energy, work & power? what do you think of when you hear the words:
TRANSCRIPT
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Energy, Work & Power?
What do you think of when
you hear the words:
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EnergyEnergy • Universe is made up of matter and energy.
• Energy is the mover of matter.
• Energy has several forms:
– Kinetic
– Potential
– Electrical
– Heat– etc.
http://tiki.oneworld.net/energy/energy.html
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• Conversions from one form of energy to another continually occur.
• Energy cannot be created or destroyed.
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Work = Force|| x Distance
In this case, the distance is the magnitude of the displacement.
Only the component of force parallel to the displacement does work
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Work• Force applied in the direction of
motion• Work = Force|| Distance• W = F||d• The unit for work is the Newton-
meter which is also called a Joule.• 1 joule is equal to a force of 1 N exerted
over a distance of 1 m
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x
F
xxdFW
xF
yF
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Work or No Work
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m= 10kg
mg = 100N
In this case, the weight does positive work
d = 2 m
Work = mgd = (100N)(2m)
Work = 200 Nm =200J
m= 10kg
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m= 10kg
mg = 100N
In this case, the weight does negative work
d = 2 m
Work = -mgd = -(100N)(2m)
Work = -200 Nm = -200J
Fa
m= 10kg
mg = 100N
FA
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• measured in watts (W)
• One watt of power is expended when one joule of work is done in one second.
Power
http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg
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Power
• Power is equal to the amount of work done per unit time.
interval time
donework Power
• The unit for power is the Joule/second which is also called a Watt.
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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.
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• Energy possessed by an object due to its motion or its stored energy of position.
• It can either be potential energy or kinetic energy.
• All forms of energy are measured in joules (J).
• Em = Ek + Ep
Mechanical Energy
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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.
• So why worry about conserving energy?
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
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Conservation of Mechanical Energy
• Ek (before) + Ep (before) = Ek (after) + Ep (after)
– When no other type of energy is present.
• Why does a dropped ball not return to its original elevation?
http://www.science-projects.com/Drop/DropBalls.htm
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Energy Transformation of a Pendulum
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html
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Potential Energy• (PE) stored chemical energy or
energy of position.
– An object’s ability (potential) to do work by virtue of its position.
– Types: elastic, gravitational and chemical.
• Examples: – Rubber bands– Springs– Bows– Batteries– Gravity?
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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?
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Potential Energy
• The same work is done on each block. What mattersis the final elevation, not thepath followed
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
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Kinetic Energy• The energy of motion.
• Kinetic Energy = ½ mass velocity2
EEkk = ½ mv² = ½ mv²
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mgh
mg
h
How much work does gravity do on the falling mass?
Work = mgh
PE = mgh
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mgh
mg
h
How much energy does the mass have at the bottom of its fall, just before it hits the ground?
Kinetic energy
PE = mgh
v
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mg
h
v
mg
KE = ½ mv2
PE = mgh
If potential energy at top of path is 100 J, how much kinetic energy does the ball have just before it hits the ground and comes to a stop?
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Object Falling from Rest
http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2
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Practice:
• Think about standing on your desk…if you jumped off, how fast would you be moving just prior to reaching the floor?
• How much work would you do on the floor? (Refer back to previous slide if needed)
http://school.discoveryeducation.com/clipart/images/in-desk.gif
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More Practice
• Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s.
• If the roller coaster car doubled its
speed, what would be its new kinetic energy?
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Calculate speed at positions B,C and D.
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More Energy Transfer
• High speed winds are used to do work on the blades of a turbine at the so-called wind farm.
• Mech. Energy from the air gives the air particles the ability to apply a force to the blades.
• As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.
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Energy Transfer...
• This diagram shows that the potential energy of the boy is changing as he swings. Explain what is happening, in other words how is the energy changing?
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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 = KE
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
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Work-Energy Relationship
• 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
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Joule to Joule
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/cs.html
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Forces in Car Crashes
• http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3
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Seatbelt Physics
http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1
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• The formula for KE is _____?• Kinetic Energy is dependent upon ___ & ____• If an object’s velocity doubles, its kinetic energy
_____. If velocity triples, it will have _____ times the kinetic energy.
• If mass doubles, kinetic energy _____. If mass triples, kinetic energy _____.
• …so what happens to KE if mass doubles and velocity doubles? What if mass is cut in half and velocity doubles?
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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.
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Tennis Ball Bounce
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• 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 – it is always directed opposite to the deformation of the object.
Relative “bounciness” of various types of
balls