INERTIA AND NEWTON’S FIRST LAW OF MOTION

DEMO TIME

History of Physics: http://vimeo.com/69381331

ARISTOTLE

Lived in Greece Born  384 BC Died: 322 BC

Believed that unequal massesReach the ground at different time

All movement required forces

Proper state was of rest – all objects will eventually come to rest.

Stood for 2000 years

GALILEO

Lived in Italy Born: 1564 Died 1642

Believed unequal masses fell at the same time

GALILEO’S THOUGHT EXPERIMENT

Galileo observed objects return to original position regardless of incline

Therefore, object’s “natural” state is not of rest They resist changes in motion - inertia

Observed that inclines change velocity The smoothness of surface directly effected motion With no friction (surface or air), how does speed

change on a level surface?

INERTIA

A property of matter that causes an object to

resist changes in its state of motion; it is

directly proportional to the mass of the

object.

Inertia = laziness = an object’s

resistance to change

NEWTON’S FIRST LAW/ LAW OF INERTIA:

Objects at rest or moving with a

constant velocity maintain their state

of rest or constant velocity unless

acted upon by an external, unbalanced

force.

INERTIA AND CAR CRASHES

This law of inertia helps us to

understand the principles behind using

seatbelts and air bags. Once an object

is moving, it tends to keep moving at a

constant velocity because of its inertia.

Air bags and seatbelts help to slow us

down safely.

EXAMPLES OF INERTIA

pulling tablecloth out from under

dishes

card and glass

Coin into cup

NEWTON’S SECOND

SHOW ME

State Newton’s first law of motion

NEWTON’S SECOND LAW

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Fnet = ma

Force: Any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction.

DERIVING THE NEWTON

The unit of Force is the N (Newton) N = kgm/s

FNET = VECTOR SUM OF ALL FORCES

EXAMPLE: A mass of 6 kg is accelerated to 3.2 m/s2.

What force was applied?

A 14 N [left] force is applied to a 5 kg mass. What is the acceleration?

A force of 250 N [right] is applied to an object that causes it to accelerate to 4.5 m/s2. What is the mass of the object?

PRACTICE

In Motion: Newton’s Laws Practice Booklet

Pages 3-4

SHOW ME

What is Newton’s Second Law?

NEWTON’S THIRD LAW

NEWTON’S THIRD LAW

For every action there is an equal and opposite reaction.

http://www.youtube.com/watch?v=JGO_zDWmkvk

MASS VS. WEIGHT

Mass never changes Weight is the force of gravity acting on

a mass. Measured in Newtons (N)

Weight = (mass) x (acceleration due to gravity)

W = mg

Sally has a mass of 60 kg. What is her weight on Earth?

What is her mass on Mars?What is her weight on Mars?

What is her mass on Jupiter?What is her weight on Jupiter?

Joshua weighs 100 N on Earth. He wants to bulk up so he goes to the Venus. Is this a good idea?

1) What is his mass on Earth?

2) What is his mass on Venus?

3) What will his weight be on Venus?

RECAP NEWTON’S LAWS

http://www.youtube.com/watch?v=mn34mnnDnKU

MOMENTUM

Which scenario will cause the bigger

dent in your car?

A sports car travelling at 50 km/h

crashes into your car?

A tank travelling 50 km/h crashes

into your car?

The tank will cause more damage because it has more momentum

Which scenario will cause the most

damage to you?

A 5 g bullet travelling at 30 m/s hits

you in the leg? (fired from a gun)

Your friend tosses you a 5 g bullet

at a speed of 1 m/s.

Obviously the bullet fired from the

gun will cause you greater pain.

In this case the bullet from the

gun has a greater momentum.

Momentum depends on two factors,

mass and velocity

p = mv

p = momentum

m = mass

v = velocity

momentum is a vector (it has

magnitude and direction)

EXAMPLES

Example: Calculate the momentum of a 1000 kg car travelling at 6m/s to the right

CONSERVATION OF STUFF

COLLISION EXAMPLE A 1000 kg car travelled at 6 m/s to the right

collides with a 1500 truck travelling at 5 m/s to the left.

1) What is the momentum of the car?

2) What is the momentum of the truck

3) What is the total momentum of the system before the collision?

4) What is the total momentum of the system after the collision ?

THE ONLY RULE

Energy is always conserved The total amount of energy before an event

will be equal to the energy after an event This concept applied to momentum

MOMENTUM BEFORE A COLLISION = MOMENTUM AFTER A COLLISION

INELASTIC COLLISIONS

Inelastic = objects collide and stick together

Example: A 1000 kg car travelled at 7 m/s to the right collides with a 2000 kg semi travelling at 5 m/s to the left. If the collision in inelastic,

1) What is the total momentum of the system after the collision

2) What is the velocity of the system after the collision

ELASTIC COLLISIONS Elastic = the objects do not stick together

Example: Jenny (mass = 80kg) is skating with a velocity of 2m/s to the right. She runs into Justice (mass=70 kg) who was too busy texting to move (thus he was stationary)

1) What is the total momentum of the system after the collision?

2) What is the momentum of Jenny after the collision?

3) What is the momentum of Justice after the collision?

HOMEWORK In Motion: Newton’s Laws Practice Booklet: Pages 5-6 ***Add in*** What is Paul’s final velocity? 7) A car with mass 1500 kg, travelling at 10m/s

collides inelastically into a stationary smart car (m= 500 kg).

a) What is the initial momentum of both carsb) What is the initial momentum of the system?c) What is the final momentum of the system?d) What is the final velocity of the system after the

collision?

Paul (mass = 90kg) is running 5 m/s [Right] and collides with Laura (mass = 70 kg) running 5 m/s [Left]. After the collision Laura is moving with a velocity of 2 m/s [right]

IMPULSE

IMPULSE = I

Objects in motion stay in motion unless you apply a force against the motion for a period of time

The greater the momentum of an object, the harder it is to stop the object’s motion.

To stop an object with a large momentum, you will need to either apply a large force or apply a force for a long period of time

FANCY DERIVATION

Inpulse causes a change in momentum I = Δp = pfinal – pinitial

I = Fnet Δt

Example: In football, the defensive

player applies a force for a given

amount of time to stop the momentum

of the offensive player with the ball.

Impulse causes a change in

momentum.

EXAMPLES

A water skier lets goof the tow rope

and coasts to a stop. If the water

exerts an average force of 280 N on

her, and she stops in 5.0 seconds,

what is the impulse that the water

exerts on the skier?

A pool ball of mass 0.125kg rolls toward the cushion at 3 m/s and bounces back at 2 m/s

a) What is the initial momentum of the ball?

b) What is the final momentum of the ball?

c) What is the change in momentum of the ball

d) What is the impulse of the ball?

TRY IT OUT

In Motion: Newton’s Laws Practice Booklet

Pages 6-7