Work W = the transfer of energy E

W =

Work done ___ a system ____________ its total energy.

Work done ___ a system ____________ its total energy

box

pilebox

d

Box nowhas ______energy

W done on system: W done by system:

d

F

Box nowhas ______ energy

Fd = ET

on increasesby decreases

moreless

F

F

d

Pile drivers:

Ex. A 200-N box is pushed 8.0 m along a floor by a horizontal 75-N force. How much work was done?

200N

F = 75 NW =

=

=

•1 newton·meter equals 1 ____________ :

1 Nm = 1 ____

•So the answer can be written: W = _______

• Both Nm and J are ______________.

•To express them in terms of fundamental units,

write: 1 J = 1 Nm = 1 (________)(____)

= 1 ______________

joule

J

derived

600 J

kgm/s2 m

kgm2/s2

Fd

(75 N)(8.0 m)

600 Nm

Plot the force and distance from the previous example.

F (N)

d (m)

What does the grey area represent?

area = L W = ( )( ) = =

area = LW = =

75

8.0

8.0 m75 N 600 Nm 600 J

Fd W the work done

Note:

1/ In the equation: J = Ft, J represents the quantity _________.

But in the answer: W = 600 J, J represents the unit ____________.

2/ If you substitute F = 1 N and d = 1 m, you get:

W = Fd = ( )( ) =

1 joule is the work done (or energy needed) in lifting 1_________________________to a height of 1 ___________.

3/ Joules are metric __________ units. Americans use ____________ for energy units. Foods rated in calories in the US are rated in joules in the _______________________ .

impulse

joules

1 N 1 m 1 J

stick of butter meter

energycalories

Rest of the World

1 m

w ≈ 1 N

F =1 N

w =1 N

= 1 joule

No work.

4/ Work W and energy E are __________. They do NOT have _______________, even though F and d do.

6/ The equation W = Fd is only valid when F and d are parallel:

Fd

F

d

or

5/ What is the value of work W = Fd when d = 0?

W = ____. This is b/c there is no if d = 0.

scalarsdirection

0 E

…then the "real" equation for W must be used:

W = where is the angle between F and d

F

d

Fx= Fcos

The equation W = Fd is ok to use here only if ____ is replaced by the ___________________ in the direction of d:

W = Fd = _________ , which is the same as the one above.

When F and d are NOT parallel…

Fdcos

component of FF

Fcosd

The equation W = __________ explains why the work W that you do on a box is _________ when you carry it horizontally at _____________________ .

F you exert to hold

wd

Another way to see that W must equal 0:

Given W = ET, doing work should change _________ ..

But the energy E of box ________________________ as it

is carried horizontally, so ET = ___ and so W = ____ .

W = Fdcos

=

=

=

Fdcoszero

constant velocity

Fdcos900

Fd(0)

0

energy

does not change

0 0

Ex. Which does more work, lifting a 30-N box a verticaldistance of 2-meters at a constant speed of…

…. 0.5 m/s or …1.0 m/s?

30N

In both cases, constant v a = ____ Fnet = ____.

w = 30 N

F exerted against gravity = _____

The W done against gravity:

W = F d = ( ) ( ) =

The distance 2 m will be covered in less _____ in case A, but W is ______________ for both cases.

BOTHCASES:

0 0

30 N

30 N 2 m 60 Nm

time

the same

What’s he doing?

Working Time

= power

the rate at which ______________________the rate at which _________ is transferred

P = W t

Power is a _____________—it has ___________________

If time t is constant:

P

W

directP

t

inverse

If work W is constant:

Power P:work is done

energy

scalar no direction.

What did the motor say to the generator when heasked her out for a date?

Don’t give me that old “power” line again.

Ex: When Harry Potter lifts Draco Malfoy on his broomstick, 1400 J of work is done in 2.5 seconds. How much power did his broomstick develop?

Given:

W =

t =

Unknown:

P = ?

Equation:

P = = =

•1 joule/second equals 1 ___________

1 J/s =

•So the answer can be written: P =

• Both J/s and W are _________________

•To express them in terms of fundamental units,

write: 1 W = = =

=

1 (kgm/s2)(m)/s

1400 J

2.5 s

W/t

1400 J/2.5 s

560 J/swatt

1 W

560 W

derived

1 J/s 1 Nm/s

1 kg m2/s3

Oh, Quit it (ch)!

Because work W = _____ and avg. speed vavg = ______ ,

the equation for power P can be written:

P = W = = t

Ex. How much time will it take a 1500-W (about 2horsepower) motor to lift a 2200 N box a distance of 40. meters?

P = Fd t

Given:

P = 1500 W

F = 2200 N

d = 40. m

Unknown: Equation:

t = ?

1500 = (2200)(40) t

t =

Fd d/t

Fd t

F vavg

59 s

tide power:

Notes:

1/ In the equation: P = W/t, W represents the quantity __________

But in the answer: P = 560 W, W represents the unit ____________

And in: w = 30 N w represents thequantity ___________

2/ If you substitute F = 1 N, d = 1 m and t = 1 s, you get:

P = Fd/t = ( )( )/ = =

1 watt is the power developed in raising 1 _________

__________ to a height of 1 ________ in 1 ___________.3/ To remember that 1 watt = 1 joule per second:

Watt? Don't worry, _______ get it in a _________!joule second

work

watts

weight

1 N 1 m 1 s 1 J/s 1 W

stick

of butter meter second

All that energy going up in smoke…

And don’tforget.....

Kimberly!

Open up your Energy Packet to pages 77-8.

Do #1-25 odd on a separate sheet of lined paper.