward; while catching a cricket ball coming with a great speed. Actually,
while catching a cricket ball the momentum of ball is reduced to zero. If the ball is stopped suddenly,
its momentum will be reduced to zero instantly causing the instant rate of change in momentum due
to which ball will exert great force on the hands of player due to which the player’s hand may get
injured. Therefore, by pulling the hand backward a fielder gives more time to the change of
momentum to become zero. This prevents the hands of fielder from getting hurt.
• For athletes of long and high jump, sand bed or cushioned bed is provided at the place of landing. This
is because when an athlete falls on the ground after performing a high or long jump, the momentum of
his body is reduced to zero. If the momentum of an athlete will be reduced to zero instantly, it will
result in the production of a large force which may hurt the player. Whereas, by providing a cushioned
bed, the momentum of player’s body is reduced to zero in a delayed period due to which less force
acts on his body hence, preventing the athlete from getting hurt.

14. NEWTON'S THIRD LAW OF MOTION
• It states that for every action there is equal and opposite reaction, in the opposite direction and but
of equal magnitude, i.e., action and reaction forces are equal and opposite.
APPLICATIONS OF NEWTON'S THIRD LAW OF MOTION
• Recoil of gun: When bullet is fired from a gun, it moves ahead. By the Newton’s 3rd law of
motion, the bullet apply same force on gun in backward direction. Due to this force, gun moves
back giving a jerk to the shoulder of the gunman. This is called recoil of gun.
• Walking of a person: A person is able to walk due to the Newton’s Third Law of Motion. During
walking, a person pushes the ground in backward direction and in the reaction the ground also
pushes the person with equal magnitude of force but in opposite direction. This enables him to
move in forward direction against the push.
• Swimming in water: Man pushes water back by applying force. By Newton's 3 Law, water applies
equal and opposite force on swimmer. Due to this force man moves ahead.

15. CONSERVATION OF MOMENTUM
• In the absence of external force the total momentum of a system will be constant. i.e.,
• Total momentum before collision = Total momentum after collision