energy, work & power? what do you think of when you hear the words:

Energy, Work & Power?

What do you think of when

you hear the words:

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

• Conversions from one form of energy to another continually occur.

• Energy cannot be created or destroyed.

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

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

x

F

xxdFW

xF

yF

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 expended 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.

• 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

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

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

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

• 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

EEkk = 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?

Object Falling from Rest

http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2

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

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?

Calculate speed at positions B,C and D.

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.

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?

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

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

• 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?

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.

Tennis Ball Bounce

• 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