Newton's three laws of motion are summarized. Newton's First Law states that objects at rest stay at rest and objects in motion stay in motion unless acted upon by an unbalanced force. Newton's Second Law states that force equals mass times acceleration. Newton's Third Law states that for every action, there is an equal and opposite reaction. Examples are provided to illustrate each law, such as friction slowing moving objects or gravity accelerating objects at the same rate but with different forces depending on their mass.

1. UNIT # 10
NEWTON’S LAWS OF MOTION
RAKHSHANDA HASHMI
Education Department
NUML,Isb

2. I. Law of Inertia
II. F=ma
III. Action=Reaction

4. Newton’s First Law
(law of inertia)
An object at rest tends to stay at
rest and an object in motion tends
to stay in motion unless acted upon
by an unbalanced force.

6. Equal forces in opposite
directions produce no motion
(1st diagram)
Unequal opposing forces
produce an unbalanced force
causing motion
(2nd diagram)

7. If objects in motion tend to stay in motion, why
don’t moving objects keep moving forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.

8. Newton’s First Law
(law of inertia)
• MASS is the measure of the
amount of matter in an object.
• It is measured in Kilograms

9. Newton’s First Law
(law of inertia)
• INERTIA is a property of an object
that describes how
______________________ the
motion of the object
• more _____ means more ____
much it will resist change to
Mass inertia

10. These pumpkins will not move unless
acted on by an unbalanced force

11. 1st Law
• Once airborne,
unless acted on
by an
unbalanced
force (gravity
and air – fluid
friction), it
would never
stop!

12. 1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tree forever.

13. Why then, do we
observe every day objects
in motion slowing down
and becoming motionless
seemingly without an
outside force?
It’s a force we sometimes cannot
see –

14. • There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water resistance
– Static friction: initial friction when moving an object
What is this unbalanced force that acts on an
object in motion?

15. Slide a book
across a table and
watch it slide to a rest
position. The book
comes to a rest
because of the
presence of a force -
that force being the
force of friction -
which brings the book
to a rest position.

16. • In the absence of a force of friction, the book
would continue in motion with the same
speed and direction - forever! (Or at least to
the end of the table top.)

17. Inertia

18. Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist
changes in their motion. When the car going 80
km/hour is stopped by the brick wall, your body
keeps moving at 80 m/hour.

19. Newton’s First Law is also called
the Law of Inertia
Inertia: the tendency of an object to
resist changes in its state of motion
The First Law states that all objects
have inertia. The more mass an object
has, the more inertia it has (and the
harder it is to change its motion).

20. More Examples from Real Life
A powerful locomotive begins to pull a
long line of boxcars that were sitting at
rest. Since the boxcars are so massive,
they have a great deal of inertia and it
takes a large force to change their
motion. Once they are moving, it takes
a large force to stop them.
On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).

21. UNIT # 11
NEWTON’S 2ND LAW OF MOTION

22. Newton’s Second Law
Force equals mass times
acceleration.
F = ma

23. 2nd Law

24. Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.

25. What does F = ma mean?
Force is directly proportional to mass and acceleration.
Imagine a ball of a certain mass moving at a certain
acceleration. This ball has a certain force.
Now imagine we make the ball twice as big (double the
mass) but keep the acceleration constant. F = ma says
that this new ball has twice the force of the old ball.
Now imagine the original ball moving at twice the
original acceleration. F = ma says that the ball will
again have twice the force of the ball at the original
acceleration.

26. More about F = ma
If you double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples the
force.
So . . . what if you decrease the mass by half? How
much force would the object have now?

27. What does F = ma say?
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.

28. 2nd Law
• When mass is in kilograms and acceleration is
in m/s/s, the unit of force is in newtons (N).
• One newton is equal to the force required to
accelerate one kilogram of mass at one
meter/second/second.

29. 2nd Law (F = m x a)
• How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
• Write the formula
• F = m x a
• Fill in given numbers and units
• F = 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800 N

30. If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.

31. Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
98 N = 10 kg x 9.8 m/s/s
F = ma
9.8 N = 1 kg x 9.8 m/s/s

32. Check Your Understanding
• 1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine
the mass.
16 N = 3.2 kg x 5 m/s/s
• 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
• 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
• 9800 kg-m/sec/sec or 9800 N

34. UNIT # 12
NEWTON’S 3RD LAW OF MOTION

35. Newton’s Third Law
For every action there is an
equal and opposite reaction

36. What does this mean?
For every force acting on an object, there is an equal
force acting in the opposite direction. Right now,
gravity is pulling you down in your seat, but
Newton’s Third Law says your seat is pushing up
against you with equal force. This is why you are
not moving. There is a balanced force acting on
you– gravity pulling down, your seat pushing up.

38. Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).

39. 3rd Law
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.

40. 3rd Law
There are two forces
resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.

41. Newton’s 3rd Law in Nature
• Consider the propulsion of a
fish through the water. A fish
uses its fins to push water
backwards. In turn, the water
reacts by pushing the fish
forwards, propelling the fish
through the water.
• The size of the force on the
water equals the size of the
force on the fish; the direction
of the force on the water
(backwards) is opposite the
direction of the force on the
fish (forwards).

42. 3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.

43. • Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the
bird upwards.
• The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on
the bird (upwards).
• Action-reaction force pairs make it possible for birds
to fly.

44. Newton’s Third Law
• A bug with a mass of 5
grams flies into the
windshield of a moving
1000kg bus.
• Which will have the
most force?
• The bug on the bus
• The bus on the bug

45. Newton’s Third Law
• The force would be the
same.
• Force (bug)= m x A
• Force (bus)= M x a
Think I look bad?
You should see
the other guy!

46. Action: earth pulls on you
Reaction: you pull on earth
Action and Reaction on Different Masses
Consider you and the earth

47. Action: tire pushes on road
Reaction: road pushes on tire

48. Action: rocket pushes on gases
Reaction: gases push on rocket

49. Consider hitting a baseball with a bat. If we
call the force applied to the ball by the bat the
action force, identify the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
(b) the force applied to the bat by the ball

50. Newton’s 3rd Law
• Suppose you are taking a space walk
near the space shuttle, and your
safety line breaks. How would you
get back to the shuttle?

51. Newton’s 3rd Law
• The thing to do would be to take one of the tools
from your tool belt and throw it is hard as you can
directly away from the shuttle. Then, with the help of
Newton's second and third laws, you will accelerate
back towards the shuttle. As you throw the tool, you
push against it, causing it to accelerate. At the same
time, by Newton's third law, the tool is pushing back
against you in the opposite direction, which causes
you to accelerate back towards the shuttle, as
desired.

52. Other examples of Newton’s Third Law
• The baseball forces the
bat to the left (an
action); the bat forces
the ball to the right (the
reaction).

53. 3rd Law
• Consider the motion of
a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip
the road and push the
road backwards.

54. 3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.

55. What Laws are represented?

57. Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.

58. Vocabulary
Inertia:
the tendency of an object to resist changes
in its state of motion
Acceleration:
•a change in velocity
•a measurement of how quickly an object is
changing speed, direction or both
Velocity:
The rate of change of a position along
a straight line with respect to time
Force:
strength or energy