i nertia and n ewton ’ s f irst l aw of m otion. demo time history of physics:
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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
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