Physics - dissolving a compressed spring in acid

[person]

I have a physics question. If a spring were to be compressed into a box, and a person were to pour a substance that would dissolve the spring inside the box, where would the energy from the spring go.

Note: all parts of the spring would dissolve at the same rate.

[Martin]

It works to heat the solution (ie. kinetic energy) - the heat released by dissolving a compressed spring would be greater than dissolving a spring in equilibrium.

[Tony]

I'd imagine that as the spring is dissolving, the spring particles will snap out due to them being in compression. This release of kinetic energy will heat up the dissolvant solution due to friction.

[MysticVegeta]

person wrote:

Note: all parts of the spring would dissolve at the same rate.

Yes but some parts would dissolve before some parts, the result would be the spring would jump inside the box I think

[md]

A spring is really just a coiled, uniform diameter wire coil. it would disolve at an equal rate along all surfaces exposed, so except for the bits actually touching the box everything would go at the same rate.

However, at some point impurities in the actual metal will cause the spring to fail as the the diameter of the remaining metal gets smaller, so the spring will release whatever energy it has. I think.

[person]

When i read the question, im assuming that it meant the spring doesnt fall apart and release it as kinetic but dissolve at a constant rate, and is still compressed the moment before it gets dissolved.

[Delos]

That's one hell of a box if an acid with a small enough pKa will dissolve the metal spring but leave the box unscathed! My vote for the box gets corroded and falls to pieces before the spring does.

[person]

Fine, the box is nolw made out of glass. And the spring is made of Mg. And the acid is HCl.

[md]

See the thing about the spring remaining a spring (and in one peice) until it disolves completely is that a spring made of metal bad 1/8" diameter has significantly more potential energy then a spring of say 1/32" diameter can sustain without breaking. The spring will fail before it disolves, there is no way arround it.

[Martin]

In the real world, this would be impossible. At some point the spring would snap thrashing the acid and increasing its kinetic energy. However, it's still a valid question. A thiner spring has less potential energy than a ticker one, and from the beginning to the time the spring snaps it is still losing potential energy. So what happens to this potential energy? Remember that when something is dissolved in acid nothing is being destroyed, but rather particles are being broken apart when they react with the acid (ie. they don't just disappear). The kinetic energy released by this reaction works to heat the acid solution.

For example, let's say from t=0 to t=5, the spring is dissolving and at t=5 the spring snaps. We'll say that at t=0 potential energy is 10, and P(t)=10-t.

From t=0 to t=5, the potential energy decreases from 10 to 5, releasing 5 units of kinetic energy into the solution. At t=5, the spring snaps releasing the other 5 units.

[md]

Would it though? Or would the spring keep it's potential energy until it snaps, though it will be under ever more strain...

I'm inclined to think your theory is correct martin.

[Martin]

Cornflake wrote:

Would it though? Or would the spring keep it's potential energy until it snaps, though it will be under ever more strain...

I'm inclined to think your theory is correct martin.

The equation for a spring is F = -kx, with k representing the 'strength' of the spring.

The problem is that you're thinking of the spring as being under direct pressure - such as if a box were placed on top of the spring. In that case, the distance that the spring would be compressed would increase as the spring was dissolved, and the potential energy would remain constant. In this case, k decreases (as the spring dissolves) but x increases such that F remains constant.

In this case however, the spring is in a box, so the distance that it is compressed remains constant. So we have x remaining constant, and k decreasing as the spring dissolves. So therefore potential energy is lost.

[rizzix]

if you test it.. you should notice an increase in temperature.. i.e. potential energy gets converted to kinetic energy.

[1of42]

Martin wrote:

The equation for a spring is F = -kx, with k representing the 'strength' of the spring.

That's the force the spring exerts in opposition to a force compressing (or lengthening) it, no? Wouldn't the more applicable one in this case be (1/2)kx^2, or elastic potential energy?