Forces and law of motions 2.pdf class 9 CBSE

Forces and law of motion class 9

1. Force and
law of
Motion
Ms. Smita

2. Topics Covered
FORCEGALILEO’S ANALYSIS
ABOUT MOTION
NEWTON’S LAW OF
MOTION

3. Rest and
Motion
An object is at restif it’s position relative
to it’s surrounding doesn’t change over
time
Ex: Tree, House, Box
An object is in motionif it’s position
does change with time.
Ex: Car, Earth, Water

4. Force
We cannot see, touch or smell the force
Force is defined as a push or pull on the
body.
Force is an external effort that can
change an object’s state of motion or
rest or change its shape.
Force is a vector quantity
SI unit-Newton (N)
CGS unit-dyne ( not used)
1N= 100000 dyne

5. Effects of forces
Stopping a moving objectStopping
Moving a stationary objectMoving
Changing the speed of a moving objectChanging
Changing the direction of a moving objectChanging
Changing the size and shape of an object.Changing

6. Balanced and Unbalanced forces

7. Balanced forces
•Can be at rest and or moving with the constant speed
Unbalanced forces
Moving with inconstant speed

8. Resultant (Net)
force
The resultant force is the total force
acting on an object when multiple forces
are combined. The resultant force has
the same effect on an object as the
original system of forces.

9. Galileo's analysis of motion
•Aristotle’s Belief: Natural state of bodies is at rest
•Galileo’s Opposition: CHALLANGED
Galileo’s observation:
•Ball rolling down an inclined plane: Speed increases
•Ball rolling up an inclined plane: Speed decreases
Experiment on Horizontal plane
•On smooth surface, ball continues to move
•Speed remains constant with no external force or friction
Conclusion: Bodies naturally oppose changes in their state of rest or motion

10. Inertia
•Inertia is defined as a property of matter by which it tries to maintain its state of rest or
of uniform motion along a straight line.
•Inertia is directly proportional to the massInertia of
rest
Object stays at rest
until external force
affects it
Body moving
backwards whrn
car starts
Inertia of
motion
Object continue
moving until
external force
affects it
Hockey puck
continue across the
ice until scted upon
by outside force

15. Between a football and rock of same size,
which one will have more inertia?

16. Newton’s First Law of
Motion
•Newtons, first law of motion states that a body remains in
the state of rest or uniform motion in a straight line unless
until an external force acts on it.
•There are two conditions on which the first law of motion
is dependent:
• Object at rest: When an object is addressed velocity (V
=0) and acceleration (a =0) is equal to zero. Therefore,
the object continues to be at rest.
•Object in motion: When an object in motion velocity is
not equal to zero, (V≠0) while acceleration is equal to
zero (a=0) therefore an object will continue to be
motion with constant velocity and in the same motion.

17. Example: Newton’s 1st
law of motion
•A person standing in a bus falls backward when
bus starts moving suddenly.
•A person standing in a bus falls forward if driver
applies brakes suddenly

19. Momentum
Momentum is the quantity of motion an object has
The product of velocity and mass is called the momentum (p)
p= m × v
Where,
p= momentum
m= mass
v= velocity of the object

Impulse = force × time (out of syllabus)

20. Momentum:
Examples
Even a small bullet even a small bullet is
able to kill a person when it is fired from a
gun because of its momentum due to
great velocity.
A person get injured in the case of hitting
by a moving object, such a stone paper or
anything because of momentum of the
object.
Person a person gets injured seriously
when hit by a moving vehicle because of
the moment of due to mass and velocity.

22. What is the momentum of a body of mass 5 KG
moving with a velocity of 0.20 m/s. (ans: 1 kg m/s)

23. The mass of good lorry is 4000 KG and mass of
goods loaded on it is 20,000 KG. If the lorry is
moving with a velocity of 2 m/s, what will be its
momentum? (48,000 Kg m/s)

24. Newton’s second law of motion
THE RATE OF CHANGE OF
MOMENTUM OF AN OBJECT IS
DIRECTLY PROPORTIONAL TO THE
APPLIED AND BALANCED FORCES
IN THE DIRECTION OF THE FORCE.
NEWTON’S SECOND LAW OF
MOTION STATES THAT THE
ACCELERATION OF AN OBJECT IS
DIRECTLY PROPORTIONAL TO
THE NET FORCE ACTING ON IT
AND INVERSELY PROPORTIONAL
TO ITS MASS.
IF ANY OBJECT IS IN THE STATE
OF REST, THEN IT WILL BE
REMAINING INTEREST UNTIL AN
EXTERNAL FORCE IS APPLIED TO
CHANGE ITS STATE. SIMILARLY,
AN OBJECT WILL REMAIN IN
MOTION UNTIL ANY EXTERNAL
FORCE IS APPLIED OVER IT TO
CHANGE ITS STATE. THIS MEANS
ALL OBJECTS RESIST TO IT,
CHANGING THEIR STATE.
THE STATE OF ANY OBJECT CAN
BE CHANGED BY APPLYING
EXTERNAL FORCES ONLY.

25. Mathematical Expression: Newton’s 2nd law of
motion
•Newton, second law of motion states that the
force exerted by a body is directly proportional
to the rate of change of its momentum
u v
m m
m= mass

26. A person is prone to more serious injuries when
volume on from certain height on a card concrete
floor then on Sandy floor. Explain why
When a person falls from a height on a hard concrete floor, he immediately comes in rest
position. It means change in momentum is taking place in an extremely short time in
consecutively force exerted by the floor on the person to destroy its momentum is
extremely large. Hence, chances of more injuries
When a person volume falls on the sandy surface, the surface gets compress downward
and it increases the time of fall. As a result, for same change in momentum force exerted
by sandy surface on a person is less and chances of his being heard or less.

27. Q: Why does a continuous push over time helps to accelerate a car
with a dead battery more effectively than a sudden push?
Continuous, pushing applies a study force over the time gradually
changing the course momentum which is more effective than a sudden
push. That does not provide enough time for momentum change.
Q; Why does a fast-moving cricket ball hurt a spectator more than a
table tennis hitting a player?
A fast-moving cricket ball has a greater momentum due to its higher
mass and velocity which result in more force will impact compared to
the table tennis.