# Calculating the velocity of a head-on impact between two suns

• ShaunPereira
In summary, the two solutions to the problem show that the velocity of the planet can be calculated by either integrating the distance between the two stars over time or using the concept of reduced mass.
ShaunPereira
Homework Statement
Two stars each of one solar mass (=2×10 ^
30 kg) are approaching each other for a head on collision. When they are a distance 10^9 km, their speeds are negligible. What is the speed with which they collide ? The radius of each star is 10^4 km. Assume the stars to remain undistorted until they collide. (Use the known value of G).
Relevant Equations
F=Gm1m2/r^2
Energy conservation principle
Firstly I would like to start with solving the problem with energy conservation principle which most solutions to the question show.
-Gmm/r= 1/2 mv^2 +1/2mv^2 -Gmm/2R
Where m= mass of planet
r= initial seperation
v= final velocity.
If we calculate to find v be about 2.6× 10^6 m/s
So far so good
The problem I encounter is when I try to use integration and integrate the infinitesmal distances over which acceleration changes to find final velocity

a= vdv/ds
a ds = v dv
a as a function of distance is Gm/r^2
On integrating we get
V^2/2 = GM/r where r extends from initial to final seperation

Comparing this equation and the one we get by using conservation of energy gives us different answers where velocity is divided by a factor of 2 in the former case and is not in the latter which gives me two answers

I have tried hard to think over the problem even assuming the velocity to be relative velocity between the two planets in the case of the integration but I just can't wrap my head around it

A little help would be appreciated

You need to take into account that both stars move.

Yes that is why I have mentioned my attempt at looking at the velocity as relative as both planets move towards each other with the same velocity but I haven't been able to develop the mathematics for it and need help with that

ShaunPereira said:
Yes that is why I have mentioned my attempt at looking at the velocity as relative as both planets move towards each other with the same velocity but I haven't been able to develop the mathematics for it and need help with that
There are a couple of ways to do this. Both stars are subject to the same force, which means that they both have the same acceleration. The relative acceleration, therefore, is double the acceleration of each star. Note also that the if the distance between the stars is ##r##, then the relative/separation speed of the stars is ##\frac{dr}{dt}## and this is not the speed of each individual star.

Another approach is to think about the motion of each star relative to the centre of mass of the system.

A more sophisticated approach is to use the concept of the reduced mass of the system. You could do an internet search for reduced mass.

PeroK said:
There are a couple of ways to do this. Both stars are subject to the same force, which means that they both have the same acceleration. The relative acceleration, therefore, is double the acceleration of each star. Note also that the if the distance between the stars is ##r##, then the relative/separation speed of the stars is ##\frac{dr}{dt}## and this is not the speed of each individual star.

Another approach is to think about the motion of each star relative to the centre of mass of the system.

A more sophisticated approach is to use the concept of the reduced mass of the system. You could do an internet search for reduced mass.
Thank you so much
I understood my mistake of not taking the relative acceleration there which was 2a and that the velocity we thus obtain is the relative velocity. Arrived at the right answer.

Reduced mass is a concept I am familiar with(a little) . I have used it sometimes for problems like where two masses connected by a spring oscillate. Never knew that I could use it in a problem like this too.

However I didn't get your approach regarding the centre of mass. All I know is that the centre of mass doesn't move ( in this case) and that the velocity obtained would be the actual velocity in which I am interested and not relative to the other body but I don't know what to take the acceleration as and with what limits should I integrate the acceleration with respect to displacement to get v

ShaunPereira said:
However I didn't get your approach regarding the centre of mass. All I know is that the centre of mass doesn't move ( in this case) and that the velocity obtained would be the actual velocity in which I am interested and not relative to the other body but I don't know what to take the acceleration as and with what limits should I integrate the acceleration with respect to displacement to get v
If ##r## is the distance of one star from the centre of mass, which is a fixed point, then we have:$$\frac{d^2r}{dt^2} = -\frac{GM}{(2r)^2}$$

Got it finally! Thank you very much

## 1. How do you calculate the velocity of a head-on impact between two suns?

The velocity of a head-on impact between two suns can be calculated using the formula v = √(2GM/R), where G is the gravitational constant, M is the mass of one sun, and R is the distance between the two suns.

## 2. What units should be used when calculating the velocity of a head-on impact between two suns?

The units used for the mass (M) should be in kilograms (kg), the distance (R) should be in meters (m), and the velocity (v) will be in meters per second (m/s).

## 3. Can the velocity of a head-on impact between two suns be negative?

No, the velocity of a head-on impact between two suns cannot be negative. It represents the speed at which the two suns are approaching each other.

## 4. What assumptions are made when calculating the velocity of a head-on impact between two suns?

When calculating the velocity of a head-on impact between two suns, it is assumed that the two suns are point masses (meaning their size can be ignored) and that there are no external forces acting on them.

## 5. How accurate is the calculated velocity of a head-on impact between two suns?

The calculated velocity of a head-on impact between two suns is an idealized value and may not be completely accurate in real-world scenarios. Factors such as the shape and rotation of the suns, as well as any external forces, can affect the actual velocity of the impact.

• Introductory Physics Homework Help
Replies
7
Views
214
• Introductory Physics Homework Help
Replies
7
Views
1K
• Introductory Physics Homework Help
Replies
55
Views
2K
• Introductory Physics Homework Help
Replies
6
Views
1K
• Introductory Physics Homework Help
Replies
6
Views
735
• Introductory Physics Homework Help
Replies
1
Views
910
• Introductory Physics Homework Help
Replies
5
Views
1K
• Introductory Physics Homework Help
Replies
34
Views
3K
• Introductory Physics Homework Help
Replies
5
Views
686
• Introductory Physics Homework Help
Replies
1
Views
879