Laws of motion

3. Jeho B. Caballes
Science Teacher
DBTC - Cebu
LAWS OF MOTION
Inertial Frames, Acceleration, Interaction

4. Overview
• Session 1: Inertial Frames, Law of Inertia, Forces, Friction
• Session 2: Law of Acceleration
• Session 3: Law of Interaction, Action-Reaction Pairs
• Session 4: Applying Laws of Motion (Quipper Task)

5. Session 1
Forces, Free-body Diagram,
Law of Inertia

6. Learning Targets
• I can differentiate between contact and
non-contact forces.
• I can draw free-body diagrams.
• I can define inertial frames of reference in
relation to the concept of inertia.

7. Forces
Force is usually referred to as the pull or push exerted upon an
object as it interacts with another object.
has magnitude and
direction
SI unit = Newton (N)

8. Forces
Contact Force
direct physical contact
Non-Contact Force
action-at-a-distance forces

9. Contact Forces
Normal force
It refers to the force exerted upon an
object by the surface with which it is in
contact with.
perpendicular to the
surface regardless of the
angle

10. Contact Forces
Friction force
It refers to the force exerted upon an
object by a surface, such that the force
is parallel to it.
parallel to the surface

11. Contact Forces
Tension force
It refers to the pulling force exerted upon
an object by a cord it is attached to.

12. Non-contact Forces
Magnetic force It refers to the repulsion or attraction
between moving charged particles.

13. Non-contact Forces
Electric force
It refers to the force exerted by one
electric charge to another, both for
stationary and moving charges.

14. Non-contact Forces
Gravitational
force
It refers to the force responsible for
attracting two objects that have mass.

16. Forces
Contact Force
direct physical contact
Non-Contact Force
action-at-a-distance forces
Normal force
Friction force
Tension force
Magnetic force
Electric force
Gravitational force

17. Check Your Understanding
Identify what is being referred to in each of the following statements.
1. It is the pull or push exerted upon an object by another object.
2. These forces occur when two objects are physically in touch with
each other.
3. Give an example of a contact force that opposes the relative motion
between surfaces.
4. These forces act between objects without direct physical interaction.
5. Name the force responsible for the attraction between two masses.

18. Free-body Diagram
A diagram used to show the relative magnitude and direction of
all forces acting upon an object in a given situation.

19. Free-body Diagram
Objects DO NOT necessarily always have four forces
acting upon them.

20. Steps for Drawing FBD
1. Identify the object (represent it as a box).
2. Identify all forces acting on the object and
their direction.
3. Draw each force as an arrow (the length
should be proportional to its magnitude).
4. Label each arrow with force name and
magnitude.

21. Example
Draw a free-body diagram for a paperweight that is at rest on a desk.
Fn
Fg

22. Example
Gregory pulled a 2 kg box attached to a rope along the frictionless surface
of the floor with a force of magnitude 30.0 N. He exerted this force at a 30-
degree angle to the east.
Fn
Fg
FT 30 N

23. Example
A block of cheese is resting on an incline.
Fn
Fg
Ff
Fg
Ff
Fn

24. Example
The carrot hangs from the ceiling by two ropes.
FT2
Fg
FT1

25. Worksheet 4
1. A book is at rest on a table top.
2. A girl is suspended motionless from a bar which hangs from the ceiling
by two ropes.
3. An egg is free-falling from a nest in a tree. Neglect air resistance.
4. A rightward force is applied to a book in order to move it across a desk
at constant velocity. Consider frictional forces. Neglect air resistance.
5. A car is coasting to the right and slowing down.

26. Consider the following scenario:
After it has been released, the book will slow down and eventually
stop.
What caused the book to stop?

27. "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."
First Law of Motion

28. This tendency of objects to remain in motion is
referred to as inertia, which is also the reason
why Newton’s first law is also called the law of
inertia.
First Law of Motion

29. Inertial Frame of Reference
A frame of reference which is at rest or which is moving with
a uniform velocity along a straight line is called an inertial
frame of reference.
Example:
1. Observer at rest on the earth.
2. A lift moving up/down with constant velocity.

30. Session 2
Law of Acceleration

31. Learning Targets
• I can determine the relationships between force,
mass, and acceleration.
• I can state the Second Law of Motion.
• I can solve problems using the Second Law of
Motion.

32. “The acceleration of a moving
object depends upon its mass
and the force acting on it”
Second Law of Motion

33. F = ma
Second Law of Motion
Heavier objects require
more force to accelerate.
Lighter objects require
less force to accelerate.

34. Limitations of Second Law
• The Second Law applies only to external forces,
defined earlier as the forces from outside the system of
interest of the object under investigation.
• The Second Law is only relevant when the object’s
mass is constant.
• The Second Law may only apply in inertial frames of
references

35. Examples
• What net external force must be applied to make a vehicle
accelerate at 2.5 m/s2 if it has a mass of 3,250 kg?

