# Homework Help: Elevator Problems

1. Sep 16, 2007

### BuBbLeS01

1. The problem statement, all variables and given/known data
A 56.0 kg girl weighs herself by standing on a scale in an elevator. What is the force exerted by the scale when the elevator is descending at a constant speed of 10 m/s?

What is the force exerted by the scale if the elevator is accelerating downward with an acceleration of 2.4 m/s2?

If the elevator's descending speed is measured at 10 m/s at a given point, but its speed is decreasing by 2.4 m/s2, what is the force exerted by the scale?

2. Relevant equations
Fnet=ma

How do we find acceleration without having a time?

2. Sep 16, 2007

### Staff: Mentor

In all cases you are given the acceleration (or all the information needed to figure it out). You don't need the time.

3. Sep 16, 2007

### nrqed

Draw a free body diagram and then apply $$\sum F_y = m a_y$$. That is all there is to it, really. Just be careful to include the correct sign of $$a_y$$.

4. Sep 16, 2007

### BuBbLeS01

Okay so I have F=(m*-a)+w for the second part

5. Sep 16, 2007

### BuBbLeS01

Now for the first part I am not sure what to do with the speed??

6. Sep 16, 2007

### Staff: Mentor

All you care about is acceleration.

7. Sep 16, 2007

### BuBbLeS01

I don't understand

8. Sep 16, 2007

### Staff: Mentor

Reread the statement of the first problem. How is the velocity changing?

9. Sep 16, 2007

### BuBbLeS01

it's descending at 10 m/s

10. Sep 16, 2007

### Staff: Mentor

Acceleration is the rate of change of velocity. How is the velocity changing? (Hint: Read it carefully. )

11. Sep 16, 2007

### BuBbLeS01

it's not its constant

12. Sep 16, 2007

### Staff: Mentor

Exactly! So what is the acceleration?

13. Sep 16, 2007

### BuBbLeS01

0 m/s^2

14. Sep 16, 2007

### Staff: Mentor

You got it.

15. Sep 16, 2007

### BuBbLeS01

ohhh so the only force acting is the weight.

16. Sep 16, 2007

### Staff: Mentor

No. If the only force acting on her was her weight, the girl would be in free fall.

17. Sep 16, 2007

### BuBbLeS01

so how is the third part any different from the second if the acceleration is the same?

18. Sep 16, 2007

### Staff: Mentor

The accelerations are not the same. Direction counts!

19. Sep 16, 2007

### BuBbLeS01

but they are both going down?

20. Sep 16, 2007

### Staff: Mentor

No. The second one has acceleration going down (it tells you that), but the third one you have to figure out the direction of acceleration by reading carefully.

21. Sep 16, 2007

### BuBbLeS01

ok so since the speed is decreasing then the force is acting to slow it down and bring the elevator eventually to a stop?

22. Sep 16, 2007

### Staff: Mentor

Sure. And which way must it be accelerating, since it happens to be moving downward as it slows?

23. Sep 16, 2007

### BuBbLeS01

it is accelerating up

24. Sep 16, 2007

Right!