# Two masses and spring oscillation

OK...Please leave it for a while.

The potential consists of an elastic part and a Coulomb-part. Expand the potential function into Taylor series about the equilibrium point. Keep up to the second order term. From the equilibrium condition and from the coefficient of the second-order term, you get the equivalent force constant.

ehild
Could you show me how you would get the equivalent force constant using your above approach ?

ehild
Homework Helper
The second derivative of the potential is the negative of the first derivative of force. It would not be a different approach.
Anyway, you have to choose a frame-of reference first.

Sorry, I have to leave now, but I am soon back.

ehild

Congratulations on your 10000 post ehild
Homework Helper
Thanks I did not notice.

Back to equivalent force constant. The potential energy is a function of some variable x. Assume U(x) has a local extreme somewhere, at x0. You want to find the behaviour of the system near x0.
The Taylor series of a function f(x) about x0 is defined as

$$T(x)=\sum_0^∞{f^{n}|_{x=x_0}\frac{(x-x_0)^n}{n!}}$$
where f(n) means the n-th derivative of f(x), and you have to take all derivatives at x=x0.

There are some criteria when you can do the expansion and the series converges to f(x).

Now we assume that U(x) can be expanded and we stop at the second-order term.

$$U(x)≈U(x_0)+\frac{df(x_0)}{dx} (x-x_0)+\frac{1}{2}\frac{d^2 f(x_0)}{dx^2}(x-x_0)^2$$

f(x) has local extrem at x0, so the first derivative has to be zero. We can choose the zero of the potential energy at x0. Our approximate potential function reduces to one term $$U(x)≈\frac{1}{2}\frac{d^2 f(x_0)}{dx^2}(x-x_0)^2$$

If the second derivative of the potential function is positive, it has a minimum at x0. The potential is the same as that of a spring, and the effective spring constant is equal to the second derivative of U at the equilibrium point.

$$D=\frac{d^2 f(x_0)}{dx^2}$$

The force is negative gradient of the potential : F=-dU/dx = -D(x-x0)). You can switch over to the variable x-x0=s, and the the equation of motion is mds2/dt2+Ds=0. The angular frequency of the SHM is ω=√(D/m) (in our case, use the reduced mass).

ehild

Last edited:
I have a very basic question that when do we expand a function and keep terms up to first order term (i.e first two terms) just as in post #42 and when do we expand up to second order (i.e first three terms) just as in above post ?

ehild
Homework Helper
You can ignore the second order term in an expansion if it and all the other terms are much smaller than the first order one. If the first order-term is zero, as in the case at the bottom of a potential well, you have to keep the second order term.

How many terms are needed, it depends on the deviation from x0, the centre of the expansion.
In this problem, we were interested in small oscillation about an equilibrium position. |x-x0|could be sufficiently small.

ehild

• 1 person
You can ignore the second order term in an expansion if it and all the other terms are much smaller than the first order one. If the first order-term is zero, as in the case at the bottom of a potential well, you have to keep the second order term.

ehild
Okay...thanks for enlightening me :)

Another naive question

But what if the first order term is not present ?

What is approximate value of ##(1-(\frac{d}{x})^2)^2## under the assumption x<<d ? Should it be ##1## or should it be ## 1-2(\frac{d}{x})^2 ## ?

ehild
Homework Helper
Okay...thanks for enlightening me :)

Another naive question

But what if the first order term is not present ?

What is approximate value of ##(1-(\frac{d}{x})^2)^2## under the assumption x<<d ? Should it be ##1## or should it be ## 1-2(\frac{d}{x})^2 ## ?
If the first term is not present expand to the second-order term.

Remember cosx. If x is small, usually it is approximated by 1 or by 1-x2.

##(1-(\frac{d}{x})^2)^2## under the assumption x<<d : if x<<d then d/x >> 1 and you can ignore 1.

If you mean (1-(x/d)2)2 it is approximately 1-2(x/d)2.

But it depends what else is in the expression you want to approximate. If it is 1-(1-(x/d)2)2, you have to keep the second-order term. If it is 4+(1-(x/d)2)2, for example, you might ignore (x/d)2

ehild

• 1 person
I was initially hesitant (rather afraid) asking these questions .But after reading the explanations I am quite glad and very satisfied that I put my doubts in front of you .