36. Examples
• A wind-driven iceboat, initially at rest on a horizontal ice
rink, is pushed by the wind such that 3.5 seconds after its
release, it is travelling eastward at 20 kph. The boat and
its rider has a combined mass of 215.50 kg. Calculate the
force that the wind exerted on the boat.

37. Worksheet 5
1. A skydiver of mass 70-kg has jumped out of a plane two miles above the
surface of the earth. After 20 seconds, he has reached terminal velocity,
meaning he is no longer accelerating. What is the force of the air on the
skydiver's body?
2. A block of mass 3 kg is moving on a surface. A force of 25 N acts on it in the
positive x-direction and 15 N in the negative x-direction. In which direction is it
moving? What is its acceleration?
3. A net external force of 7.5 N is exerted upon a 4.65-kg object. If the object’s
initial velocity is 2.3 m/s when the force is applied, determine its final velocity
after 6 seconds.

38. 1. A skydiver of mass 70-kg has jumped out of a plane two
miles above the surface of the earth. After 20 seconds, he
has reached terminal velocity, meaning he is no longer
accelerating. What is the force of the air on the skydiver's
body?
2. A block of mass 3 kg is moving on a surface. A force of 25 N
acts on it in the positive x-direction and 15 N in the negative
x-direction. In which direction is it moving? What is its
acceleration?
3. A net external force of 7.5 N is exerted upon a 4.65-kg
object. If the object’s initial velocity is 2.3 m/s when the force
is applied, determine its final velocity after 6 seconds.

39. Let's Summarize
• Acceleration refers to change in velocity. When the net external
force (or the combination of individual forces exerted on an object) is
not zero, acceleration occurs.
• The magnitude of acceleration a is directly proportional to the
magnitude of the net external force F exerted on an object of a given
mass m.

40. Let's Summarize
• Kilogram is the SI unit of mass.
• Newton IS the SI unit for force.
• N = kg•m/s2

41. Examples
Which object exerts greater force?

42. Session 3
Law of Interaction and Action-
Reaction Pairs

43. Learning Targets
• I can state the third law of motion.
• I can explain the concept of action-and-reaction
pairs.
• I can identify action-and-reaction pairs.

44. Third Law of Motion
"For every action (force), there is an equal and
opposite reaction (force)."
Fa = - Fr

45. Action-Reaction Pairs
Forces always come in pairs known as
"action-reaction" force pairs.
Fa = - Fr

46. Action-Reaction Pairs
Action-reaction pairs are forces of equal
magnitude and opposite direction that
act on different object.
They never act on the same object.

47. Examples
foot is pushing against the
ground backward
ground is pushing against
the foot forward
"Walking on the ground"

48. Examples
the ball is exerting force
on the pin to the left
the pin is exerting force
on the ball to the left
"Bowling Alley"

49. Examples
Which object exerts greater force?

50. Check Your Understanding
1. State the third law of motion.
2. In your own words, explain the concept of action-
reaction pair.
3. Give a real-life scenario and identify the
action-reaction pair.

51. Worksheet 6
Answer the following on 1/2
crosswise yellow pad.

53. Let’s Process
1. The diver moves "forward" and dives into the
water. The raft moves "backwards" in the water
because of the reaction force. The action force is
the diver pushing off of the raft, and the reaction
force is the raft pushing back on the diver
(causing the diver to go forward and into the
watery.

54. Let’s Process
2. The racquet does not swing backwards because
the force of your arm keeps it from going back.
The action force is the ball hitting the racquet
(which your arm "absorbs"), the reaction force is
the racquet pushing back on the ball causing it to
go back across the net

55. Let’s Process
3. The action force is the rocket pushing out the
"hot" gases produced by the engine. The reaction
force is the hot gas pushing back on the rocket
propelling it into outer space. There is no need for
air to push on because the hot gases produced by
the rocket allow the action and reaction forces to
operate.

56. Let’s Process
4. The forces on the book are gravity pulling the
book down and the table pushing the book back
up. These two forces are equal and opposite
(action-reaction) forces.

57. Let’s Process
5. When the two people push off of each other they
move away from each other on the scooters
because of the equal and opposite action and
reaction forces. The speed at which and the
distance each person moves depends on their
mass, friction of the scooter, etc.

58. Let’s Process
6. The person with less mass would move away
faster and would most likely move a greater
distance if the two scooters were identical (had
the same amount of friction). This can be
explained by Newton's 2nd Law (F=-ma) because
they both have the same force (action-reaction)
the person with less mass will accelerate more.

59. Let’s Process
7. The person moves away from the wall. In terms of
action-reaction the person applies a force on the
wall and the wall pushes back on the person with
an equal and opposite amount of force (reaction
force). Hopefully the person does not apply
enough force to make the wall move.

60. Let’s Process
8. When shooting a gun the action force is the
gunpowder in the shell pushing on the shotgun
shell. The reaction force is the recoil pressing
backwards against your shoulder.

61. Let's Summarize
• Newton’s Third Law of Motion states that if an object A applies
a force on object B, object B applies a force on object A, such
that the two forces comprise a respective action and reaction
pair.
• These two forces are of equal magnitude but of opposite
direction.