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