Q:

Okay so someone asked what the pull of the sun on us is and they got this answer: ,..........Out here, at the distance we orbit the sun, the gravitational pull of the sun is only 0.0006 of the strength of the earth's gravity on the surface of the earth .............My question is what would the moon pull on us be compared to sun cause you on find m per sec but that applies to the on surface gravity, this question arose from me hearing somewhere in a video (can't find it now) that they said if a person was standing up against your back they would have as much pull on you as the moon does on you... I commented this in a video and someone just called me dense lol saying a human has almost no mass.. Is the above scenario plausible or accurate and what's the sun's pull on us compared to the moon or vice versa

- Padraic Mills (age 23)

Edmonton Alberta Canada

- Padraic Mills (age 23)

Edmonton Alberta Canada

A:

The pull from the Sun and Moon is simple enough to calculate because you can look up their masses and their distances and use that the gravitational field falls off as 1/distance^{2}. I'll do a rough calculation without looking anything up.

You know the moon tides are bigger than sun tides, but not a huge amount bigger. I'll guess roughly x3. The distance to the sun, if I remember right, is around 300x the distance to the moon. That means that the tidal effects from the moon, compared to the moon gravity here, are enhanced by a factor of around 300 compared to the same comparison for the sun. So I think the sun's field here is about 100 times as big as the moon's. You can check that by looking up the precise numbers.

How big is the sun's field? Just using the approximate distance to the sun and the radius of the earth's orbit, I get something around 10^{-3} m/s^{2}. So for the moon it would be roughly 10^{-5}m/s^{2}. The earth's surface field, for comparison, is close to 10 m/s^{2}.

If I assume that you and your friend have typical sizes of around 1m and density not all that different from the earth, I'd get that your pull on each other would only be around 10^{-7}m/s^{2}. That's a little rougher than the other calculations, even with accurate numbers, because you probably aren't spherical. Even with these rough numbers, however, it looks like the person-to-person gravity pull is less than the gravity pull from the moon.

Mike W.

*(published on 02/07/2017)*