Transcript Energy - Vincent Sapone

Torque, Momentum, Impulse,,
Energy
2-9 is Torque
10-16 is momentum
Weight vs. Mass
• Weight is mass times
gravity
• W= mg or Fw=mg
• Mass in kilograms
• Gravity is 9.8m/s2 on
earth
• Mass how much stuff or
matter you are made up
of. Mass is also measure
of your inertia.
YOUR WEIGHT?
ON VARIOUS PLANETS
http://www.exploratorium.edu/ronh/weight/
Assignment: Two forks.
• Center of Mass: The
point in an object
where all of its mass
balances around.
• When a force is exerted
on an object outside of
its center of mass, it will
rotate.
Hammer: Center of Mass
Two bodies with the same
mass orbiting a common
barycenter similar to some
asteroids (Antipoe 90)
Two bodies with a difference
in mass orbiting a common
barycenter similar to plutocharon system
Two bodies with a major
difference in mass orbiting a
common barycenter internal
to one body (similar to the
Earth–Moon system)
Two bodies with an extreme
difference in mass orbiting a
common barycenter internal
to one body (similar to the
Sun–Earth system)
Two bodies with the same mass orbiting
a common barycenter, external to both
bodies, with eccentric elliptic orbits (a
common situation for binary stars)
Torque
• Torque is a measure of
how much a force
acting on an object
causes that object to
rotate about an axis.
• Formula
• “τ = Fr”
• τ=torque (N-m)
• F=Force (N)
• r = distance from axis (m)
Opening a Door
• Its harder to open a
door closer to its
hinges.
• Its easier to open a
door far away from its
hinges because r is
greater and there is
more Torque.
EXAMPLES OF TORQUE
SEASAWS
CROWBARS --Fulcrum
PHET PHYSICS TORQUE
SEASAW PHYSICS
• To balance a seesaw, the
torque on one side must
equal the torque on the
other side.
• Problem: A 150lb adult sits
4 feet from the center of a
seesaw. Where must a
100lb person sit to balance
equally?
TORQUE
Torque Lab
• Students will be given a
ring stand and clamps,
rubber bands and several
masses. They set up and
sketch the problems on
the next page.
• Use dowels or wood
rulers to create a multilayered balanced torque
system.
 Place a a 200g mass 10cm
from the axis of rotation.
Where would you put a
100g mass on the other
side to balance it?
 Place a 200g mass 20cm
from the axis of rotation
and a 100 mass 10cm
from the axis of rotation.
Where would you have to
put another 100g mass to
balance it?
• Place the mystery mass
object 30cm from the axis
of rotation.
Torque Problem
Jenga Block Game
1.
2.
3.
4.
5.
No glue! Blocks have to be
placed and supported
entirely by their own
weights.
Only one block per level.
We're making a skewed
tower.
All blocks are of the same
shape, weight, and are of
uniform density.
Must stand for 30 seconds.
2nd attempt: you may use 10
pennies and two rubber
bands.
MOMENTUM
Momentum and Collisions
• Momentum can be
defined as "mass in
motion." If any object
with mass is moving,
then it has momentum
• p=mv
• p=momentum (kg-m/s)
• m=mass (kg)
• V= velocity (m/s)
Momentum & Collisions!
Newton’s Cradle
Billiards
Conservation of Momentum
• The total
momentum of two
or more objects
before a collision
is the same as the
momentum of the
objects after a
collision.
1) Calculate the momentum
for the following objects:
a) A 75kg speed skater
moving forward at 16m/s
b) A 135 ostrich running
north at 16.2m/s
c) A 5.0kg baby on a train
moving eastward at
72m/s
d) A 48.5kg passenger
seated on a train that is
stopped.
2) Calculate the velocity of a
0.8kg kitten with a forward
momentum of 5kg-m/s.
3) Calculate the momentum of
a 6.0kg bowling ball moving
at 10m/s down the alley
towards the pins.
1. A one kilogram ball traveling at 1m/s hits a
another stationary 1kg ball. The ball moving
initially stops. What happens to the ball that was
originally stationary?
2. A two kilogram ball moving at 2m/s hits a
stationary 1.5 kg ball. After the collision the two
kilogram ball is moving at 1m/s. How fast is the
1.5kg ball moving?
3. A toy car with a mass of 0.5kg is moving with a
velocity of 3m/s and it smashes into and sticks
to another car with a mass of 1kg. What is the
velocity of the cars after the collision?
