# Homework Help: What am I doing wrong in my calculations

1. Jan 29, 2013

### aruji73

What is the distance the upper spring is extended from its unstretched length?
Now, three masses m1 = 3.3 kg, m2 = 9.9 kg and m3 = 6.6 kg hang from three identical springs in a motionless elevator. The springs all have the same spring constant of 229.57N/m

Now the elevator is moving downward with a velocity of v = -2.3 m/s but accelerating upward with an acceleration of a = 4.8 m/s2. (Note: an upward acceleration when the elevator is moving down means the elevator is slowing down.)

What is the distance the upper spring is extended from its unstretched length?
cm

I did

(3.3(9.8+4.8))/229.57=0.20987 m =20.987 cm

2. Jan 29, 2013

### SammyS

Staff Emeritus
That looks good to me.

Is there a problem regarding significant digits?

3. Jan 29, 2013

### aruji73

Hi I am not sure my answer is supposed to be in cm but I am sure I have done it right puzzling indeed

4. Jan 29, 2013

### rl.bhat

In your problem you have mensioned three springs and three masses. Whether the springs are connected with each other? If yes, who?

5. Jan 29, 2013

### aruji73

springs are connected is m1 at top is connected to m2 in middle which is connected to m3 in the bottom

6. Jan 29, 2013

### haruspex

So, the spring at the top is supporting, directly or indirectly, all three masses? Won't that increase the tension in it?

7. Jan 29, 2013

### aruji73

this is the pic

#### Attached Files:

• ###### masseshangingfromsprings2smaller.png
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8. Jan 29, 2013

### haruspex

Right, so what is the tension in the top spring? If you're not sure, start with the free body diagram of the bottom mass and work up.