Upload
porter-navarro
View
40
Download
2
Embed Size (px)
DESCRIPTION
What do you think of when you hear the words:. Work, Power & Energy?. Work = Force || x Distance. W = Fd The distance is the displacement. Only the force parallel to the displacement does work. Work. Force MUST be applied in the direction of motion - PowerPoint PPT Presentation
Citation preview
Work = Force|| x DistanceW = Fd
The distance is the displacement.
Only the force parallel to the displacement does work
Work• Force MUST be applied in the
direction of motion
• 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
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
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
• measured in watts (W)
• One watt of power is used when one joule of work is done in one second.
Power
http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg
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.
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.
EnergyEnergy • Universe is made up of matter and energy.
• Energy is the mover of matter.
• Energy has several forms, some examples:
– Mechanical• Kinetic• Potential
– Electrical
– Heat– Chemical
http://tiki.oneworld.net/energy/energy.html
• Energy contained 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).
• ME = KE + PE
Mechanical Energy
Conservation of Energy
• Energy cannot be created nor destroyed
• Energy 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
Conservation of Mechanical Energy
• KE (before) + PE (before) = KE (after) + PE (after)
• Why does a dropped ball not return to its original elevation?
http://www.science-projects.com/Drop/DropBalls.htm
Energy Transformation of a Pendulum
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html
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?
Gravitational Potential Energy
• PE = Weight height = Fwh = mgh
• 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?
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
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
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?
Check for Understanding
• Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s.
More Energy Transfer
• High speed winds are used to do work on the blades of a turbine at the so-called wind farm.
• Mechinacal Energy from the air gives the air particles the ability to apply a force to the blades.
• As the blades spin, their energy is converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.
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?
Mechanical Energy Conversion• Explain the following diagram:
a. What is happening
b. Explain the conversion of KE to PE to KE to PE…
c. http://www.funderstanding.com/coaster
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
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
Truck Collision
• In a head-on collision:
• Which truck will experience the greatest force?• Which truck will experience the greatest change in
velocity?• Which truck will experience the greatest acceleration?• Which truck would you rather be in during the collision?
WHY?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1