Work & PowerWork & Power

Work = Newton · Meters

Work = Joules

WorkForce distance

Caution!

The force must be in the same direction as the distance moved or no work is done on the object.

Force Force

(weight)(weight)

DistanceDistance

Normal Normal ForceForce

Since the force and distance moved are not in the same direction, no work is done on the tray!

Effort vs Work

Work

Have you done work on anything recently?

I pushed a grocery I pushed a grocery cartcart

How do you know How do you know you did work?you did work?

ForceForce

I pushed I pushed horizontallyhorizontally

DistancDistancee

The cart The cart moved moved horizontallyhorizontally

How do you know How do you know you did work?you did work?

ForceForce DistancDistancee

Is work being done on the burrito?

Yes! Yes!

W = FdW = Fd

W = (7 N)(1 m)W = (7 N)(1 m)

W = 7 JW = 7 J

7 N 1 m

Is work being done on the egg?

DistanceDistance

Normal Force

No work is done on the egg

(On the wall)

on the wall.on the wall.

Power = Watts

Power is the rate at which work is done.

Power = Joules / seconds

Power Worktime

How Powerful is she?

When doing a chin-up, Courtney lifts her

425 N body a distance of 0.25 meters in 2 seconds. What is the power delivered by her biceps?

What is the work done by Hajji who lifts a 30 kg bag straight up 1.5 meters? If it takes 0.5 seconds, what was his power rating?

W = ?m = 30 kgd = 1.5 mt = 0.5 sP = ?

W = FdW = FdFFww = mg = mg

FFww = (30kg)(9.8 m/s = (30kg)(9.8 m/s22))

FFww = 294 N = 294 N

W = (294 N)(1.5 m)W = (294 N)(1.5 m)

W = 441 Nm or JW = 441 Nm or J

P = W/tP = W/t

P = 441 J / 0.5 sP = 441 J / 0.5 s

P = 882 J/s or 882 WattsP = 882 J/s or 882 Watts

How much time did it take for Marx to lift a 10 kg object 4 meters if his power rating was 160 Watts?

POWER LAB

WorkWork

The work done is:

W = F d

= force x distance= weight x height

= (mg)(h)

Weight

Normal Force

Where to?

1st floor

2nd floor

3rd floor

4th floor

Group 1

Group 2

Group 3

Group 4

Group 5 Group 6

Group 7

Where to?

• Group 1: Stairs from 1st floor to middle 1st and 2nd • Group 2: Stairs from middle of 1st and 2nd to 2nd floor• Group 3: Stairs from 2nd floor to middle of 2nd & 3rd • Group 4: Stairs from middle of 2nd & 3rd to 3rd floor• Group 5: Stairs from 3rd floor to middle of 3rd and 4th • Group 6: Same as group 5• Group 7: Stairs from middle of 3rd and 4th to 4th floor

Work, Power, Energy Hockey

• http://www.nbclearn.com/portal/site/learn/science-of-nhl-hockey

Work = Force x Distance

Work = 30 N x 4 m

Work = 120 Nm or 120 J

So the horse has done 120 J of work ON the bucket

30 N

Bucket goes up a distance of 4 m

Bucket’s weight

30 N

Simple machines can be used to change the direction of force

Images Cited

• http://a.abcnews.com/images/WN/pd_scream_071012_ms.jpg

• http://xpigs.files.wordpress.com/2007/12/latest-design-waiter-waitress-trays.gif

• http://www.celticstrongman.co.uk/weightlifter_barbell_squats_hg_wht.gif