The gravitating of a small mass towards a big mass

AI Thread Summary
The discussion centers on the gravitational interaction between a small mass and a larger mass, focusing on the equations governing their motion. The gravitational potential energy is expressed as GMm/x, and the velocity of the smaller mass is derived as v(x) = √(2GM/x). The main challenge is solving the differential equation d²x/dt² = GM/x² to find the position and velocity as functions of time. There is a concern about the absence of the larger mass's radius, which is crucial for determining the collision point. The work-energy theorem is suggested as a method to relate kinetic and potential energy in this context.
brotherbobby
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Homework Statement
A large mass of ##10^8## kg is held in free space at a certain point. A small mass of 1 kg exists at a point 1000 km (##10^6\;\text{m}##) away from the larger mass at the start of motion. Answer the following questions :
(1) Write the velocity of the smaller mass as function(s) of its space coordinate and time, assuming the location of the larger mass to be at the origin, the motion taking place along a line and the start of motion to be at ##t=0##.
(2) What is the location of the smaller mass at a given time ##t##?
(3) When will the smaller mass collide with the bigger mass?
(4) With what velocity will the bigger mass collide with the smaller mass?
Relevant Equations
From Newton's gravitation formula, the acceleration due to gravity ##g=\frac{d^2x}{dt^2}= \frac{GM}{x^2}##, where ##x## is the distance of separation between the masses ##M## and ##m## at a given instant.
Gravity.png
Diagram :
I draw a picture of the problem situation and paste it to the right.

Attempt : Let me assume that the position of the smaller mass ##m## at a given instant of time ##t## is ##x(t)##.

(1) Gravitational potential energy ##\frac{GMm}{x} = \frac{1}{2}mv^2(x)##, where ##v(x)## is the velocity of the mass at that position. This simplifies to ##\boxed{v(x) = \sqrt{\frac{2GM}{x}}}##.
However, I do not know how to find the velocity as a function of the time ##t##, or ##v(t)##.
More crucially, I do not know how to solve the differential equation given in the Relevant Equations above : ##\frac{d^2x}{dt^2}= \frac{GM}{x^2}##. If I could, it would yield ##\frac{dx}{dt} = v(t)## and ##x(t)##, thereby answering questions 1 and 2 above.
Solving the differential equation would also lead to finding when will the masses collide and with what velocity, which would answers questions (3) and (4) above.

Request : A help or hint would be very welcome.
 
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For solving the differential equation, note that you have $$v(x) = \sqrt{\frac{2GM}{x}}=\frac{dx}{dt}.$$ Can you separate variables and integrate?

I am bothered by the fact that the radius of the larger sphere is not given. How will one know where the smaller mass is when it collides? Check your source to see if you missed it. If the radius is not given, assume that it is ##R## and proceed.

On edit
Actually it should be $$\frac{dx}{dt} = -\sqrt{\frac{2GM}{x}}$$ because the distance ##x## decreases as time increases.
 
Last edited:
brotherbobby said:
(1) Gravitational potential energy ##\frac{GMm}{x} = \frac{1}{2}mv^2(x)##, where ##v(x)## is the velocity of the mass at that position.
GPE is not ##\frac{GMm}{x}## (you forgot something) and there is no law that says GPE equals KE (or -KE). What does it say?
 
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Can you prove this?
If the object with mass ##m \ll M## is dropped at rest at the distance ##x_1## from the centre of the object with mass ##M##, then
## \dfrac{mv^2}{2} - \dfrac{GMm}{x} = -\dfrac{GMm}{x_1}## where ##x \leq x_1## and ##v## is the velocity at position ##x##.
Hint: work-energy theorem.
 
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