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Question about choice of objects when calculating potential energy

Physics Asked by Pranav Jain on October 14, 2020

The formula for change in potential energy of a system made up of object A and B is:

change in potential energy of system = – (dot product of [conservative force vector of A on B] and the [displacement vector of B])

But what if we instead consider:

change in potential energy of system = – (dot product of [conservative force vector of B on A] and the [displacement vector of A])

Will the result be the same either way we do it?

An example would be appreciated.

One Answer

If you'd physically think about such a situation, where we have A and B as two point masses at rest, being attracted by the gravitational force, nature doesn't care what you think is the relative position of A with respect to B or B with respect to A.

But even if you think about this problem mathematically, the expression for the forces on particles A and B would be -

$$vec{F_B}=frac{-G m_A m_B}{r^2} hat{r} qquad qquad vec{F_A}=frac{+G m_A m_B}{r^2} hat{r}$$

Here, $vec{r_B}=r hat{r}$ (displacement vector of B) and $vec{r_A}=-r hat{r}$ (displacement vector of A) and $hat{r}$ is the unit vector pointing from A to B.

Now, if you try to calculate the potential energy difference using either A or B as the origin, it's $$P.E.= int_{infty}^{r}vec{F_A} cdot dvec{r_A}= int_{infty}^{r}vec{F_B} cdot dvec{r_B}=frac{-Gm_A m_B}{r}$$

Therefore, this again proves that nature doesn't care what you choose as the displacement vector direction or the origin. The potential energy expression would be consistent anyway.

Correct answer by Tachyon209 on October 14, 2020

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