Thanks ehild ehild
Homework Helper
I read just now : "the only stupid question is which is not asked". ehild

ehild
Homework Helper
Returning to your energy consideration, and assuming the CM is in rest: If you have two masses with coordinates x1 and x2 corresponding velocities v1 and v2, and you introduce the variable x=x2-x1, the corresponding velocity v=v2-v1:

CM is in rest: m1v1+m2v2=0, and v2-v1=v →
##v_1=-\frac{m_2v}{m_1+m_2}\\v_2=\frac{m_1v}{m_1+m_2}##

The kinetic energy is

##KE=\frac{1}{2} \left(m_1 (\frac{m_2 v}{m_1+m_2} )^2+ m_2( \frac{m_1 v}{m_1+m_2})^2 \right)##

##KE=\frac{1}{2}\frac{m_1 m_2}{m_1+m_2} v^2=\frac{1}{2} μ v^2##

Assuming SHM about the equilibrium value of x: x-xe=Asin(ωt), v=Aωsin(ωt) and expanding the potential energy about xe: PE=0.5 D(x-xe)2

KE+PE=E →

##\frac{1}{2} A^2\left((μω)^2\cos^2(ωt)+D^2 \sin^2(ωt)\right)=E##

The expression can be constant only when μω=D.

Hello ehild

Here is the derivation how a two-body problem can be reduced to one body-problem. I write it in one dimension,but the derivation is the same for 3D.
I would like to understand how this concept of reduced mass is applied in case of a Planet of mass m moving around sun of mass M in circular orbit

Suppose the Sun is considered stationary .In that case ## \frac{GMm}{D^2}=\frac{mv^2}{D}## where D is the distance between them .This gives ##v=\sqrt{\frac{GM}{D}}##

Now if we want to take into account the motion of Sun also i.e both Sun and the planet orbiting about their common COM then this is equivalent to replacing the mass of planet 'm' by reduced mass 'μ = Mm/(M+m)' and replacing the Sun with a central mass 'M+m' .

Using this we have ## \frac{G(M+m)\mu}{D^2}=\frac{mv^2}{D}##

Solving we get ##v=\sqrt{\frac{GM}{D}}## .

I am wondering why hasn't the result changed since now the Sun is also considered to be orbiting ?

ehild
Homework Helper
What is v now?

ehild

Speed of the planet in circular orbit.

ehild
Homework Helper
No. The reduced-mass approach started by introducing new variables instead of the position vectors of the planet and Sun. You might look at my post in this thread about it.

ehild

vela
Staff Emeritus
Homework Helper
I would like to understand how this concept of reduced mass is applied in case of a Planet of mass m moving around sun of mass M in circular orbit

Suppose the Sun is considered stationary .In that case ## \frac{GMm}{D^2}=\frac{mv^2}{D}## where D is the distance between them .This gives ##v=\sqrt{\frac{GM}{D}}##
By assuming the Sun is stationary, you've reduced the two-body problem to a one-body problem. In this case, you're assuming the Sun doesn't move because its mass M is so much larger than the mass of the planet.

Using the reduced-mass approach instead, you get
$$\frac{GMm}{D^2} = \frac{\mu v^2}{D}.$$ The force between the two bodies remains unchanged, but the mass of moving body is replaced by the reduced mass ##\mu##. When ##M \gg m##, you have ##\mu \cong m##, and you recover the previous result.

Now if we want to take into account the motion of Sun also i.e both Sun and the planet orbiting about their common COM then this is equivalent to replacing the mass of planet 'm' by reduced mass 'μ = Mm/(M+m)' and replacing the Sun with a central mass 'M+m' .
I'm not sure how you came up with this.

ehild
Homework Helper
The two-body Kepler problem can be solved by using the CM mass of reference. The difference of the position vectors obey the same differential equation as a single body with the reduced mass orbiting about a stationary mass of m+M. The gravitational force between the mass and the planet is F= GmM/D2. That between the reduced mass and a central mass of m+M: G μ(M+m)/D2= G(mM/(m+M))(M+m)/D2=GmM/D2.

ehild

• 1 person
vela
Staff Emeritus
Homework Helper
Ah, now I understand what Tanya was getting at in her post.

