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1. Newton’s Three
Laws of Motion
Physics: Force, Motion, and Energy

2. Sir Isaac Newton
1. An English
scientist and
mathematician
2. Described 3 laws
that relate forces to
motion

3. 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

4. Relationship Between
MASS and INERTIA
Every object possesses inertia. An
object’s inertia depends on its mass.
Mass is the measure of inertia.
The larger the mass of an object, the
greater is its inertia.

6. Newtown’s 1st Law of Motion
“Law of Inertia”
 An object at rest remains at
rest and an object in motion
remains in motion at constant
speed and in a straight line
unless acted on by an
unbalanced force.

7. 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).

8. What does this mean?
Basically, an object will “keep doing what
it was doing” unless acted on by an
unbalanced force.
If the object was sitting still, it will remain
stationary. If it was moving at a
constant velocity, it will keep moving.
It takes force to change the motion of an
object.

9. 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 it.
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.

10. Some Examples from Real Life
Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite
directions. This balanced force results in no
change of motion.
A soccer ball is sitting at rest. It
takes an unbalanced force of a kick
to change its motion.

11. 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

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

13. In outer space, away from gravity
and any sources of friction, a
rocket ship launched with a
certain speed and direction would
keep going in that same direction
and at that same speed forever.

14. Bell Work
What is Newton’s 1st Law of
Motion?
Answer: The Law of Inertia

15. Question
What is the relationship
between mass and inertia?
Answer: Mass is a measure
of how much inertia
something has.

16. Bell Work
What is inertia?
Answer: Inertia is the
tendency of objects to stay at
rest or in motion.

17. Question
Is inertia a force?
Answer: No, inertia is a property
of matter. Something has
inertia. Inertia does not act on
something.

18. Question
 A force of gravity between the sun and
its planets holds the planets in orbit
around the sun. If that force of gravity
suddenly disappeared, in what kind of
path would the planets move?
Answer: Each planet would move in a
straight line at constant speed.

19. Question
 The Earth moves about 30 km/s
relative to the sun. But when you
jump upward in front of a wall, the
wall doesn’t slam into you at 30
km/s. Why?
 Answer: both you and the wall are
moving at the same speed, before,
during, and after your jump.

20. Assignment:
Answer the following in your notebook.
1. Why does a jeepney continue to move for some
time even after its engine has been switched off?
2. Which of these vehicles moving at the same speed
is difficult to stop- a car, a bus, or a train? Explain your
answer.
3. When you are inside a bus or jeepney,
a) what do you experience when the vehicle starts
from rest?
b) What do you also experience when, while the
vehicle is moving fast, it stops suddenly?
c) Explain these experiences in terms of the law of
inertia.

21. VOCABULARY
1. Force- a push or pull, all forces have size and
direction
2. Mass- the amount of matter an object is made
of
3. 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

22. 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

23. Newton’s Second Law of Motion
Force = mass x acceleration
F=ma

24. 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.

25. In Other Words…
F
a
Small Force = Small Acceleration

26. In Other Words…
Large Force = Large Acceleration
F
a
So….if you push twice as hard, it accelerates twice as much.

27. But there is a twist….
Acceleration is
INVERSELY related to the
mass of the object.

28. In other words…..using the same
amount of force….
F
Large Mass a
Small acceleration
F
Small Mass
Large acceleration
a

29. 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.

30. What does F = ma say?
F = ma basically means that the force of an
object comes from its mass and its
acceleration.
Force is measured in
Newtons (N) = mass (kg) x acceleration (m/s2)
Or
kg m/s2

31. High Mass
Something very massive
(high mass) that’s changing
speed very slowly (low
acceleration), like a glacier,
can still have great force.

