Transcript Inertia & Newton`s First Law

2.2: Inertia & Newton’s First Law
What happens when...
• You’re driving along and suddenly slam on the
brakes?
– Your car stops, your body...
• You make a sharp right turn.
– The car goes right, your body...
• You’re stopped at a red light and the light turns
green. You hit the gas.
– The car jerks forward, your body...
Object Observed
Initial State
Coin on a billiard
ball
(ball knocked away
horizontally)
at rest
Plate and cup on a
silk tablecloth
(tablecloth pulled
horizontally)
at rest
Prediction
Observation
Motion of
Object
at rest
at rest
Teddy bear on cart
(cart jerked
forward quickly)
at rest
Ball launched from
ballistics cart
(ball launched as
cart moves at
constant velocity)
in motion
in motion
Teddy bear on
moving cart (cart
hits the wall and
stops)
in motion
in motion
at rest
Why?
Galileo’s Thought Experiment
• The ball speeds up as it goes down the ramp
and slows down as it goes up the ramp.
• Imagine no friction at all between a ball and a
ramp.
• The ball stops on the other side when it
reaches its initial height.
• What if the ramp continued on in a straight
line?
• Without friction or any other force to stop it,
the ball would continue moving at a constant
velocity.
• Inertia causes the ball to resist changes to its
motion.
Inertia
• The property of matter
– Anything that has mass has inertia
• Causes an object to resist changes to its state of
motion
– If at rest, it will want to stay at rest
– If in motion, it will want to stay in motion
• Directly proportional to the object’s mass (the
more mass, the more inertia)
Make a prediction...
Vcar = 80 km/h
Will the car be able to make it up the ramp and be able to come
to a complete stop inside the trailer?
The Mythbusters find out...
http://www.youtube.com/watch?v=dYlstdCWzCY
Newton’s First Law of Motion
If the net force acting on an object is zero, the
object will maintain its state of rest or
constant velocity.
This means:
1. Objects at rest will remain at rest unless acted upon by a
net external force.
2. Objects in motion will remain in motion unless acted upon
by a net external force.
3. If the velocity of an object is constant (or zero), the net
external force acting on it must be zero.
4. If the velocity of the object is changing in magnitude,
direction, or both (acceleration), the change must be
caused by a net external force on the object.
Some applications of Newton’s 1st Law:
• Seatbelts
– Don’t allow your body to move forward
• Airbags
– Slow the deceleration of your body
• Concussions
– the brain collides with the inside of the skull
– Shaken Baby Syndrome
Homework
Examples on the Board
Pg. 126 #1-5
Pg. 129 # 2-10, 12,13