Download Newton`s Laws

Chapter 3
Force, Mass and Acceleration
Newton’s Laws
Sir Isaac Newton
1642-1727
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An English Mathematician and Physicist
that formulated the three laws of motion,
law of universal gravitation and invented
calculus before the age of 30
Newton’s discoveries helped to answer
many questions such as: what causes
tides, how do the planets move and why
do objects of different masses fall at the
same rate?
What is Force?
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Force is a push, pull or any action
that has the ability to change motion
(This doesn’t mean that a force will
always make something move– can
you think of a situation such as this?)
There are two units that are
commonly used to measure force,
Newtons and Pounds. Scientists
prefer Newtons(N)
Force vs. Mass
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The Newton relates to force and
motion. A force of 1 Newton causes
a 1 kg mass to have an acceleration
of 1 m/s2
We call it a Newton because that is
somewhat easier to say than “1 kg
times 1 m/s2”
There are 4.48 Newtons per pound
Newton’s First Law
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Known as the Law of Inertia
“An object at rest will stay at rest until
acted upon by an unbalanced force, an
object in motion will continue with
constant speed and direction until acted
upon by an unbalanced force”
Basically – An object at rest remains at
rest and an object in motion remains in
motion unless acted on by an unbalanced
force.
Inertia
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Objects ability to resist changing its
state of motion
Inertia is dependant on mass,
therefore an object with more mass
would have a greater inertia
Example: Which is harder to move,
a bowling ball or a beach ball? The
ball with the greater mass is harder
to move because it has greater
inertia
Newton’s 2nd Law
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Relates the applied force to an
object’s mass and its acceleration
It says that force causes
acceleration, mass resists
acceleration and the acceleration is a
ratio of force over mass
Common sense: If something has
more mass, it would take more force
to cause the object to accelerate
Using Newton’s 2nd Law
F = ma
F
M
A
An object’s Force equals its mass times its acceleration
2nd Law
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A car engine produces a force and
you can calculate the acceleration of
the car when you know this force
and the mass of the car
If you want to increase the
acceleration of the car you must
decrease the cars mass or increase
the force of the engine
Balanced vs. Unbalanced Forces
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The motion of an object depends on
the total of all forces acting on it –
Net force
If the forces are balanced(net force
=0), then the object is in equilibrium
and there is no chance of movement
changing because the net force is
zero
If there is a net force, then the
forces are unbalanced, and
movement changes.
Weight vs. Mass
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Mass is the amount of matter in an object,
usually measured in kilograms
Weight is the force of gravity acting on a
certain mass, measured in Newtons or
pounds
A certain mass of something has a
“weight”, which is the force acting on that
mass. It varies depending on the gravity.
Gravity, Mass and Weight
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Imagine that you travel to the moon.
Which of these will be the same as on
earth: Gravity, your mass or your weight?
An object will have the same mass on the
moon as on earth – it still has the same
amount of matter
An object weighs less on the moon but
has the same mass. It weighs less
because there is less gravity.
You would weigh more on Jupiter than
Earth because Jupiter has more gravity
than Earth
Gravity
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A force that pulls every mass toward
another mass
Earth is the biggest mass for us so
other masses are pulled toward
earth’s center
On earth, every kilogram of mass
weighs 9.8 Newtons
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You can figure out the weight by
multiplying the mass times the
acceleration of gravity
F= m X a
Weight is a force (pulled to earth’s
center)
The acceleration due to gravity on
Earth is 9.8 m/s2
Gravity, Cont.
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On Earth, every object will fall at the same
rate (not counting air friction)
The Acceleration of gravity is 9.8 m/s2
meaning that every second, a falling
object accelerates 9.8 m/s
In other words, every second something is
falling it is moving 9.8 m/s faster
If you drop a bowling ball and a match
box car at the same time, they will hit the
ground at the same time
Newton’s Third Law
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“For every action, there is an equal and
opposite reaction”
There must be a PAIR of objects
The action and reaction act on separate
objects
If you press down on the table, you are
exerting a force on it. Is the table
exerting a force on your hand?
Your force on the table is action force, the
table’s force on you is reaction force
3rd Law
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Skateboard – foot on ground is
action – earth against skateboard is
reaction
Throw ball on a skateboard
Recoil – backward acceleration from
reaction force
Law of Universal Gravitation
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The force of attraction (gravity)
between 2 objects depends on the
mass of the objects and the distance
between them
More noticable between large objects
(planets and stars)
Force = G x mass 1 x mass 2
d2
G = 6.67 x 10-11
Friction
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Forces that work against the motion
of the object
Force of friction is always in the
opposite direction of the motion
Must consider when finding net force
Will cause motion to stop eventually
unless force is continually supplied
Types of Friction
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Air Friction – caused by air moving
around objects
Sliding Friction – two objects sliding
against one another
Rolling Friction – one object rolling
over another object
Viscous Friction – object moving in
water or fluid