32. Low Mass
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.

33. 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

34. Examples of Newton’s 2nd Law

35. Examples of Newton’s 2nd Law

36. In Summary
 The acceleration of an object is directly
proportional to the net force &
inversely
proportional to its mass.
 F = ma
 Force = Mass x Acceleration

37. CHECK YOUR
UNDERSTANDING
Net Forces and Newton’s Second Law

38. Question
 What is the second law of motion?
Answer: Law of Acceleration

39. Question
 State the formula of the Law of
acceleration?
Answer: F = ma or
Force= mass x acceleration

40. Question
 Suppose that the acceleration of
an object is zero. Does this mean
that there are no forces acting on
it?
Answer: No, it means the forces acting on it
are balanced and the net force is zero.
 Think about gravity and normal force acting
on stationary objects.

41. Question
 When a basketball player dribbles a ball, it
falls to the floor and bounces up. Is a
force required to make it bounce? Why?
If a force is needed, what is the agent.
 Answer: Yes, when it bounced it changed
direction. A change in direction =
acceleration. Acceleration requires a
force. The agent was the floor.

42. How Does Weight Tie In?
– Mass is the quantity of matter in an object. More
specifically, mass is a measure of the inertia, or
“laziness,” that an object exhibits in response to any
effort made to start it, stop it, or otherwise change its
state of motion.
– Weight is the force of gravity on an object.
– If force is equal to mass x acceleration then, Weight
is equal to mass x acceleration due to gravity

43. Weight
– So, on earth, your weight is
– Your Mass x 9.8 m/s/s
 For example, if I say a 2kg book is resting
on a table…
– The force due to gravity (weight) is 2 x 9.8
– The normal force would be the same but
opposite direction

44. Example
What net force will give a 30
kg body an acceleration of 6
m/s2.

45. Example
Find the mass of a
horizontal force of 16 N
acting on a acceleration of
8.0 m/s2.

46. Example
An 80.0 N horizontal force
acts on an object with a
mass of 4.00 kg and is
initially at rest. Find the
magnitude of the
acceleration produced.

47. Check Your Understanding
1. What acceleration will result when a 12 N net force applied to a 3 kg
object? A 6 kg object?
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.
Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1 m/s2?
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2?
5. Find the weight of a 5.0 kg cement block.
6. Find the mass of a 980.0 N object.

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

49. 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.

50. Think about it . . .
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).
Why does it hurt so much when you
stub your toe?

51. When you push on the wall, the wall pushes on you.
Forces and Interactions

52. Newton’s third law describes the relationship
between two forces in an interaction.
• One force is called the action force.
• The other force is called the reaction force.
• Neither force exists without the other.
• They are equal in strength and opposite in
direction.
• They occur at the same time
(simultaneously).
Newton’s Third Law

53. Examples of Newton’s 3rd Law
2 cars hit head on

54. When the girl jumps to shore, the boat moves backward.
Newton’s Third Law

55. When action is A exerts force on B, the reaction is simply B exerts force on A.
Identifying Action and Reaction Pairs

56. Earth is pushed down
by the boulder with
just as much force as
the boulder is pulled
down by Earth.
Action and Reaction on Different Masses

57. Question
1. A force interaction requires at least
a(n)
a. single force.
b. pair of forces.
c. action force.
d. reaction force.

58. Question
3. The force that directly propels a motor
scooter along a highway is that
provided by the
a. engine.
b. fuel.
c. tires.
d. road.

59. Question
 We know that Earth pulls on the moon. Does
the moon also pull on Earth? If so, which pull
is stronger?
 Asking which pull is stronger is like asking
which distance is greater—Manila and Tarlac
or Tarlac to Manila. The distances either way
are the same. It is the same with force pairs.
Both Earth and moon pull on each other with
equal and opposite forces.

60. Question
 Suppose a friend who hears about Newton’s third
law says that you can’t move a football by kicking it
because the reaction force by the kicked ball would
be equal and opposite to your kicking force. The net
force would be zero, so no matter how hard you
kick, the ball won’t move! What do you say to your
friend?
 If you kick a football, it will accelerate. No other
force has been applied to the ball. Tell your friend
that you can’t cancel a force on the ball with a force
on your foot.

61. Review on the three laws of motion