Newtons Law of Motion

1. Newton’s
Laws of
Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction

3. While most people know
what Newton's laws say,
many people do not know
what they mean (or simply do
not believe what they mean).

4. Background
 Sir Isaac Newton (1643-1727) was an
English scientist and mathematician
famous for his discovery of the law of
gravity also discovered the three laws of
motion.
 Today these laws are known as Newton’s
Laws of Motion and describe the motion of
all objects on the scale we experience in our
everyday lives.

5. “If I have ever made any valuable discoveries, it has
been owing more to patient attention, than to any
other talent.”
-Sir Isaac Newton

6. Forces and Motion
 Forces can create cha
nges in motion (accel
eration or deceleratio
n).

7. Definition of a Force
 A force is a p
ush or a pull.

8. Balanced Force
 A force that prod
uces no change i
n an object’s mo
tion because it is
balanced by an e
qual yet opposite
force.

9. Unbalanced Forces
Are forces that
result in an ob
ject’s motion b
eing changed.
+

10. Motion can be described as
:
 A change in an obje
ct’s position.

11. Newton’s Laws of Motion
 1st Law – An object at rest will stay at
rest, and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
 2nd Law – Force equals mass times
acceleration.
 3rd Law – For every action there is an
equal and opposite reaction.

12. Vocabulary
 1. Friction- a force that opposes motion
between 2 objects that are touching
 2. Inertia-tendency of all objects to stay at rest
or in motion
 3. Mass- the amount of matter an object is
made of

13. 1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.

15. 1st Law
 Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move unless acted on
by an unbalanced force.

16. Examples of Law of Inertia
 The body moves to the side when a car
makes a sharp turn.
 Seat belts tightening in a car when it
stops quickly.
 A ball rolling down a hill will continue to
roll unless friction or another force stops
it.
 If pulled quickly, a tablecloth can be
removed from underneath of dishes

17. 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 –
friction.

18. Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.

19.  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?

20. 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.

21.  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.)

22. 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.

24. Examples of Newton’s 1st Law of Motion
 1. Car suddenly stops and
you strain against the seat
belt
 2. Car turns left and you
appear to slide to the right
 3. The difficulty of pushing
a car that won’t start

25. Examples of Newton’s 1st Law of
Motion

26. Examples of Newton’s 1st Law of
Motion

27. Write about it!
 Write down one
example of inertia you
see in the room.

28. Bell Work
 What is Newton’s 1st Law of Motion?
 Is an example of Newton’s 1st Law when a car
turns left and you appear to slide to the right?
 What is inertia?

29. 2nd Law

30. 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.

31. Newton’s Second Law of Motion
 The acceleration of an object depends on the
mass of the object and the amount of the force
applied

32. Newton’s Second Law of Motion
 1. Force = mass x acceleration F=m x a
 2. Force- a push or pull, all forces have size and direction
 3. Mass- the amount of matter an object is made of
 4. Acceleration-the rate at which velocity changes; and object
accelerates if its speed changes, if its direction changes and if
both speed and direction changes

33. Newton’s Second Law of Motion

34. Newton’s Second Law of Motion
Sample Problem
Suppose a ball of mass 0.60 kg was hit with a force of 12 N.
What is its acceleration?
Solution: a=F/m
a=12N/0.60kg
a= 20 m/s2
System of Units Force Mass Acceleration
SI (mks) Newton (N) Kg m/s2
SI (cgs) Dyne G cm/s2
English Pound (lb) slug ft/s2

35. Newton’s Second Law of Motion
2. If the force was increased to 24 N for the same ball, what is its
acceleration?
Solution: a=F/m
a=24N/0.60kg
a= 40 m/s2

36. Examples of Newton’s 2nd Law
 1. Hitting a softball, the harder the hit, the
faster the ball goes
 2. Football players and their positions
 3. Loaded versus an unloaded truck

37. Examples of Newton’s 2nd Law

38. Examples of Newton’s 2nd Law

40. Bell Work
• What is Newton’s 2nd Law of Motion?
• What is an example of Newton’s 2nd Law?
• What is friction?

42. 3rd Law
 For every action, there is an
equal and opposite reaction.
Law of Interaction

44. Newton’s Third Law of Motion
 1. For every force, there is an equal and
opposite force
 2. Action and Reaction

45. Examples of Newton’s 3rd Law
 1. 2 cars hit head on
 2. Jumping out of a boat onto a dock
 3. Astronauts in space

46. 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.

47. 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.

48. 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).

49. 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.

50.  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.

68. MASS AND WEIGHT
 The difference between mass and weight is one of
the most frequently asked questions. Some students
often use the terms mass and weight
interchangeably, which is completely wrong.

69. MASS AND WEIGHT

70. Comparison and differences
between mass and weight
Definition
 Mass is simply the
measurement of the
amount of matter in a
body.
Definition
 Weight is the
measurement of the
amount of force acting
on a mass due to
acceleration due to
gravity.

71. Comparison and differences
between mass and weight
Denotation
 Mass is denoted by
“M.”
 The SI unit of mass is
Kilogram (Kg).
Denotation
 Weight is denoted by
“W.”
 The SI unit of weight is
Newton (N).

72. Comparison and differences
between mass and weight
Gravitational
 Mass does not depend
upon gravity and is
constant everywhere.
 Mass can never be zero.
Gravitational
 Weight is dependent on
gravity, and so, it varies
from place to place.
 Weight can be zero
where there is no
gravity (like space).

73. Comparison and differences
between mass and weight
Measuring Instrument
 Mass can be easily
measured using any
ordinary balance like
beam balance, lever
balance, pan balance,
etc.
Measuring Instrument
 Weight can be measured
by a spring balance or
by using its formula.