Work, Power & Energy?

What do you think of when

you hear the words:

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

Work or No Work

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

Kinetic Energy• The energy of motion.

• Kinetic Energy = ½ mass velocity2

KEKE = ½ mv²= ½ mv²

mgh

mg

h

How much work does gravity do on the falling mass?

Work = mgh

PE = mgh

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.

Determine the Unknown Information:

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

Joule to Joule

http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/cs.html

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

• Imagine 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?

• What information do you need to know? – G– U– E– S– S

http://school.discoveryeducation.com/clipart/images/in-desk.gif