Transcript Newton`s First Law of Motion

Newton’s First Law of Motion
INERTIA
First Law of Motion

also called the “law of inertia”
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“If the net force acting on an object is
zero, the object will maintain its state of
rest or constant velocity.”

in other words, unless the forces change,
the object will keep doing what its doing!
Inertia
a property of matter that causes an
object to resist changes in its state of
motion
 it is proportional to the mass of the
object
 ie: the greater the mass, the greater the
inertia an object possesses, the harder it
is to change the object’s motion

interaction between a frictionless
ball and frictionless ramp
**Force is not needed to keep an
object in motion!**
example: standing on
the bus
as the bus starts to move, your body wants to
stay at rest because of inertia, so you might
fall towards the back of the bus
 once the bus reaches a constant velocity, you
have no trouble standing because you are also
moving with a constant velocity
 if the bus slows or stops unexpectedly you
will likely fall towards the front of the bus
because your body wants to keep moving
forward

Net Forces
the net force is the sum of all the forces
acting on an object
 as long as the forces acting on an object
are balanced (offset), then the motion of
the object will
not change

Applications of the 1st Law

Seat Belts
◦ when a car suddenly stops, you continue to
move forward until an object exerts a net
force on you (the dashboard!!)
◦ a seat belt is designed to exert a force on you
to slow you down
◦ in order to allow you to move about freely
with your seat belt on, the belt is designed to
activate upon sudden decreases in velocity
only
the belt strap is attached to a spool which is
attached to a gear
 beneath the gear is a pendulum that swings
freely
 when the car suddenly stops, the pendulum
swings forward due to inertia and locks the
gear

other applications
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Blood rushes from your head to your feet while
quickly stopping when riding on a descending elevator.
The head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a hard
surface.
When drinking hot coffee in the car, and sudden acceleration
will cause you to spill coffee into your lap.
To dislodge ketchup from the bottom of a ketchup bottle, it
is often turned upside down and thrust downward at high
speeds and then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries
during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly
forward off the board when hitting a curb or rock or other
object that abruptly halts the motion of the skateboard.
Summary
1.
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4.
objects at rest remain at rest
unless acted upon by a net force
objects in motion remain in motion unless
acted upon by a net force
if the velocity of an object is constant (or
zero), the net external force acting on it
must be zero
if the velocity of an object is changing
either in magnitude, direction or both, the
change must be caused by a net external
force on the object