Download The Laws of Motion

The Laws of Motion
Newton’s Three Laws
What is a Force?
It is something we experience every single day.
You are exerting a force on your pencil right now as you
copy these notes or perhaps you are pressing down on
computer keys
Even if you think to yourself, “well I am not taking notes,”
you are still experiencing the force of gravity pulling you
down and your chair exerting a force up so…there is no
escape! Well there is…but we will get to that another day
Newton’s First Law
If an object does not interact with other objects, it is
possible to identify a reference frame in which the
object has zero acceleration
What does this mean?
It’s complicated…but we can easily simplify it
“Any intelligent fool can make things bigger, more
complex and more violent. It takes a touch of genius –
and a lot of courage – to move in the opposite
direction” -Albert Einstein
Newton’s First Law (cont.)
In terms of a reference frame, imagine you have an air
hockey table inside a train moving 70 mph. A hockey
puck on the table, experiencing almost friction,
appears to not be moving but we know the train is
moving so the puck must also be moving at 70 mph
In reference to the train, the puck is not moving at
all…but in reference to the earth it is moving 70 mph
Newton’s First Law (cont.)
If the train were to speed up or slow down, the puck
would then begin to move and maintain its 70 mph
speed because there is no friction between it and the
table
So the first law can be simplified – A body at rest or at
constant velocity in the absence of an external force
remains at rest or at constant velocity
In other words, when no force acts on an object, the
acceleration of the object is zero
Newton’s Second Law
What happens when to an object when a force acts on
it?
Imagine exerting a force on a block and moving it
along a horizontal, frictionless surface. The block
experiences a change in velocity (acceleration) because
it went from stationary to moving. The more force you
apply, the more the block accelerates
So there is obviously a direct relationship between
force and acceleration
Newton’s Second Law (cont.)
What else affects the acceleration?
Would a heavier block accelerate as quickly with the
same amount of force it took to move the first one?
The answer is, of course, no – it would take more force
to move the heavier block at the same acceleration
So there is an inverse relationship between acceleration
and the mass of the object
Newton’s Second Law (cont.)
So, when viewed from an inertial reference frame, the
acceleration of an object is directly proportional to the
net force acting on it and inversely proportional to it’s
mass
This brings the formula:
Newton’s Second Law (cont.)
Practice Problem
If that first block weighed 10 kg and the force you
applied accelerated it by 10 m/s2, how much force
would you need to apply to the heavier block that
weighs 50 kg in order to achieve the same acceleration?
Newton’s Third Law
The law of Action and Reaction
If two objects interact, the force exerted by object 1 on
object 2 is equal to the magnitude and opposite in
direction to the force exerted by object 2 on object 1
The sum of the two forces is zero because they equal
and opposite which would mean they cancel out
So, if there is an equal and opposite force, how does
anything ever move?
Newton’s Third Law (cont.)
Imagine book sitting on a table. It is not moving
because the table is exerting a force up equal the
amount of force down that the earth is pulling on the
book. This force the table is exerting is not the
reaction force.
The reaction force is that the book is also pulling on
the earth with the same amount of force that the earth
is pulling on the book – equal and opposite!
Newton’s Third Law (cont.)
So, the reaction force must act on two different objects
and must be of the same type.
In order for the object(s) to move, an external force is
needed
Review
First Law: A body at rest tends to stay at rest unless
acted on by an external force and an object in motion
tends to stay in motion unless acted on by an external
force
Second Law: Acceleration is directly proportional to
force and inversely proportional to mass  F = ma
Third Law: For every action there is an equal and
opposite reaction