Tuesday, September 18, 2012

Gravity is cool!

So we were doing a lab in AP Physics today.  It involved two different masses hanging side by side from a pulley, and then letting it accelerate and timing how long it took, then calculating the acceleration.  That may not have made sense, but the point is, we got a percent error of about 27%, which is not very good.

So we started brainstorming what could possibly have caused this error.  It's not the mass or friction of the pulley, because that would skew the results in the other direction.  It was probably due to human error in timing the experiment, but our eventual conclusion is way cooler.

I came up with this.  See, all masses create a gravitational field.  The more massive an object, the greater the force exerted by its gravitational field on other masses.  That's why the Earth, which is very massive, can exert force on us when we are so far away from the core; whereas we don't feel inexplicably drawn to, say, a bowling ball.  (Though if you feel an inexplicable attraction to a bowling ball, I'm not judging you, don't worry.)

So therefore, the most massive member of our lab group must be exerting gravitational force on the masses in the experiment, and causing the results to be screwy (because when calculating our theoretical value, we only took account of the Earth's gravitational force.  Adding another force on the system would change the results).

Someone proposed measuring the masses of each group member and banishing the most massive member, but they were overruled.  Unfortunately, the period ended before we could try the experiment again, this time standing several meters away to prevent any gravitational interference.  Oh, well.

For those who don't do physics, let me clarify: this is entirely preposterous, because the amount of gravitational force caused by a human would be incredibly, incredibly small.  We can even calculate the force between, say, a 60 kg person and a 1 kg mass that are 1 meter apart.

F = GMm/r^2

Where G = 6.67300 × 10-11 m3 kg-1 s-2

So we plug in the two masses for M and m, and divide by 1 sqaured, which is still one, and get that the force exerted by the person on the mass is 4x10^-9 N.  Which is very very very small.  Negligible.

....I just realized that this was probably not at all interesting.  I apologize.  I promise, no more equations.  I just find this so COOL!

Well, actually, this concludes this post.  I was gonna say one more thing about our Science Olympiad team, but I just realized it involves equations.  I'm already gonna lose all my non-science/math readers, so I'm not going to push it.  Over and out, yo.

P.S. Your computer is probably pulling you towards it right now, with a force of somewhere around 3.2 x 10^-8 N.  Don't let it get you!

P.P.S. This is Bill Amend drawing for xkcd.  The second comic is related.  The others are also funny.


  1. Just have to mention - since (I presume) the lab was set up to measure time taken for a vertical displacement (and thus vertical acceleration) only, a mass of a group member, however large, would have had no impact if standing beside the apparatus as the ensuing force would be perpendicular to gravity.

    Was your largest group member lying on the floor beneath the apparatus as a landing pad for the weight? Now that would be a whole 'nother story!

    (As you allude to, of course, a group member or small black hole hovering above the apparatus can be ruled out as it would have skewed the results in the opposite direction.

    Bahaha thanks for the laughs. =)

  2. The moon! THE MOON! Was the moon over the opposite side of the earth at the time by any chance? XD XD XD

  3. I think you should put more science and equations. But I'm biased. I LOVE science. :)

  4. Crazy coincidence! My AP Physics class did a very similar Atwood's Machine lab just yesterday in class! We were finding the mass of pennies and it was great! Though no one came up with quite as good explanations as to why our percent error of acceleration was so much!
    Thank you for the post!


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