To find mass of falling object

  • Thread starter lighteyes
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  • #1
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A friend asked this question and it got me thinking.. and I can't seem to think of any solution. :bugeye:

Out of curiosity, he asked me.. is it possible to find the mass of a free falling object without knowing the force it hits the ground? (assuming we know the displacement s=0.91m, v= vms-1) That object is also said to rebound to a height of 0.61m.

So I know mass doesn't affect motion of free falling object.. but is it possible to calculate mass? I tried whatever I know and m is always cancelled out.
 

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  • #2
HallsofIvy
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If all you have to go on is the motion itself, no, it is not possible. Objects of any mass will free-fall exactly the same.
 
  • #3
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If all you have to go on is the motion itself, no, it is not possible. Objects of any mass will free-fall exactly the same.
Would that be true of two Earth mass bodies as they get close? It would seem to me if they both have a terminal velocity of say, 10 km/sec,(a small meteor attracted to Earth) they would both move together, wouldn't that mean a higher terminal velocity?
 
  • #4
Drakkith
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Would that be true of two Earth mass bodies as they get close? It would seem to me if they both have a terminal velocity of say, 10 km/sec,(a small meteor attracted to Earth) they would both move together, wouldn't that mean a higher terminal velocity?
Terminal velocity is an effect that the atmosphere produces on objects falling through it. It creates drag, which causes objects to reach an equilibrium where the force of gravity on the acceleration is counterbalanced by the drag on the object, creating a maximum free falling velocity that it can reach at a given altitude.

The acceleration would still be the same no matter the mass. Increasing an objects mass relative to the earth only makes it so that the earth itself falls toward that object at a greater acceleration, which would look like an object was falling toward the other much faster than was actually happening unless you could tell that both bodies were falling towards each other. Hope that makes sense.

Edit: Ignoring drag, the acceleration is the same for all objects, no matter the mass.
 
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  • #5
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Hitting the ground is one of many possible interactions a falling object can experience. Consider drawing your data from others. :)
So if it is falling through air, consider the effect of drag.
If you happen to know the mass of the object causing the acceleration with sufficient accuracy, use some newtonian mechanics.
If it is subject to them, you could also use electromagnetic forces.

And any others or a combination thereof. :)
 
  • #6
K^2
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Terminal velocity depends on the mass and shape of the object. If you have a simple shape, like a smooth sphere, you can estimate its drag coefficient and use that to find the mass from terminal velocity. For some random object, the problem is incredibly complex, since most objects tend to tumble as they fall making computations of drag coefficient even using numerical methods incredibly complex.
 

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