Discovered the law of gravity
Discovered the three laws of motion.
Today these laws are known simply as Newton’s Laws
of Motion.
Published findings in the book Philosophiae Naturalis
Principia Mathematica (mathematic principles of
natural philosophy) in 1687.
Background
Sir Isaac Newton (1643-1727)
English scientist and mathematician
1st Law of Motion (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 mean?
Basically, an object will “keep doing what it was doing” unless a force comes along to change it.
- If the object was sitting still, it will remain stationary.
- If it was moving at a constant velocity, it will keep moving.
- It takes force to change the motion of an object.
First we need to define the
word FORCE:
The cause of motion (what causes objects to move)
Two types of forces
.
.
Forces may be
balanced or unbalanced Balanced forces – all forces acting on an object are
equal
There is NO MOTION
Unbalanced forces – one or more forces acting on
an object are stronger than others
There is MOTION
A NET FORCE
Newton’s First Law is also called the
Law of Inertia
Inertia: the tendency of an object to resist changes in its state of motion
1st Law – all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).”
Ex. Semi Truck compared to motorcycle
Example
In Pawn Stars episode, Corey puts a chair in
the back of his pick up truck. Slams on gas to
leave parking lot because he is in a hurry. The
chair falls out the back of the truck
Other Video Clips
http://www.youtube.com/watch?v=HRq-v4Gmzxg
Eureka – Inertia
Fast forward to 3.57 min left
Examples Law of Inertia Ex. A powerful train engine begins to pull a long line of
boxcars that were sitting at rest. Since the boxcars are so
massive, they have a great deal of inertia and it takes a
large force to get them going. Once they are moving, it
takes a large force to stop them.
Ex. On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).
Examples Law of Inertia
Unless acted upon by an
unbalanced force, this golf ball
would sit on the tee forever.
Once airborne, unless acted on
by an unbalanced force
(gravity and air resistance
(fluid friction), it would never
stop!
The truck is in motion.
What is the force that causes it to stop?
The push of the stopped car.
The car is at rest.
What is the force that causes it to move?
The push of the truck.
Why does a book that is sliding across
a table eventually slow down/stop?
Answer: There are forces we cannot see, such as
friction and gravity.
A book sliding across a table slows down and stops because of the force of friction.
Other Forces?
If you throw a ball
upwards it will
eventually slow down
and fall because of
the force of gravity.
In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction
would keep going in that same direction and at that same speed
forever.
Force of Gravity
Attraction between any two objects in the universe
Force of gravity increases as...
mass increases (gravitational force exerted by the moon is
less than that exerted by the earth)
distance decreases
There are four main types of friction:
Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
Static friction: initial friction just before an object begins to move
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.
Car Safety and 1st Law of Motion
Car Safety features
http://www.youtube.com/watch?v=8zsE3mpZ6Hw
&feature=related
Unit of Force = Newton (N)
One newton (N) is equal to the force
required to accelerate one kilogram
of mass at a rate of one
meter/second/second.
N = kg·m/s2
What does F = ma mean? Force is directly proportional to mass for a given acceleration.
Greater mass requires greater force for the same acceleration.
If a heavy SUV and a small sports car accelerate at the same rate, the SUV requires greater force.
Force is directly proportional to acceleration for a given mass.
Greater force causes greater acceleration of a given object.
A tennis player with a strong (more forceful) serve will accelerate the ball more than a player with a weaker serve.
Mass and acceleration are inversely proportional for a given force.
Greater mass undergoes smaller acceleration with a given force.
If the same force is applied to a golf ball and a bowling ball, the golf ball will have a greater acceleration.
Problem Solving using F = m a
How much force is needed to accelerate a 1400 kilogram car 2 m/s2?
F = m x a
F = 1400 kg x 2 m/s2
F = 2800 kg-m/s2 or 2800 N
Check Your Understanding
1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?
12 N = 3 kg x a = 4 m/s2
12 N = 6 kg x a = 2 m/s2
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
16 N = m x 5 m/s2 =3.2 kg
3. How much force is needed to accelerate a 66 kg skier 1 m/s2?
F = 66kg x 1 m/s2 =66 N
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2?
F = 1000kg x 9.8 m/s2 =9800 N
Mass, weight, and acceleration of gravity
Weight is a force, specifically the gravitational force
exerted on an object.
In this case, Fw = m g
A 10 kg mass has weight = 98 N.
A 1-kg mass has weight = 9.8 N.
What about the apple?
F = ma
Weight
the force of gravity on an object
MASS always the same
(kg)
WEIGHT depends on gravity
(N)
W = mg W: weight (N)
m: mass (kg)
g: acceleration due
to gravity (m/s2)
F = ma Mrs. M weighs 557 N. What is her
mass?
GIVEN:
W = 557 N
m = ?
g = 9.8 m/s2
WORK:
m = W ÷ g
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
m
W
g
3rd Law Newton’s Law States: whenever 2 objects
interact, they exert forces upon each other.
Ex. When you sit in your chair,
your body exerts a downward
force on the chair and the chair
exerts an upward force on your
body.
This is called action –reaction
Everything occurs in pairs.
Newton’s 3rd Law in Nature
Ex. 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.
Newton’s 3rd Law in Nature
Equal but opposite
Size of force on the water =size of the force on the fish
Direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).
3rd Law
Ex. As the birds push
down on the air with
their wings, the air
pushes their wings up
and gives them lift.
Other examples of Newton’s Third Law
The baseball forces the
bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
As the wheels turn, they “grip”
the road (think friction) and
push the road backwards as the
road pushes the car forward.
3rd Law
Ex. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube.
As the gases move downward, the rocket moves in the opposite direction.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push
against a wall?
You slide in the opposite direction (away from the
wall), because you pushed on the wall but the wall
pushed back on you with equal and opposite force.
Why does it hurt so much when you
stub your toe?
When your toe exerts a force on a rock, the
rock exerts an equal force back on your toe.
The harder you hit your toe against it, the
more force the rock exerts back on your toe
(and the more your toe hurts).
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