Newton`s First Law of Motion

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Transcript Newton`s First Law of Motion

Laws of Motion
First Law of
Newton’s First Law of Motion
“Objects tend to keep on doing
whatever it is that they are
doing unless another force acts
on them”
The Law of Inertia!!
Objects will stay in motion/at rest
until acted upon by a force
Mass is a measure of Inertia
• The mass of an object corresponds to it’s
– Ex. Which has more inertia:
– An elephant or a feather?
– A car or a semi truck?
The more mass an object has, the more
inertia it has!!
Mass is not the same as Volume
Mass: a measure of the amount of
matter in an object
Volume: a measure of the amount
of space occupied by an object
Mass is not the same as weight!!
Mass: the amount of matter in an
Weight: A measure of gravity’s pull on
an object
Second Law
of Motion
Law of Motion
• The force acting on an object is equal to the
mass of the object multiplied by its
Force = mass x acceleration
Definition: the rate of change of velocity
An object is said to accelerate when it does
any one of the following:
• Slows down
• Speeds up
• Changes direction
Which of the following shows an example
of acceleration? Why or why not?
• A car going from a dead stop to 60 mph.
• A car going around a turn while maintaining
a constant speed.
• A car slowing down as it rounds a turn.
• A car coming to a stop.
• A car going down a straight stretch of road
at 35 mph.
Which car or cars (red, green, and/or blue)
are undergoing an acceleration? Study each
car individually to determine the answer.
Forces and Acceleration
• In order for an object to accelerate, a force
has to act on it (either a push or a pull)
• Usually, more than one force acts
• Acceleration depends on the net force
Third Law of
Newton’s Third Law of Motion
Whenever one object exerts a
force on a second object, the
second object exerts an equal and
opposite force on the first.
Or more simply…
For every action, there is an
equal and opposite reaction.
• Forces are not simply single actions (a push
or a pull)
• They are really interactions between two
– Ex. A car crashing into a tree:
• The car exerts force on the tree
• The tree exerts force on the car
Forces always occur
in pairs!!!
What is the force pair acting on you
as you sit in your chair at your desk?
What is the force pair acting as you
walk across the floor?
Action and Reaction Forces
• Neither can exist without the other
• They are equal in strength and opposite in
Action and Reaction Forces
• Action: Object A exerts a force on
object B.
• Reaction: Object B exerts a force
on object A.
Which exerts more force, the
Earth pulling the moon, or the
moon pulling the earth?
Give the action/reaction forces that
would occur when a football player
tackled another football player.
What is the action reaction
force pair when a
cannonball is shot from a
• The force the cannon exerts on the
cannonball is exactly the same as the force
the cannonball exerts back on the cannon.
• Why then, if the forces are the same, does
the cannon not accelerate back as fast as the
cannonball accelerates forward?