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Title: Introduction%20to%20Newton

1
Introduction to Newtons Laws
2
If you had the option of kicking a soccer ball or
a bowling ball, which would you pick?
Why? Why do you move to the side of a car
when the car turns a corner?
3
Back in the day
Aristotle tried to answer the question of why
objects fall to Earth? By explaining that objects
would seek their natural position.
-This also explain why Earth didnt move. It was
already in its natural resting place at the
center of the universe.

• This was, until Copernicus
• came along and explained that the Earth orbited
the sun.
• This was later backed up by Galileo and Newton
when they explained forces and motion in the
universe.

4
Comparison between Aristotle and Galileo
• ARISTOTLE
• Things at rest were at their natural resting
place.
• Objects that are moving away from their natural
resting place need to be continuously forced to
keep them moving
• Galileo
• Objects at rest stay at rest until something
forces them to do otherwise.
• Objects in motion stay in motion unless something
forces it to do otherwise

5
Newtons Laws
What are Newtons 3 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. (Law of Inertia)
• 2nd Law Force equals mass times acceleration.(F
ma)
• 3rd Law For every action there is an equal and
opposite reaction. (All forces come in pairs)

6
Newtons 1st LawLaw 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.
What does this all mean?
7
Inertia
Inertia is an objects ability to resist a change
in motion.
8
What gives an object Inertia?
• Volume is the amount space an object takes up.
• Not what gives an object inertia.
• Weight is the affect that gravity has on an
object.
• Not what gives an object inertia.
• Mass is the amount of matter an object possesses.
• (the amount of stuff something is made of)
• This is what gives an object inertia.

Mass is directly related to Inertia. The more
mass you have, the more inertia you have.
Some Inertia Demos
9
Newtonss 1st Law and You
Dont 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 km/hour.
10
If Newtons 1st law is true
Why then, do we observe every day objects in
motion slowing down and becoming motionless
seemingly without any outside influence?
11
Forces!
A force is a push or pull. -force is a vector
quantity (it has a size and direction)
• Friction is a type of force. It is a force that
opposes motion.
• Acts in the direction opposite motion
• In the absence of friction, an object you
• push (like a book on a table top) would continue
in
• motion with the same speed and direction -
forever!
• (Or at least to the end of the table top.)

12
Up until this point, we have only been dealing
with objects at rest or moving with a constant
velocity.
Can we describe the motion of objects that dont
move at a constant velocity?
Yes!!! Things that arent moving at a constant
velocity are accelerating.
13
Acceleration Review
Acceleration is how much velocity changes in a
certain amount of time.
Acceleration is a vector quantity.
The units for acceleration are meters per second
per second, or m/s2
14
(No Transcript)
15
Newtons 2nd Law
F ma
The net force of an object is equal to the
product of its mass and acceleration
16
Newtons 2nd Law
If F ma then the units for force are kgm/s2
We call kgm/s2 Newtons (N)
How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?
Lab !
17
Another form of F ma
We can write this as What does this mean in
English?
18
What is a net force?
The sum of all forces acting on an object in a
given direction.
1 N
3 N
2 N
19
Unbalanced forces vs balanced forces
Forces are unbalanced when the net force on an
object is not at equilibrium (or at zero).

Forces are balanced when the net force on a
object is at equilibrium.
20
So why does everything fall with the acceleration
due to gravity?
Big F over big m
Little F over little m
21
Newtons 3rd Law
For every action there is an equal and opposite
reaction
OR
All forces come in pairs
Whenever an object exerts a force on a second
object, the second object will exert an equal
force in the opposite direction on the first
object If A pushes on B then B also pushes
on A
22
• 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).

23
Can you think of other examples of Newtons 3rd
Law?
24
Horse and Cart
A horse can pull a cart with a force of 3120N.
According to Newtons 3rd, what is the force that
the cart pulls back on the horse with?
If this is true, how can the horse ever move the
cart if the forces cancel each other out?
25
Tug of War
How much force does the bird on the right pull
with if the bird on the left pulls on the worm
with a force of 2 N and it wins the tug of war?
What is the tension in the worm?
26
Free Body Diagram(Force Diagrams)
Lets draw a picture of the force of a man pushing
on a Physics Box
15N
27
Free Body Diagrams
When we draw forces acting on an object (in the
form of a free body diagram), we should draw all
the forces so that they are pulling from the
center of mass.
15N
28
Other types of forces
Normal force is the force that is always
perpendicular to the surface of what ever an
object is sitting on. It generally opposes the
downward force of gravity.
Gravitational force Force due to gravity is the
force that an objects exerts because of gravity.
On Earth, all objects gravitational forces act
downward toward the center of the planet.
Weight is an objects force due to gravity. They
are interchangeable.
Tension only observed when an object is under
the influence of a pull.
Friction a force that opposes motion (retarding
force)
29
downward force on the chair (your weight) and the
chair exerts an upward force on your body (known
as the Normal Force).
30
QuestionFor each question, make sure to draw an
accurate free body diagram!
What is the net force on a 15 kg box that is
sitting on a table?
What is the net force on a 15 kg box that is
being pulled to the left with 40 N of force and
28 N to the right?
31
QuestionFor each question, make sure to draw an
accurate free body diagram!
What is the net force on a 15 kg box on a
frictionless surface that is moving at a constant
velocity of 4 m/s to the right?
What is the net force on a 15 kg box on a
frictionless surface that is accelerating to the
right at a rate of 3 m/s2?
32
Can you do this one?
Question
Draw a free body diagram of a 20 kg box that is
being pushed to the right with a force of 50 N
and there is a 30 N frictional force. What is
the net force? What is the acceleration?
What is the acceleration of a 15 kg box sitting
on a frictionless surface if you pull it from a
rope to the right at an angle of 30 degrees from
the horizontal with 50N of force?
For this one we need TRIGONOMETRY!
33
Types of friction
Static Friction The frictional force that
exists between two surfaces when they are
stationary Kinetic Friction The frictional
force that exists between two surfaces when the
objects are moving across each other (like a book
sliding on a table)
Both of these types of friction can be calculated
by using the coefficient of friction (either the
coefficient of static friction or the coefficient
of kinetic friction.)
34
Coefficient of Friction
A value that is different for any two surfaces. -
It is a constant that can be used to help
calculate the force due to friction if you know
the normal force of an object.
Ff ?FN
Mu (like the pokemon)
? is usually in between zero and one
35
Coefficient of Friction
The coefficient of static friction tends to be
greater than that of kinetic friction
Another way to think about what ? is, is how
sticky the interaction between two surfaces
is. - The greater the ?, the greater the friction.
36
Questions
If the coefficient of friction between your tires
and the road is 0.2, and the car has a mass of
1700 kg, what is the frictional force between the
37
Questions
A car with a mass of 700 kg is breaking hard. If
the car is traveling at an initial velocity of 15
m/s, and takes 3 seconds to stop, what is the
coefficient of friction between the tires and the
road? (draw a free body diagram)
38
Heres a doozy
If this 15 kg box is sliding down the hill at a
constant velocity, what is the frictional force
between the box and the incline?
30o
39
Thought question
Why is it impossible to make a piece of string
completely horizontal and straight if you hang a
mass from the center?