Transcript Newton`s Laws

4.2 Newton’s Laws of Motion
Sir Isaac Newton
(1642-1727)
• Discovered laws of motion
and gravity
•
First Law: An object at rest stays at rest and
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an object in motion, stays in motion (same
direction & same speed) unless acted upon
by an unbalanced force
Also called the Law of inertia
Mass—A Measure of Inertia
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Inertia is a property of matter that causes it to
resist any change in its motion
The amount of inertia an object has depends
on its
more mass = more the inertia
more speed = more the inertia
Mass—A Measure of Inertia
You 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—A Measure of Inertia
Mass Is Not Volume!
Do not confuse mass and volume.
Volume is a measure of space and is measured
in units such as cubic centimeters, cubic
meters, and liters.
Mass is measured in the fundamental unit of
kilograms.
Mass—A Measure of Inertia
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—A Measure of Inertia
It’s just as difficult to
shake a stone in its
weightless state in space
as it is in its weighted
state on Earth.
Mass—A Measure of Inertia
Mass and weight are proportional to each other in a given
place:
In the same location, twice the mass weighs twice as
much.
Mass and weight are proportional to each other, but
they are not equal to each other.
Weight = (mass) x (acceleration due to gravity)
Formula: W = mg
What is a Force?
FORCE = Any push or pull acting on an
object
The SI unit of force is the newton, the symbol for the
newton is N.
One newton is equal to slightly less than a quarter
pound.
If you know the mass of something in kilograms and
want its weight in newtons at Earth’s surface,
multiply the number of kilograms by 10.
One kilogram of nails weighs
10 newtons, which is equal to
2.2 pounds.
Away from Earth’s surface,
where the force of gravity is
less, the bag of nails weighs
less.
Second Law: The acceleration of an object is
proportional to the net outside force acting
on the object
Force = mass object  acceleration of object
Formula:
F = ma
The acceleration produced depends on the mass that
is pushed.
Forces can be BALANCED or UNBALANCED
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Balanced forces are equal in size and
opposite in direction
Unbalanced forces are not equal in size
and/or opposite in direction.
If the forces on an object are unbalanced, we
say there is a NET force.
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The greater the force, the greater the acceleration
The greater the mass, the greater the force needed for
the same acceleration
Examples of Newton’s 2nd
Law
•Bunting the baseball, versus a grand slam
•The positioning of football players - massive players on
the line, lighter (faster to accelerate) players in the
backfield
•Pushing a loaded versus an unloaded truck
Third Law: For every
force, there is always an
equal and opposite
reaction force.
Example of
Newton’s 3rd Law
Identifying Action and Reaction Forces
When action is A exerts force on B, the reaction is simply B
exerts force on A.
Action and Reaction
think!
We know that Earth pulls on the moon. Does the moon also pull
on Earth? If so, which pull is stronger?
Action and Reaction
think!
We know that Earth pulls on the moon. Does the moon also pull
on Earth? If so, which pull is stronger?
Answer: Asking which pull is stronger is like asking which
distance is greater—between New York and San Francisco, or
between San Francisco and New York. The distances either way
are the same. It is the same with force pairs. Both Earth and moon
pull on each other with equal and opposite forces.
Weightlessness – free from the effects of
gravity?
Using a rocket
you could get
enough sideways
speed to keep
missing the earth
as you fall.
The faster you run from the tower the farther you go before falling
to earth
• There is gravity in
space
• Weightlessness is
due to a constant
state of free-fall