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The Law of Inertia
Galileo said…
Marbles rolled down a ramp will reach the same height as
that from which they are released no matter how long the ramp.
So, what if the ramp is flat?
Newton said…
Objects at rest remain at rest unless acted upon by
an outside force.
Objects in motion will remain in motion unless
acted upon by an outside force.
What causes the tire to
stop rolling?
TableCloth.mpg
cool video. Click HERE
Inertia is dependent on mass.
The larger the mass, the more inertia it has.
Forces can change the motion of an object.
A force is a push or pull.
What kinds of changes occur?
• Contact Forces – Any force applied when
things are in contact.
Ex. Push, pull, air resistance, friction
• Field Forces – Any force applied, but there
is no contact.
Ex. Gravity, Magnetism, electric field
The unit of force is a newton.
(Appropriately named)
One Newton = the amount of force needed to accelerate
1 kilogram by 1 meter per second squared.
1 kg
a = 1 m/s/s
Force is a vector quantity.
100 Newtons
The length of the arrow shows the strength of the force.
The direction of the arrow shows the direction that the
force is applied.
Force vectors are added and subtracted just as velocity
vectors are…
The sum of the force vectors is called the net force.
Finding Net Force:
If the net force is zero. There is no change in motion.
The system is at equilibrium. The forces are balanced.
An unbalanced force results
in a change in motion in the
direction of the larger force.
Finding Net Force:
Dumb & Dumber moving a piano examples:
http://www.youtube.com/watch?v=-s4nVg_W
Finding Net Force:
Drawing free-body diagrams
floor pushing up
Gravity pulling down
A box being pushed across a table. Draw the free diagram.
Weight – is a force.
It is the force of gravity acting on an object’s mass.
Weight (in newtons) = mass x acceleration due to gravity.
1 kg x 9.8 m/s/s = 9.8 Newtons
Can you calculate your weight in Newtons?
Friction – is a force which is caused by objects
touching and that opposes motion, but is
necessary for basic movement.
Friction helps
us stand
Friction helps go
forward and stop
Friction helps
us walk and run
Friction can also hinder motion.
Can you think of situations where friction is a disadvantage?
Friction…
…helpful or harmful?
Newton’s second law explains the relationship
between the net force on an object, it’s mass
and the acceleration that results from the force:
Let’s look at the relationships:
If mass is constant, the larger the force on an object, the
faster the objects accelerates.
Large force = mass x large acceleration
Small force
= mass x small acceleration
This is a direct proportion.
Let’s look at the relationships:
If the force is constant, the larger the mass of an object, the
slower the object accelerates.
force = small mass x large acceleration
force = large mass x small acceleration
This is an indirect or inverse proportion.
Let’s look at the relationships:
If the acceleration is constant, the larger the mass of an object,
the stronger the force must be to change the motion.
small force
= small mass x acceleration
large force = large mass x acceleration
This is a direct proportion.
Solving problems related to Newton’s second law:
The basic equation is:
Force = mass x acceleration
or F = ma
Try this:
Rearrange the variables so that you can solve for
acceleration or mass.
a=f/m
m=f/a
For Practice:
Example 1
How much force must a 30,000 kg jet plane develop in order
to achieve an acceleration of 1.5 m/s/s?
Example 2
What acceleration is produced by a force of 500 N applied
to a 1000 kg car?
Example 3
If a grocery cart is pushed with a force of 75 N, and the
cart accelerates from rest at a rate of 1.5 m/s/s, what is the mass
of the cart?
What is the weight of the cart?
What about friction?
• Friction is a force that opposes motion.
• The force applied must be greater than the
force of friction in order to accelerate an
object.
Net force
• Fnet = Fapplied - Ffriction
Ffriction
Fapplied
Consider this box. In order to make it accelerate, an applied
force larger than the force of friction is needed to make it
accelerate.
Example problem:
A 500 kg cart is pushed across the floor
with a force of 40 N. The floor and wheels
oppose the motion with a force of 10 N.
What is the acceleration of the cart?
What about upward motion?
• Gravity is a force pulling down on an
object.
• In order to accelerate an object upward, the
applied force must be larger than the weight
of the object.
Net force
• Fnet = Fapplied - Fweight
Fapplied
Fweight
Consider this rock. In order to make it
accelerate, an applied force larger than
the weight of the rock is needed to make
it accelerate.
Example problem:
A 1000 kg elevator is accelerated upward.
The weight of the elevator opposes the
motion. What force must be applied in order
to cause an acceleration of 1m/s/s?
Newton’s Third Law
• For every Action there is an equal and
opposite Reaction
• Forces ALWAYS come in pairs.
Examples:
• Hammer and nail
• Your chair and your “seat.” You and the
ground
• Pushing against the wall
• Tires and the tar
• Rocket and propelling Gasses
Why does the nail go into the board?
“A batter exerts some 6000-8000 pounds of force on the ball.
This force is required to change a 5 1/8th-ounce ball from a
speed of 90 mph to a speed of 110 mph, this distorts the baseball
to half its original diameter and the bat is compressed one
fiftieth of it's size.“
How many Newtons of force are applied to the ball?
How much force does the ball apply to the bat?
• The horse pulls on a wagon with a force of
10,000 Newtons, with what force does the
wagon pull on the horse?
• How is it possible that the horse and wagon
move?