# HELP with a mechanics question please

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#1
Its the very last bit of the question thats bothering me:

Question 2C which is on the second page: "The lift is raised by a single vertical cable. The mass of the lift is 300 kg. Find the maximum tension in the cable during this motion."

http://pmt.physicsandmathstutor.com/...9;s%20Laws.pdf

The markscheme is at the bottom.

I understand you're meant to find the greatest acceleration and in my velocity time graph i can see when the lift is accelerating uniformly in first 4 seconds you get 2(ms-1)/4(secs) = 0.5 ms-2. -This is the acceleration they used in the markscheme for this question - but i don't understand how a lift ACCELERATING upwards would cause tension in the lift cable?? Would a lift not going down cause a bigger tension in the cable since it has to be held upwards by the cable? But from my velocity time graph theres a bit where it decelerates from 2ms-1 to rest in 3 seconds and this obviously gives 2/3 = 0.6ms-2 which is a higher acceleration than the one they used in the markscheme? Is it because for this one the lift is decelerating so its actually -0.6ms-2? I just thought if a lift goes down it will have a high tension but now I think thats wrong right?? Could someone explain it to me thank youuu - much appreciated
0
5 years ago
#2
When the lift is at rest, the tension has to be equal to the lift's weight. This is also the case when the lift is moving at a constant velocity. But when the lift is accelerating upwards, the tension must be greater than the lift's weight to give a resultant upwards force (which causes the acceleration).

In the section you are looking at at the end of the motion, you correctly note that the lift is decelerating - that is, it has a negative acceleration. In terms of forces, this is because there is a resultant downwards force, which can only happen if the lift's weight is greater than the tension in the cable, and so this cannot be the point where the tension is the greatest.
2
#3
(Original post by Pangol)
When the lift is at rest, the tension has to be equal to the lift's weight. This is also the case when the lift is moving at a constant velocity. But when the lift is accelerating upwards, the tension must be greater than the lift's weight to give a resultant upwards force (which causes the acceleration).

In the section you are looking at at the end of the motion, you correctly note that the lift is decelerating - that is, it has a negative acceleration. In terms of forces, this is because there is a resultant downwards force, which can only happen if the lift's weight is greater than the tension in the cable, and so this cannot be the point where the tension is the greatest.
You explained it perfectly - thank you 0
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