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Elevator problem?

  • Thread starter tnhoots
  • Start date
  • #1
36
0

Homework Statement



A skier of mass 69.7 kg is pulled up a slope by a motor-driven cable.
(a) How much work is required to pull him a distance of 59.8 m up a 29.9° slope (assumed frictionless) at a constant speed of 1.90 m/s?
(b) A motor of what power is required to perform this task?
hp

Homework Equations



(a) w=f*change in distance*cos angle
(b) power=work/change in time

The Attempt at a Solution



gravity is the only force in play. so the equation for work would be
w=(9.8m/s^2)(59.8m)(cos29.9)=508.03

take that value and divide it by change in time to get power

*correct?
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,258
618
"cos angle" refers to what angle? With respect to your given angle cos is the wrong trig function. You are also missing a mass factor which would have been obvious if you had checked the units on your answer.
 
  • #3
No. w=f.s=m.a.s
You havent taken the mass of the skier into account. The acceleration will be along the incline (which youve taken incorrectly).
 
  • #4
Power can be calculated by p=fv
 
  • #5
36
0
Ok so the F in the problem is 9.8m/s^2. I would use cos0. However, the change in distance is unknown. Using the formula d=time * speed, I found it to be 8.75m. So, now would I use the work formula with the numbers (9.8m/s^2)(8.75m)(cos0). Which equals 85.57J. After I find that I would divide work by time (5secs) to find average power which is 17.15 W...just my thinking??
 
  • #6
F is not 9.8 m/s^2. That is a. You are forgetting to take into account mass of the skier. force = mass * acceleration
 
  • #7
36
0
Ok-so hopefully this is it:
W=(mass of skier*acceleration)(change in distance)(cos29.9)
W=(132.43)(59.8)(cos29.9)=6865.23.
Then take that value and divide by the time which is 31.55 seconds.
Which equals 217.6 hp
 
  • #8
340
0
The SI unit of power is watts. There are about 746 watts per h.p. but you better look up that number, i am pulling it out of my head.
 
  • #9
36
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Is the way I worked the problem out correct? I just need to convert the 217.6 watts to hp? I have had trouble solving this problem. It would be nice to know if this thinking is correct.
 
  • #10
340
0
The reasoning seems okay to me.
 
  • #11
36
0
UGH! I'm still stuck. Any help would be greatly appreciated!!
 
  • #12
340
0
Where are you stuck?
 
  • #13
36
0
alright...i think i've got it. I have to run to class, but I will post later witht the answer I finally came up with! YAY!
 
  • #14
36
0
actually, nevermind that answer was also incorrect. sorry to get our hopes up!:frown: Still working...
 
  • #15
340
0
whoops, you shouldnt be looking at the cosine of theta. Sorry, just caught that.
 

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