Work and Energy ENERGY The ability to do work WORK A change in energy

Work and

Energy

ENERGYThe ability to

do work

WORKA change in energy

Work

Energy

Circular Definition!

Radiant ElectricalChemical

Thermal Nuclear

Magnetic

Sound

Mechanical

At home – check out http://www.youtube.com/watch?

v=3ff_AXVlo9U for the Singing Tesla Coil!

Potential energy + Kinetic energy

=

Mechanical energy

Potential energy + Kinetic energy = Mechanical energy

Example of energy changes in a swing or pendulum.

2 Kinds of Mechanical Energy

• Kinetic energy (KE)

• Potential Energy (PE)

–Gravitational PE (PEg)

–Elastic PE (PEelastic)

Kinetic Energy

Energy due to motion

KE = ½ mv2

Potential EnergyEnergy due to

position

Bowling Ball – Conservation of Energy

Conserving Energy

Gravitational PE

PEg = mgh

A 25 kg boy runs with a speed of 6.5 m/s. How much kinetic energy does

the boy have?

530 Joules

How much potential energy does a 0.50 kg apple have if it hangs from a

tree 13m above the ground?

64 Joules

A 220 gram fly has a kinetic energy of 55 Joules. How fast is the fly

flying?

22 m/s

ENERGY IS

CONSERVED!!

WORKW = Fs

(Work = Force x displacement)

Units:Joule (J) = N· m

Work is done only when components of a force are parallel to a displacement.

Force helps motion

Positive Work

Force opposes motion

Negative Work

Force 90° to motion

No work

Object not in motion

No work

A student drags his book bag 15 meters across the

classroom by pulling on it with a force of 5.0 Newtons.

How much work does the student do on the bag?

75 Joules

Larry carries his 35N physics book across the room 25m away. How

much work does he do on the book?

0 Joules

Work-Kinetic Energy Theorem

Wnet = KE

POWERThe rate at which

work is done

P= Wt

Or…

P=t

Or…

P=Pt

Or…

P = Fv= Force * velocity

The SI unit of poweris the Watt,

named in honor ofJames WattJames Watt.

One Watt, W, of powerOne Watt, W, of poweris the power achievedis the power achieved

when when 1.0 J of work1.0 J of work is done or is done or1.0 J of energy is transferred1.0 J of energy is transferred

in a in a time of 1.0 stime of 1.0 s..

Watt = 1 J/sHorsepower (hp) = 746 W

Unit of Power

How much power is generated by a motor that can lift a 5300

kg crate of bricks with a velocity of 15 m/s?

780,000 W

How much power is generated by a car engine that does 5500 Joules of

work in 35 seconds?

160 W

How long will it take a 36 Watt motor to pull a

box across the floor 25 m with a force of 16 N?

11 s

Homework:Chapter 8, Hewitt:

Read 8.1-8.6Do RQ 1-13; P&C 1-3,5-7;

T&E 3-8;T&S 1-4