# Minimum Force ( I don't know why this is not correct)

1. Oct 24, 2007

An elevator (mass 4100 kg) is to be designed so that the maximum acceleration is 0.0300g.

What is the maximum force the motor should exert on the supporting cable?

$$\sum F=m.03g$$
$$T=mg(1+.03)$$
41385 N <--this IS correct

What is the minimum force the motor should exert on the supporting cable? I thought this would just be weight?? But that is not correct
N

I am clearly misunderstanding what the question is asking

Casey

Last edited: Oct 24, 2007
2. Oct 24, 2007

### Staff: Mentor

No. If the motor provided a force equal to the weight, then the elevator would not move. The force of the motor would be balanced by the weight.

When one feels an acceleration it is a differential acceleration, i.e. it is in addition to the acceleration of gravity.

So T = m(g+a) = mg (1 + 0.03) = 4100 kg * 9.8 m/s2 * (1.03) = 41385.4 N

3. Oct 24, 2007

But that is what I did use (exactly^^^) for the maximum force. It is the minimum that I am concerned with.

I got 41385.4 N for MAX force. And it was correct.

Casey

4. Oct 24, 2007

### Staff: Mentor

Are you supposed to get something like 38975 N as a minimum?

5. Oct 24, 2007

I don't know. It is one of those web assign problems. It is for a friend of mine that I am helping. I never used webassign in school. But I can say that I don't like it. Half the time, I cannot tell what they are asking.

Why? Where are you getting the 38975 from? What is your stream of thought here?

Casey

6. Oct 24, 2007

### Staff: Mentor

I the elevator was dropping with an acceleration, then the tension would bg m (g-a) or mg (1-0.03) = mg (0.97).

I was trying to understand the question you were being asked.

There are two ways to feel the increased weight when standing in an elevator: 1) when it starts upward with some acceleration a from rest, or 2) when it slows down to a stop during a descent. When the elevator is falling (with some acceleration) the tension is less than the weight when it is stopped.

I'm just trying to understand the reference to minimum in this problem.

7. Oct 24, 2007

So am I. It's not as explicit as I would like it to be.

Casey

8. Oct 24, 2007