Unit 6, Lesson 5 - Newton's Laws of Motion

NEWTON’S
LAWS OF
MOTION
Unit 6, Lesson 6.5

Lesson Outline
 Law 1: Law of Inertia
 Law 2: F = ma
 Law 3: Law of Interaction
 Momentum and Impulse (Introduction only)
Credits to the owner. Some slides are derived from this site:
education.jlab.org/jsat/powerpoint/newtons_laws_of_motion.ppt

Mechanics is the branch of Physics dealing with the study
of motion.
It has two areas:
• Kinematics – describing motion
• Dynamics – what causes changes in motion?

So far, we have already studied Kinematics, that is, we
have described motion in terms of the speed,
acceleration, time, and distance travelled by a certain
body by applying different formulas.

After describing the motion of an object, we will now look
into what caused the motion, in a branch called
dynamics.

What causes something to move?
What causes change in motion?
This can be answered by studying Newton’s 3 Laws of
Motion

Newton’s Laws of Motion
(Summary)
• 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.

1st Law
LAW OF INERTIA
Inertia is a resistance to a
change in its state of
motion (speed, direction, or
state of rest).
Inertia is an ability to resist
any change in its state of
motion.

1st Law
LAW OF INERTIA
In layman’s term:
Objects tend to "keep on
doing what they're doing”.
A moving object will continue moving and a nonmoving
object will remain not moving (at rest)…
UNLESS a FORCE is applied!

1st Law
LAW OF INERTIA
Which leads us to a formal
statement:
LAW NO. 1:
A body at rest will remain
at rest and a body in
motion will remain in
motion unless acted upon
by an outside unbalanced
force.

1st Law
LAW OF INERTIA
WHY UNBALANCED FORCE?
Unbalanced forces do not cancel out (in terms of vectors
via tip-to-tail method):
The resultant is a vector, thus, there is a motion going to
that direction.

1st Law
LAW OF INERTIA
IN BALANCED FORCES…
Vectors cancel out
The resultant is zero, thus, there is no motion.

1st Law
LAW OF INERTIA
CAN ALL FORCES CAUSE CHANGE IN MOTION?
No. To be able to cause a change in motion, the force
exerted must be greater than the inertia of the object.
Example:
You cannot move your house by pushing it because you
do not have enough energy to do so!. You need to exert
tremendous amount of force to surpass its inertia, which
will of course, if possible, destroy your house!

1st Law
LAW OF INERTIA
CAN FORCES CAUSE MOTION?
No! It is a big misconception even among
physics students that forces cause motion. Force
causes a change in motion, not motion. Instead,
the correct though is that, force causes
acceleration, not motion.

1st Law
LAW OF INERTIA
CAN FORCES CAUSE MOTION?
Forces are not the cause of motion, but forces
cause "a change" in motion. I mean, if something
has a straight line motion with a velocity of 3 m/s and
a second later it has a velocity of 5 m/s, Newton's
Laws would say a force interacted with that
something, changing its motion status, but Newton's
Law would not explain why that something had a
straight line motion with a velocity of 3 m/s at the
beginning.

• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
1st Law
LAW OF INERTIA

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

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.

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.)

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.

Units
Force – Newtons (N)
Mass – kg
Acceleration – m/s2
2nd Law
F = ma

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

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 (9.8 m/s2).
• 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/s2
F = ma
9.8 N = 1 kg x 9.8 m/s2

The problem solving part has already been
tackled previously.
F=ma
a=F/m
M=F/a
2nd Law
F = ma

3rd Law
LAW OF INTERACTION or
LAW OF ACTION AND REACTION

3rd Law
LAW OF INTERACTION
LAW NO. 3:
For every action,
there is an equal
and opposite
reaction.

3rd Law
LAW OF INTERACTION
Mathematically:
action = –reaction
The negative (–) sign indicates opposite reaction.
Graphically:

3rd Law
LAW OF INTERACTION
Therefore, if you
punch a wall with a
strong force, it also
punches you back
with the same force
you exerted. That’s
why it hurts.

