Transcript Work, Energy and Power - Effingham County Schools

Conservation of Energy
Watch video:
http://www.youtube.com/watch?v=BVxEEn3w688
Work = Force|| x Distance
• Distance is the magnitude of the displacement
• Only the component of force parallel to the displacement
“does” work
http://onlinephys.com/work3.html
http://www.physics4kids.com/files/motion_work.html
Power
• measured in watts (W)
• One watt of power is expended when
one joule of work is done in one second
fitsugar.com
• Walk a mile or run a mile
– For the same person
• how does power compare?
• how does work compare?
What is horsepower?
• 1 horsepower = 746 Watts
• A horse exerting 1
horsepower can raise 330
pounds of coal 100 feet in a
minute, or 33 pounds of coal
1,000 feet in one minute, or
1,000 pounds 33 feet in one
minute.
Potential (PE) VS Kinetic
Energy (KE)
• PE: stored chemical energy or
energy of position
– elastic, gravitational, and chemical
• KE: energy of motion
Gravitational Potential
Energy
• PE = Weight  height
• PE = m g h
• Question:
– How much potential energy does a 10kg
mass have relative to the ground if it is 5
meter above the ground?
Kinetic Energy
• The energy of motion.
• Kinetic Energy = ½ mass  velocity2
Ek = ½ mv²
Mechanical Energy (ME)
• Energy possessed by an object
due to its motion or its stored
energy of position.
• ME can either be potential
energy or kinetic energy.
• All forms of energy are
measured in joules (J).
• ME = KE + PE
http://bishopcook09.wikis.birmingham.k12.mi.us/Potential+and+Kinetic
+energy%3B+Chinese+Dragons
Object Falling from Rest
http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2
Conservation of Energy
• Energy cannot be created nor destroyed; it
may be transformed from one form into
another, but the total amount of energy
never changes.
• Why worry about conserving energy?
http://water.me.vccs.edu/courses/env211/lesson2_print.htm
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Energy Transformation of a
Pendulum
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html
Ball Bounce
• GPE: Gravitational Potential Energy
• KE: Kinetic Energy
• EPE: Elastic Potential Energy
http://www.gcsescience.com/pen30-energy-ball-bounce.htm
m= 10kg
In this case, the weight
does positive work
d=2m
Work = mgd = (100N)(2m)
m= 10kg
mg = 100N
Work = 200 Nm =200J
FA
m= 10kg
In this case, the weight
does negative work
mg = 100N
d=2m
Fa
Work = -mgd = -(100N)(2m)
Work = -200 Nm = -200J
m= 10kg
mg = 100N
Work/Energy Relationship
• If you want to move something,
you have to do work.
• The work done is equal (ideally) to
the change in kinetic energy.
– what is ideal? is this actual?
• W = DKE
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Kinetic and Potential Energy
Conversions
• Sketch the diagrams and fill in the blanks:
http://onlinephys.com/work3.html
Work-Energy Relationship
• W = ΔKE
• What happens to your stopping distance
when you…
• Double your speed?
• Triple your speed?
http://www.thegrid.co.uk/index_files/Mickel-crash-brands.jpg
Forces in Car Crashes
•
http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3
Seatbelt Physics
http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1
Bungee Jumping
•
•
•
•
•
•
You wish to bungee jump off of a platform.
Assuming that there is no air resistance and the
spring constant of the 40 m long bungee cord is 100
N/m. How high should your platform be?
Determine all the forces acting ON the body.
Draw a free body diagram.
Apply Newton's second law.
Think about elastic potential energy.
Solve.
http://yugawad.wordpress.com/page/2/
• Elasticity of an object
– tendency of the object to return to its equilibrium
(natural ) shape natural shape when Fnet =0
• Restoring Force
– the force required to return object to its
equilibrium shape is called the restoring force –
directed opposite to the deformation of the
object.
Relative
“bounciness” of
various types of
balls
http://www.exploratorium.edu/baseball/bouncing_balls.html
•
A 2kg mass is placed on a frictionless track at point A and releases from
rest.
(assume the gravitational potential energy of the system is zero at point E.
•
Calculate:
–
–
–
–
–
–
–
the total mechanical energy of the mass. Use g = 10m/s/s
hint: focus on point A
the PE at point B
Using prior calculations, calculate the KE at point B
Using prior calculations, calculate the speed of the mass at point B.
If you want to calculate the speed at point C, do you have to repeat the computations again?
Calculate the speed at E
Calculate the speed at F
http://onlinephys.com/work3.html
In the above diagram, a toy car having a mass of 400 kg starts from rest at point A and
travels 3.60 meters along a uniform track until coming to rest at point B. (cannot go higher)
A) How can you determine that the track is not frictionless? (there is friction between the car
and the road)
B) Suppose the PE is 0 at ground level. Calculate the PE at point A and at point B: PE A =
________________
and PEB = ____________________
C) Determine the energy loss between these two points . (Find PEA - PEB)
D) the energy lost has been transferred into thermal energy because of the work of the
force of friction.
E) If the car travels a distance of 3.60 meters along the track, calculate the magnitude of the
frictional force acting on the car. Hint: energy lost = work done by friction
http://onlinephys.com/work3.html