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 Prediction Observation Motion of Object

Coin on a billiard ball(ball knocked away horizontally)

at rest at rest

Plate and cup on a silk tablecloth(tablecloth pulled horizontally)

at rest at rest

Teddy bear on cart (cart jerked forward quickly)

at rest 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

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 MotionIf 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 #1-5Pg. 129 # 2-10, 12,13