Transcript 8.3 Newton`s Laws of Motion

8.3 Newton’s Laws of
Motion
First Law of Motion
An object at rest
remains at rest and
an object in motion
keeps moving –
unless acted on by a
force
So the First Law is all about
INERTIA
Resistance to
motion
the more mass in an
object, the harder
it is to get it moving,
OR to stop it when it’s
already moving.
Inertia = tendency of an object
to resist changes in its velocity.
Inertia = tendency of an object
to resist accelerations.
You and the car are traveling at same speed. When the car
stops (is stopped by the wall), you keep going at the same
speed – until YOU are acted on by the steering wheel. This
is why you should wear a seat belt!
Newton’s Second Law
F = ma
Forces cause
acceleration
The unit for force is NEWTON
Newton gave credit to those who
came before him...
Free Fall and Weight
As crazy as he may
look, our friend does
not wish to continue
gravitational
acceleration until he
hits the ground. He
will soon make an
effort to slow his
descent by pulling the
ripcord on his
parachute. We hope.
Free Fall
All falling objects
(disregarding the effects
of air resistance) fall at
the same rate.
All falling objects
accelerate at the same
rate. (9.8 m/s2)
Why?
INERTIA!
Sure, gravity is pulling downward.
But the object is resisting.
The more mass, the greater the pull
downward.
BUT, the more mass, the greater the
resistance.
So everything ends up coming down at the
same rate.
WEIGHT
Measure of the force
of gravity.
Weight = mass X
free-fall acceleration
weight = mass X 9.8
m/s2
w = mg
unit is NEWTON
Terminal Velocity
Remember, air has mass.
So when you are falling, there are real
molecules underneath you, that you can’t see.
They help to hold you up. Unfortunately, gravity
wins. But there is a limit to how much you can
speed up (accelerate) –because of the air
underneath you as you fall. When the pushing
up by the air, and the pulling down by gravity are
equal, acceleration (speeding up) stops, and you
keep falling, but you don’t fall any faster.
Sky divers
reach terminal
velocity at
about 320 km
/h (200 mi/h)
Newton’s Third Law
For every
action, there
is an equal
and opposite
reaction.
Action – Reaction Pairs – foot
presses on ice, you move in
opposite direction.
Swimmer pushes against the wall,
and the wall pushes back, so she
moves away from the wall!