# Vertical spring and unattached mass

1. Sep 15, 2014

### Razvan

Suppose I have a vertical spring and an object.
If I slowly put the object on the spring until it reaches the new equilibrium point, and then I push it more and after that I release it, when exactly will the object "separate" from the spring?
Thank you.

2. Sep 15, 2014

### jbriggs444

If you push it down just a little past equilibrium and release it, what kind of motion results? Ignore friction and assume an ideal spring.

Last edited: Sep 15, 2014
3. Sep 15, 2014

### Razvan

It doesn't matter how much more I push it past equilibrium, let's assume it is enough for the object to be "launched". For the part of the motion before reaching the original equilibrium point of the spring the motion is a known one, but I think that after the spring reaches its equilibrium, the elastic force acts on the spring alone (because the object is not tied to the spring). A force acting on an object with almost zero mass should result in an infinite acceleration, which means this should be the moment when the object is separated from the spring. Is this correct?

4. Sep 15, 2014

### CWatters

An ideal spring stops providing a force on the object the instant it reaches its relaxed length.

5. Sep 15, 2014

### Razvan

And that moment is the answer to my question, am I right? Thank you.

6. Sep 15, 2014

### jbriggs444

The "when" that I assumed that you were asking about is "how hard do I have to push it down before the release will ultimately launch the mass so that it loses contact with the spring?"

Are you instead asking "how much time will elapse between when I release it and when it loses contact with the spring?"

7. Sep 15, 2014

### Razvan

How much time or the position where the object is no longer in contact with the spring.

8. Sep 15, 2014

### jbriggs444

Then CWaters has provided the answer.