Download Introduction to Newton`s Laws

Introduction to Newton’s Laws
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?
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 didn’t
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.
Comparison between Aristotle and Galileo
ARISTOTLE
Galileo
• Things at rest were at • Objects at rest stay at
their natural resting
rest until something
place.
forces them to do
otherwise.
• Objects that are
moving away from
• Objects in motion stay
their natural resting
in motion unless
place need to be
something forces it to
continuously forced to
do otherwise
keep them moving
Newton’s Laws
What are Newton’s 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)
Newton’s 1st Law
Law 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?
Inertia
Inertia is an objects ability to resist a change in
motion.
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
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 km/hour.
If Newton’s 1st law is true:
Why then, do we observe every day
objects in motion slowing down and
becoming motionless seemingly
without any outside influence?
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.)

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 don’t move at a
constant velocity?
Yes!!!
Things that aren’t moving at a constant velocity are
accelerating.
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
Newton’s 2nd Law
F = ma
The net force of an object is equal to
the product of its mass and
acceleration
Newton’s 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 !
Another form of F = ma
We can write this as
What does this mean in English?
What is a net force?
The sum of all forces acting on an object in a given direction.
3N
2N
1N
Unbalanced forces vs
balanced forces
Forces are unbalanced when the net force on an object is not
at equilibrium (or at zero).
3N
2N
=
1N
Forces are balanced when the net force on a object is
at equilibrium.
3N
3N
=
No net
force
So why does everything fall with the
acceleration due to gravity?
Little F over little m
g= a=
F
m
9.8 m/s2
Big F over big m
g= a=
F
m
9.8 m/s2
Newton’s 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”
• 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).
Can you think of other examples of Newton’s 3rd Law?
Horse and Cart
A horse can pull a cart with a force of 3120N. According to
Newton’s 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?
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?
Free Body Diagram
(Force Diagrams)
Lets draw a picture of the force of a man pushing on a Physics Box
15N
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
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 object’s 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)
When you sit in your
chair, your body exerts
a downward force on
the chair (your weight)
and the chair exerts an
upward force on your
body (known as the
Normal Force).
Question:
For 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?
Question:
For 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?
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?
Can you do this one?
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!
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.)
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
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.
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 tires and the road?
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)
Here’s 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?
FN
Ff
F
F
30o
Fg
Thought question
Why is it impossible to make a piece of string completely
horizontal and straight if you hang a mass from the center?