Work done by the tension on a cable

  • Thread starter discosucks
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  • #1
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Homework Statement



A crane lifts a 1250 kg load directly upward with a constant speed through a vertical
distance of 8 m. How much work is done by the tension in the crane cable?

Homework Equations



Im thinking its the work equation but this is were i got stuck

W = m(as)


The Attempt at a Solution



got confused right away on this one my thinking is

m = 1250
s = 8
a = is this zero seeing as its constant?
 

Answers and Replies

  • #2
Doc Al
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Im thinking its the work equation but this is were i got stuck
W = m(as)
Where did you get that formula?

What's the most basic definition of work?
 
  • #3
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work is when a force moves its point of application through a distance .

isnt that the same as W = Fs?
 
  • #4
Doc Al
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work is when a force moves its point of application through a distance .

isnt that the same as W = Fs?
That's the equation that you need. What's "F" in this problem?
 
  • #5
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That's the equation that you need. What's "F" in this problem?
force? and that would be the mass x the acceleration

but what do i say the acceleration is seeing as im told its constant?
 
  • #6
Doc Al
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force?
What force? (What force is doing the work in this problem?)

and that would be the mass x the acceleration
No. Mass x acceleration gives you the net force on something.

but what do i say the acceleration is seeing as im told its constant?
The acceleration would be zero.
 
  • #7
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Im pretty confused now lol but I'm going to say its gravitational force bearing down ?
 
  • #8
Draw a FBD. Find the FT and then plug and chug into W = F x d
 
  • #9
Stephen Tashi
Science Advisor
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Im pretty confused now lol but I'm going to say its gravitational force bearing down ?
In order to be simple, physics exercises are often unrealistic. To lift a real object, a real crane probabily doesn't raise it at a constant velocity. In a real situation there is a fluctuating imbalance of forces, so there are small accelerations (and decelerations) of the load as it rises.

In this problem, you are told the obejct is lifted with a constant velocity. Theoretically, to lift a object with a constant velocity, you could exert a force on it that exactly cancels gravity. (Tie a big helium ballon to it.) The object would be "levitated". Then you give the object a nudge and it would move upward at a constant velocity. There would be no net force on the object. One can still ask what work is done just by force that is canceling gravity. That work is non-zero.

You have to pretend the crane is levitating the object and you have to ignore whatever work was done to start the object on its way up with a constant velocity.
 
  • #10
Doc Al
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Im pretty confused now lol but I'm going to say its gravitational force bearing down ?
You're asked to find the work done by the tension, so you need the tension. (Since the acceleration is zero, that tension will equal the weight of the object.)
 
  • #11
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So its just the weight by the distance seing as the acceleration is zero?

1250 x 8 = 10000 J ?
 
  • #12
Doc Al
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So its just the weight by the distance seing as the acceleration is zero?
Yes.

1250 x 8 = 10000 J ?
1250 is not the weight.
 
  • #13
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Sorry i think i get it now

the weight is the mass times gravity

1250 x 9.81 =12262.5

12262.5 x 8 = 98100 J .
 
  • #14
Doc Al
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Good!
 

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