Momentum
A 500kg canon at
rest fires a 10kg
ball at 100 meters
per second. What
happens to the
canon?
Work
• Energy is the ability to do work.
• Work is the amount of force used to move an
object a given distance.
• Formula for Work is W = Fd
– F = force in Newtons
– d = distance in meters.
• The unit for energy and work is the joule.
Working Hard or Hardly Working
• Energy is the ability to
do work.
• Work is the amount of
force used to move an
object a given distance.
• Formula: W = Fd
– F = force in Newtons
– d = distance in meters.
• The unit for work is the
joule.
Lifting Objects:
Work done against gravity
Mass (g)
Work (joules)
W = mgh
Height object raised (m)
Gravity (m/sec2)
• Mr. Sapone who is
super awesome and
super strong deadlifts a
220kg barbell a total
distance of 0.7meters
before dropping it. How
much work did he do?
 If you carry a
800N could 10
meters across
a room, how
much work
did you do?
• If you carry an object at
a constant velocity you
do no work on the
object because there is
no net force in that
direction.
• You might be tired but
you didn’t do any work.
Two cases: No work done.
 A crane uses an average
force of 5,200N to lift a
girder 25m. How much
work does the crane do
on the girder?
 An apple weighing 1N
falls distance of 1m.
How much work is done
on the apple by the
force of gravity?
 A bicycle’s breaks apply
125N of frictional force
to the wheels as the bike
moves 14.0m. How
much work do the
breaks do?
 A mechanic uses a
hydraulic lift to raise a
1,200kg car 0.50m off
the ground. How much
work does the lift do on
the car?
Work against gravity: W=mgh
1
2
A crane lifts a steel
beam with a mass of
1,500 kg. Calculate how
much work is done
against gravity if the
beam is lifted 50 meters
in the air.
How much time does it
take to lift the beam if
the motor of the crane
can do 10,000 joules of
work per second?
Power
• Power is the rate at
which work is done.
• Power is work divided
by time.
• P = W/t (J/s)
• 1 J/s = Watt
• P = W/t
• W = Fd
• P = Fd/t
• 746watts is one
horsepower
Power Problems
Quick Lab: Calculate your Power
1.
2.
3.
4.
5.
Convert your body weight into
Newtons by dividing by ~4.5_____
Measure the time it tales you to
run up the steps in seconds ____
Measure the height in cm of one
step____ and multiply that by the
total number of steps____ to get
the total height ________. Convert
that into meters by dividing by
100__________
Calculate your power by multiplying
your body weight in Newtons by
the height of the steps in
meters______
Divide your Power by 746 to
convert it into horsepowerc____
Kinetic Energy
• The energy of motion.
• KE = 1/2 mv2
– m = mass of the object
– v = object’s velocity
KE Sample Problems
• A tomahawk missile has a
velocity of 250m/s and a
mass of 1300kg, calculate
its Kinetic energy.
• A 9mm bullet with a mass
of 0.08kg has a velocity of
400m/s, what is its Kinetic
energy?
• Which one has greater
kinetic energy? A 1000kg
car moving 90Mi/h (40m/s)
or a 36,000kg 18 wheeler
going 30Mi/h (13.4m/s)
Potential Energy: Stored energy.
Elastic Potential Energy
• U = ½ kx2 (Spring or elastic
potential energy)
– k = spring constant
– x = distance stretched in
meters
A stretched spring stores energy
Potential Energy: Stored energy.
Gravitational Potential Energy
• PE = mgh
– m = mass of object
– g= gravitational acceleration
– height from surface in meters
Potential Energy Problems
• A 60kg woman is in a
helicopter at a height of
2500 meters. What is her
potential energy?
• A 0.4kg apple is hanging
from a tree 3.4 meters
from the ground. What is
its potential energy?
• What height must I raise a
20kg object to in order
for it to have 1000 joules
of potential energy?
Conservation of Energy
• Energy is neither
created or destroyed.
• If you cut the object to
the left all of its
potential energy will
turn into kinetic energy
before it hits the floor.
Conservation of Energy
• PE (top) = KE (bottom)
1 2
mgh  mv
2
http://phet.colorado.edu/en/simulation/energy-skate-park
PE-KE Problems
• If I throw a 1kg object
out of a window 15
meters off the ground.
What will its velocity be
just before impact?
• In the figure on the
right, label from
greatest to least,
potential energy and
kinetic energy.
PE-KE Lab
• Student Led Lab