# Homework Help: Spring prob

1. Jan 2, 2013

### assaftolko

A body with a mass of 2 kg is released from rest a height of 0.4m above the edge of a vertical rested spring with k= 1960 N/m. Assume there is no energy loss in the collision:

1. what's the maximal contraction of the spring?
2. to what height will the body reach back?

about 2 - Because I have energy coservation it seems reasonble that the body will reach the same height it was released from right? but... in 1 I found that the maximal contraction with respect to the loose length of the spring is 0.1 m and that it's maximal extension with respect to the loose length is -0.08 m... so it seems that when the spring is at full strech upwards - it's still lower than the initial height of 0.4 m above the loose length of the spring... so how does it all add up? does it have anything to do with the fact that maybe the body doesn't stay attached to the spring? if it doesn't stay attached - how can I know what's the point where it is released from the spring in its way up?

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2. Jan 2, 2013

### PhanthomJay

the body does noy stay in contact with the spring...When it loses contact with it, the spring no longer exerts an upward force on it, which occurs at what point ? Note that you don't need to find this point to solve the problem and achieve your intuitive answer to part 2.

3. Jan 2, 2013

### assaftolko

Ok thanks man! I still want to know at what point this seperation happens... is it the point where the spring is at loose length? if so - why is that?

4. Jan 2, 2013

### PhanthomJay

yes, that is correct
When the spring rebounds to its loose length, what is the force exerted by the spring on the body, per Hookes law?

5. Jan 2, 2013

### assaftolko

0, but how can you conclude from this that the body will seperate from the spring? I mean for a mass that is attached to a spring - it will experience a lot of times 0 force from the spring and still will stay attached to it - why if the mass isn"t attached (by something) to the spring then I know for sure (either for vertical or for horizontal spring) that the mass will depart from the spring at loose length point?

6. Jan 2, 2013

### PhanthomJay

The spring force on a body is not only a conservative force, but it is also a normal contact force acting perpendicular to the body. Unattached normal contact forces always push in toward an object, never away from it. Its like a book resting on a table, with a normal force of the table acting up on it with a force of mg. When you now lift the book slowly from the table, the normal force gets less and less until it reaches zero, at which point, the book loses contact with the table because the normal force cannot act downwards.

7. Jan 2, 2013

### assaftolko

So the reason a body "glued" to a spring doesn't depart from it at loose length point is just the constraint from the spring? this pivot or whatever exerts a force on the body that doesn;t allow it to depart?

8. Jan 2, 2013

### MrWarlock616

Oh no that's just because of the glue.

When it's not glued to the spring, the upward acceleration due to the restoring force causes it to move upward and finally leave the spring when the force is zero.

9. Jan 2, 2013