Tanya, you have a mistake in the equation of motion in the second approach, which is why you're ending up with identical results for ##v##. You have to replace ##m## by ##\mu## on the righthand side.

Ah, now I understand what Tanya was getting at in her post.

Tanya, you have a mistake in the equation of motion in the second approach, which is why you're ending up with identical results for ##v##. You have to replace ##m## by ##\mu## on the righthand side.
$$\frac{G(M+m)\mu}{D^2}=\frac{\mu v^2}{D}$$

$$v^2 = \frac{G(M+m)}{D}$$

$$v = \sqrt{\frac{GM(1+\frac{m}{M})}{D}}$$

Now ,since ##m<<M## , neglecting term ##\frac{m}{M} ## , ## 1+\frac{m}{M} ≈ 1 ##

$$v = \sqrt{\frac{GM}{D}}$$

But again this is the same result as before .

vela
Staff Emeritus
Homework Helper
Yes, that's correct. You see to that level of approximation, both approaches yield the same result, as you should expect. If you expand ##\sqrt{1+\frac mM}## as a series, you can calculate the higher-order corrections that come from relaxing the assumption that M is stationary.

Suppose I do not consider this two body problem as a one body problem .

Let r1 be the distance of M(sun) from CM and r2 be distance of m(planet) from CM.Then ## r_1 = \frac{mD}{m+M}## and ## r_2 = \frac{MD}{m+M}##

$$\frac{GMm}{D^2} = \frac{mv^2}{r_2}$$

$$\frac{GMm}{D^2} = \frac{m(m+M)v^2}{MD}$$

$$v^2 = \frac{GM^2}{(m+M)D}$$

$$v = \sqrt{\frac{GM}{(1+\frac{m}{M})D}}$$

Again ## 1+\frac{m}{M}≈ 1 ##

So,

$$v = \sqrt{\frac{GM}{D}}$$

Is this analysis correct ?

ehild
Homework Helper
The speed v what you get with the reduced mass approach is not the speed of the planet along its orbit about the CM. It is the speed of the hypothetical planet of mass μ orbiting the stationary Sun of mass m+M. Remember we introduced the variable r=r(planet)-r(Sun), and the time derivative of r is the relative velocity of the planet with respect to the Sun.

It is better to get the angular speed. It does not change with the reduced mass approach. So ω^2=G(M+m)/D3. The radii of the orbits both of the planet and the Sun is obtained from the condition that the CM is in rest in the applied frame of reference.

ehild

• 1 person
vela
Staff Emeritus
Homework Helper
$$v = \sqrt{\frac{GM}{D}}$$

Is this analysis correct ?
Yes, but as ehild pointed out, this ##v## isn't the same ##v## as in your previous analyses.

ehild
Homework Helper
Suppose I do not consider this two body problem as a one body problem .

Let r1 be the distance of M(sun) from CM and r2 be distance of m(planet) from CM.Then ## r_1 = \frac{mD}{m+M}## and ## r_2 = \frac{MD}{m+M}##

$$\frac{GMm}{D^2} = \frac{mv^2}{r_2}$$

$$\frac{GMm}{D^2} = \frac{m(m+M)v^2}{MD}$$

$$v^2 = \frac{GM^2}{(m+M)D}$$

$$v = \sqrt{\frac{GM}{(1+\frac{m}{M})D}}$$

Again ## 1+\frac{m}{M}≈ 1 ##

So,

$$v = \sqrt{\frac{GM}{D}}$$

Is this analysis correct ?
Yes, that is correct. But you want the difference between the cases with stationary Sun and with moving one. And what would you get for double stars, that is two nearly equal masses orbiting about their common CM? Like theese http://astrobob.areavoices.com/2010/04/10/how-to-double-your-enjoyment-of-the-heavens/
ehild

Last edited:
But you want the difference between the cases with stationary Sun and with moving one.
Yes.

And what would you get for a double star, that is two nearly equal masses orbiting about their common CM?
$$v = \sqrt{\frac{Gm}{2D}}$$ ,where m is the mass of a single star and D is the distance between them.