Physics for Forcestt CSEC Caribbean.pptx

Section #6
Newton’s Laws of
Motion

Newton’s Laws of
Motion
“If I have seen farther than others, it is because I
was standing on the shoulders of giants.”
-Sir Isaac
Newton

Backgroun
d
Sir Isaac Newton (1643-1727) an English scientist and
mathematician famous for his discovery of the law of
gravity also discovered the three laws of motion.
He published them in his book Philosophiae
Naturalis Principia Mathematica (mathematic
principles of natural philosophy) in 1687. 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.

Newton’s First
Law
An object at rest wants to stay at rest and an object in
motion wants to stay in motion unless acted on by an
unbalanced force.
“Law of
Inertia”

Newton’s first law, usually called the law of inertia, is a
restatement of Galileo’s idea that a force is not needed
to keep an object moving.
What is meant by unbalanced force?
If the forces on an object are equal and opposite, they are
said to be balanced, and the object experiences no
change in motion.
If they are not equal and opposite, then the forces
are unbalanced and the motion of the object
changes.
Newton’s Law of
Inertia

Objects at Rest
Simply put, things tend to keep on doing what they’re
already doing.
• Objects in a state of rest tend to remain at rest.
• Only a force will change that state.

Objects in Motion
Now consider an object in motion.
• In the absence of forces, a moving object tends to move
in a straight line indefinitely.
• Toss an object from a space station located in the
vacuum of outer space, and the object will move
forever due to inertia.

Be Careful!!!
If an object is remaining at rest, it is
incorrect to assume that there are no
forces acting on the object.
We can only conclude that the net
force on the object is zero.
The net force acting on an object is the vector sum of
all the forces acting on it.
Examples:
?
8
lb
8
lb
4
lb
6
lb
12 lb
7
lb
9
lb
4
lb
8
lb

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 greater its inertia and the more
force it takes to change its state of motion.
The amount of inertia an object has depends on its mass -
which is roughly the amount of material present in the
object.

Inertia Example
Y
ou can tell how much matter is in a can when you kick it.
Kick an empty can and it moves. Kick a can filled with sand
and it doesn’t move as much.

Mass Is Not Volume
Do not confuse mass and volume.
Volume is a measure of space and is measured in units such as
cm3, m3, and liters.
Mass is measured in the fundamental unit of kilograms.

Which has more mass, a feather pillow or a common
automobile battery?
The pillow has a larger size (volume) but a smaller mass
than the battery. But, clearly an automobile battery is more
difficult to set into motion. This is evidence of the battery’s
greater inertia and hence its greater mass.

Mass Is Not Weight
Mass is often confused with weight.
• Mass is a measure of the amount of material in an
object.
• Weight, on the other hand, is a measure of the
gravitational force acting on the object.

Mass Is Inertia
The amount of material in a particular stone is the same
whether the stone is located on Earth, on the moon, or in
outer space.
• The mass of the stone is the same in all of these
locations.
• The weight of the stone would be very different on Earth
and on the moon, and still different in outer space.

We can define mass and weight as follows:
• Mass is the quantity of matter in an object.
• Weight is the force of gravity on an object.
It is common to describe the amount of matter in an object by
its gravitational pull to Earth, that is, by its weight.
In most parts of the world the measure of matter is
commonly expressed in units of mass, the kilogram (kg).
At Earth’s surface, 1 kilogram has a weight of 9.81
Newton’s.

The SI unit of force is the Newton (kgm/s2)
The SI symbol for the Newton is N.
If you know the mass of something in kilograms and want
its weight in Newton's at Earth’s surface, multiply the
number of kilograms by 9.81 m/s2.

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 unbalanced forces’ 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.

Frictio
n
Friction is a force that arises due to the relative motion of
two surfaces.
1. T
wo solid surfaces: Sliding friction, rolling friction
2. A solid and a fluid: Air resistance
3. Two fluids
The direction of the friction force always acts in the
opposite direction of motion.

The amount of sliding friction depends on:
1. The surface material
(smoothness/roughness)
2. The normal force between the surfaces
The amount of sliding friction does not depend
on:
3. Area of contact
4. Relative speed
Frictio
n

Physics for Forcestt CSEC Caribbean.pptx

Question #1
If gravity between the Sun and Earth suddenly vanished,
Earth would continue moving in a(n):
a. curved path.
b. straight-line path.
c. outward spiral path.
d. inward spiral path.
6.1 Assessment
Questions

Question #2
T
o say that 1 kg of matter weighs 9.81 N is to say that 1 kg of
matter
a. will weigh 9.81 N everywhere.
b. has ten times less volume than 9.81 kg of matter.
c. has ten times more inertia than 9.81 kg of matter.
d. is attracted to Earth with 9.81 N of force.
6.1 Assessment
Questions

Question #3
If the sum of all forces acting on a moving object is zero, the
object
will:
a. Slow down and stop
b. Change the direction of motion
c. Accelerate uniformly
d. Continue moving with a constant velocity
6.1 Assessment
Questions

Question #4
Which object has the greatest
inertia?
a. A 5-kg mass moving at 10
m/s
b. A 10-kg mass moving at 1
m/s
c. A 15-kg mass moving at 10
m/s
d. A 20-kg mass moving at 1
m/s
6.1 Assessment
Questions

Question #5
Which object has the greatest
inertia?
a. A 5-kg mass moving at 5
m/s
b. A 10-kg mass moving at 3
m/s
c. A 15-kg mass moving at 1
m/s
d. A 20-kg mass at rest
6.1 Assessment
Questions

Question #6
Which object has the greatest inertia?
a. A 15-kg mass traveling at 5
m/s
b. A 10-kg mass traveling at
10 m/s
c. A 10-kg mass traveling at 5
m/s
d. A 5-kg mass traveling at 15
m/s
6.1 Assessment
Questions

Question #7
Which object has the greatest inertia?
a. A falling leaf
b. A softball in flight
c. A seated high school student
d. A rising helium-filled toy
balloon
6.1 Assessment
Questions

Physics for Forcestt CSEC Caribbean.pptx

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