The load of an object on a pivot

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Discussion Overview

The discussion revolves around calculating the load on one end of a platform when attempting to lift it from a pivot. The scenario involves a 3.2m platform weighing 175kg, with one end on the ground and the other on a pivot 2.5m high. Participants explore the forces involved in lifting the platform and considerations for selecting a winch or hoist.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants propose that the force required to lift the platform depends on the angle at which the force is applied, suggesting that pulling straight up results in half the weight being supported by the winch.
  • Others argue that if the force is applied at right angles to the platform, the required force will initially be less but will increase until the platform is level, after which it will again be half the weight.
  • A participant mentions expecting about 2 degrees of deviation from a direct vertical lift and assumes that half the weight is a safe estimate for the winch's specifications.
  • Concerns are raised about the cable's angle when lifting, with a participant noting that the cable's height and distance from the pivot could affect the load calculations.
  • One participant suggests creating a scale drawing to visualize the angles involved, indicating that small deviations from vertical can significantly impact the force required.
  • Questions are posed regarding the uniformity of the platform and the potential additional weight on it, which could affect the load calculations.

Areas of Agreement / Disagreement

Participants express varying views on the calculations and assumptions regarding the load on the cable and the angles involved. No consensus is reached on the exact load or the implications of the platform's uniformity and additional weight.

Contextual Notes

Limitations include the dependence on the angle of force application, assumptions about the uniformity of the platform, and the potential impact of additional weight on the platform. The discussion does not resolve these uncertainties.

simonwait
Hi

I have a 3.2m platform which weighs 175Kg. One end of the platform is touching the floor but the other end is on a pivoting bolt 2.5m in the air.

I need to figure out what the "load" would be on the end which is on the floor when I try to lift that end 4.5m in the air. Basically so I can spec a winch or hoist to lift it.
 
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simonwait said:
Hi

I have a 3.2m platform which weighs 175Kg. One end of the platform is touching the floor but the other end is on a pivoting bolt 2.5m in the air.

I need to figure out what the "load" would be on the end which is on the floor when I try to lift that end 4.5m in the air. Basically so I can spec a winch or hoist to lift it.
Do you mean, what force is required to lift it at the free end?
It depends on the angle at which you apply the force. If you pull straight up, it will be half the weight of theplatform, regardless of the platform angle.
If you pull at right angles to the platform it will be less at first but will increase until the platform is level. At that point it will once again be half the weight of the platform. After that it will reduce again.
 
OK great.

Yeah meant lifting the free end

Because of how it moves as its lifted then i would expect about 2 degrees either side of a direct lift vertically but based on it therefore not being far off then i will assume 1/2 the weight which is well within the limits of the winch I am looking at with an SWL of 125kg.

Thanks for your help
 
simonwait said:
OK great.

Yeah meant lifting the free end

Because of how it moves as its lifted then i would expect about 2 degrees either side of a direct lift vertically but based on it therefore not being far off then i will assume 1/2 the weight which is well within the limits of the winch I am looking at with an SWL of 125kg.

Thanks for your help
Wait a moment...
With the lengths you quoted, you might have trouble keeping the cable within a few degrees of vertical. How high is the top of the cable?
 
When the platform is down the cable is paid out about 14m from the drop point

Its also about 250mm further away from the pivot than the end of the platform so there is about of an arc.
 
simonwait said:
Its also about 250mm further away from the pivot than the end of the platform
Is that when the platform is resting on the ground or when it is horizontal?
 
When its on the ground
 
simonwait said:
When its on the ground
Ok.
In general, finding the max load on the cable is quite a tricky optimisation problem, but with that 14m height I think you are ok.
 
It may be worth while doing a scale drawing (for both conditions) and checking that the angles are near enough 90 degrees. (If you don't want to get involved with the Trig.) If the angle gets as low as 80 degrees, there's only a couple of percent of difference. 45 degrees will only make a force 40% more.
Btw, has it been established that the platform is uniform? Also, what will be on the platform? With that weight be significant?
 

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