# I Find the mass if you can

1. Aug 10, 2016

### Thejas15101998

A object is moving in free space with a constant velocity. How will you find the mass of the object without changing its velocity ?

Last edited: Aug 10, 2016
2. Aug 10, 2016

### Haborix

3. Aug 10, 2016

### jbriggs444

Without loss of generality, consider the object to be at rest. Move two test masses close to it on either side and measure their acceleration.

4. Aug 10, 2016

### Thejas15101998

could you please be more elaborate

5. Aug 10, 2016

### jbriggs444

6. Aug 10, 2016

### Thejas15101998

the second sentence

7. Aug 10, 2016

### jbriggs444

Do you know Newton's universal law of gravitation?

8. Aug 10, 2016

### Thejas15101998

yes

9. Aug 10, 2016

### jbriggs444

Perhaps you have noticed that the formula has two mass terms appearing within?

10. Aug 10, 2016

### Thejas15101998

yes, but then?

11. Aug 10, 2016

### jbriggs444

Sorry, you are going to have to show some effort here. Take Newton's law of universal gravitation and solve for M. What's on the right hand side?

12. Aug 10, 2016

### Thejas15101998

GMm/rsquare

13. Aug 10, 2016

### Thejas15101998

I am not able to comprehend of what you do with finding the acceleration.......and wouldnt the accelerations be same for both the test masses when they are close to the rest mass?

14. Aug 11, 2016

### jbriggs444

Please quote Newton's universal law of gravitation in full. Do not leave out the F. Solve for M.

15. Aug 11, 2016

### jbriggs444

The point of having two test masses is so that they have no net effect on the motion of the mass that you do not wish to affect. Yes, the magnitude of their accelerations would be the same. But would their accelerations be the same regardless of the size of the mass that we wish to evaluate?

16. Aug 13, 2016

### Andreas C

An object with larger mass will accelerate the other test masses more, because it has a "stronger" gravity.

17. Aug 16, 2016

### Jenab2

Catch up with it. Match velocity with it. Go into orbit around it at a known distance. Measure the period of the orbit. Find the mass from Newton's form of Kepler's Third Law.

18. Aug 16, 2016

### nasu

Yes, but this will change the velocity of the body.
So the question is pretty much asking to measure some quantity for a system without any alteration in the state of the system.
Which will mean without interacting with the system.
You need to allow for some amount of change, if you want to measure something. In classical mechanics we assume that we can make this change as small as we want. But it is not zero for any real experiment.

In your case, if the probe mass is much smaller than the mass of the moving body, the change in its velocity will be small.

19. Aug 16, 2016

### Jenab2

You can team up with someone else who will approach the planet from the opposite side, such that the distances are always the same, so that the other person's motion is the mirror image of yours in the reference frame of the planet.

20. Aug 16, 2016

### Andreas C

jbriggs gave the answer. You put an object left, another identical object right, the objects both pull it equally, so its velocity doesn't change, but you can measure how much they accelerate towards the body.

21. Aug 16, 2016

### Staff: Mentor

Is this question for your schoolwork?

22. Aug 23, 2016

### Thejas15101998

no...my friend it was just a hot discussion which i wanted to spread.

23. Aug 30, 2016

### Fernandopozasaura

With such a succint question, a just correct answer is: it's not possible. Heisenberg dixit.

24. Sep 21, 2016

### Let'sthink

The experiment that has been suggested seems ok but it gives the mass of the object in question provided you the identical mass of test objects.

25. Sep 21, 2016

### A.T.

You could also use light beams and measure how much they will be bent.