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# Application work -energy equation3

## Application work -energy equation

(OP)
Topic: Application work -energy equation.
In the solution, a reference point (a zero level) is chosen. Unfortunately, I absolutely did not understand how to determine the reference point. I would choose from point A to point B. How do I determine the reference point?

The solution also says that the kin. Energy of point 0 and B is 0. How does this come when the mass m is pressed against the spring and released. At point 0 the speed is 0, I understand that, but at point B it is also zero???

### RE: Application work -energy equation

Disclaimer: I don't speak German.
The reference point (for potential energy) can be anywhere, choose one that is convenient to the solution: ground level or the lowest level reached by anything in the problem makes it so you don't have to deal with negative elevations. Or choose it such that the potential energy of something in the problem is zero at a point you are interested in.

Google translate says question a) is "How big does the stiffness C of the spring have to be so that m1 reaches point B exactly"
If m1 reaches B 'exactly' what would it's speed be at B?

### RE: Application work -energy equation

The short answer is that you can pick any reference point you like and make the maths work around it.

A longer answer is that a well-chosen reference point will simplify the maths a great deal. For a start, make sure it's on the line that m1 travels along. Since there's change in the system at A, I'd probably choose that point as my reference to simplify the spring elements of the equation. Having said that, since the solution you've been given talks about zero KE at point 0, it looks like whoever wrote that chose the initial point of release as their reference.

Why is KE zero at B? Looking at the shape of the question (but without working through the numbers to check), I think the scenario is that there is too much friction for M2 to be able to pull M1 up the slope by itself (so m2g < (m1g component along slope) + (m1g component Normal to slope) x µ. Give it a shove from the spring to get it started and it will move a bit, but friction will stop it eventually. The aim of Part a) of the question is to make that happen at point B (hence zero KE).

Finally, I don't think the question has been copied out correctly. There are a couple of things in the "Given that" section at the end that don't make sense:

m1 = 2m1. In this case, I wonder if the second m1 was originally a unit of mass

r, a, b = 5a. Perhaps the second comma was originally a plus sign?

A.

### RE: Application work -energy equation

Hi akan.oo

What is it you are trying to find with the calculation ? How much the spring stretches?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Application work -energy equation

the references mean exactly what they say. Pick a reference point, any point, it doesn't matter. To prove this, pick a point, do the sums, then repeat with a different point. Some of the detail calcs will change, but the results will be the same.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

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