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Mechanics - Maximum Tension

There's a lift with a mass of 5600kg and it is currently descending at a speed of -6.4/7. It's a minus because it's decelerating, I've taken down as positive. I've got the answer, but I don't understand it.

The answer is:

Maximum tension = mass x (9.8+(6.4/7))
Maximum tension = 5600 x (75/7)
Maximum tension = 60000

It looks like a force=mass x acceleration equation to me, but why is the weight of the lift not included in the equation? (5600g / 5600 x 9.8 / 54880N).

Why is the 6.4/7 positive when it's accelerating in the opposite direction of gravity?

Edit: The lift is held by a string, it doesn't stretch and it has no weight. Why is the tension in it 60000N?
(edited 10 years ago)
Reply 1
Original post by lifebook02
There's a lift with a mass of 5600kg and it is currently descending at a speed of -6.4/7.


The lift does not appear to be accelerating or decelerating

Perhaps you could tell us the actual question

Having read again

Is the lift descending
Is it decelerating at 6.4/7 m/s/s
(edited 10 years ago)
Reply 2
Original post by TenOfThem
The lift does not appear to be accelerating or decelerating

Perhaps you could tell us the actual question


Sorry, I thought that typing out all the question would be messy and maybe confuse people. Anyway, here's the question, it's split into five parts, but I'm just stuck on the last one.

Question: The mass of a lift is 5600 kg. The lift starts from rest and descends with uniform acceleration for 8 s until it reaches a speed of 6.4 ms–1. The tension in the lift cable is 50400 N.

The lift maintains this constant speed of 6.4 ms–1 for 25 s before decelerating uniformly to rest. The total time for descent is 40 s.

Find the maximum tension in the lift cable during the motion.

That's the question^
-------------------------------
The maximum tension is obviously at the part where the lift is slowing down. And the speed that it's slowing down is at 6.4/7 ms-2.

It's travelling at 6.4m/s and then it takes the lift 7 seconds to completely stop.
(edited 10 years ago)
Reply 3
Original post by lifebook02
Sorry, I thought that typing out all the question would be messy and maybe confuse people. Anyway, here's the question, it's split into five parts, but I'm just stuck on the last one.

Question: The mass of a lift is 5600 kg. The lift starts from rest and descends with uniform acceleration for 8 s until it reaches a speed of 6.4 ms–1. The tension in the lift cable is 50400 N.

The lift maintains this constant speed of 6.4 ms–1 for 25 s before decelerating uniformly to rest. The total time for descent is 40 s.

Find the maximum tension in the lift cable during the motion.

That's the question^



So, as I said, the 6.4/7 is deceleration, not a speed

So you just need the equation of motion

Taking up as +

T - mg = ma

T = mg + ma

In your answer there has been a factorising to give

T = m(g+a)

Since the lift is moving down and decelerating the up acceleration is +ve
Reply 4
Original post by TenOfThem
So, as I said, the 6.4/7 is deceleration, not a speed

So you just need the equation of motion

Taking up as +

T - mg = ma

T = mg + ma

In your answer there has been a factorising to give

T = m(g+a)

Since the lift is moving down and decelerating the up acceleration is +ve


Thanks for helping me. But wouldn't the gravity be negative since it's going down? Why're they both positive when they're going in opposite directions?

Edit: Actually, I think I get it now. Thanks for your help :smile:
(edited 10 years ago)

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