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.
3rd Law
LAW OF INTERACTION

• Mass in motion
Mathematically:
p = mv
Where p is the momentum,
from the Latin petere
meaning pressure.
m is mass (kg)
v is velocity (m/s)
MOMENTUM

The higher the
momentum, the
harder it is for
the object to
stop.
MOMENTUM

From the formula
p=mv
We can see the following relationships:
1. Mass is directly proportional to momentum
2. Velocity is directly proportional to momentum
MOMENTUM

Therefore:
-The greater the mass, the greater the
momentum (Converse is also true)
-The faster the velocity, the greater the
momentum (Converse is also true)
MOMENTUM

It makes sense
because its
indeed hard to
stop a heavy train
from moving
compared to
stopping a rolling
ball.
MOMENTUM

Problem Solving:
A fat man weighing 80 kg is running at 4 m/s, while a
thin man weighing 40 kg is running at 10 m/s. Who
has a larger momentum?
MOMENTUM

Problem Solving:
MOMENTUM
Fat man:
p = mv
= (80 kg)(4 m/s) = 320 kg m/s
Thin man:
p = mv
= (40 kg)(10 m/s) = 400 kg m/s

Problem Solving:
MOMENTUM
Fat man: 320 kg m/s
Thin man: 400 kg m/s
Therefore, the thin man has the larger momentum!
Note that even the fat man is far heavier than the thin man, the
thin man’s momentum is greater because it is running at a high
velocity. Therefore, it is harder to stop the thin man.

• Something that
changes the
momentum of an
object
To change the momentum,
you have to apply a force for
a period of time, which gives
us the formula for impulse
(on the next slide)
IMPULSE

Mathematically:
I = Ft or
I = m∆v = m(vf – vi)
I is impulse (Ns) m is mass
F is force (N) ∆v is change in velocity:
T is time (s) final velocity – initial velocity
IMPULSE

1. Which of Newton's Laws is demonstrated by a ball rolling to a
wall then stopping? (1 pt)
2. It is the tendency of an object to continue doing what it is
currently doing. (1 pt)
3. Calculate the force of a moving body of mass 45 kg
accelerating at 3 m/s2. (3 pts)
4. Refer to the experiment on p. 223-224. Answer no. 1. (4 pts)
5. Answer no. 2 (6 pts)
ASSIGNMENT: 1 whole sheet of
paper (submit tomorrow)= 30 pts

7. Solve:
From the data given by LRT System Line 1, the maximum
speed allowed for these trains is 22.22 m/s. If the mass of a
train is 40,000 kg moving to the west:
a) Calculate the momentum of the train at its maximum
speed. (3 pts)
b) Calculate the momentum of the train at 15 m/s. (3 pts)
c) Find the impulse if the train slowed down from its
maximum speed to 15 m/s. (5 pts)

1. Which of Newton's Laws is demonstrated by a ball rolling to a
wall then stopping? (1 pt)
FIRST LAW: LAW OF INERTIA
2. It is the tendency of an object to continue doing what it is
currently doing. (1 pt)
INERTIA
3. Calculate the force of a moving body of mass 45 kg
accelerating at 3 m/s2. (3 pts)
F= ma = (45 kg)(3 m/s2) = 135 N

For nos. 4-6, answers may vary but must be rational.
4. Refer to the experiment on p. 223-224. Answer no. 1. (4 pts)
The air from the balloon rushes out (action) and propels the
car forward (reaction). (Law of Action and Reaction)
Law of Inertia = the unbalanced force from the air coming out
of the balloon caused the car to move
Second Law = The greater the mass placed on the car, the
slower it moves.
Third Law = same to answer in no. 4

The greater the mass, the greater the momentum, that is, the
harder for the object to cease motion.

7. From the data given by LRT System Line 1, the maximum
a) Calculate the momentum of the train at its maximum
speed. (3 pts)
p=(40,000 kg)(22.22 m/s)
p=888,800 kg m/s, west

b) Calculate the momentum of the train at 15 m/s. (3 pts)
p=(40,000 kg)(15 m/s)
p=600,000 kg m/s, west

c) Find the impulse if the train slowed down from its
maximum speed to 15 m/s. (5 pts)
I = m(vf – vi)
I = (40,000 kg)(15 m/s – 22.22 m/s)
I = -288,800 Ns, west or 288,800 Ns, east

Unit 6, Lesson 5 - Newton's Laws of Motion

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Unit 6, Lesson 5 - Newton's Laws of Motion