# Magnitude and gravitation?

1. Oct 15, 2004

### kimikims

anyone know how to do this problem??

Tom has a mass of 65.7 kg and Sally has a
mass of 52.9 kg. Tom and Sally are standing 28.7 m apart on a massless dance foor. Sally looks up and she sees Tom. She feels an attraction. If the attraction is gravitation, find its magnitude. Assume both can be replaced by spherical masses and that G=6.67259 x 10^-11 Nm^2/kg^2.

2. Oct 15, 2004

### Pyrrhus

Use

$$F = G\frac{m_{1}m_{2}}{d^2}$$

3. Oct 15, 2004

### kimikims

So... would this be correct?

F= 6.67259 x 10^-11 [(65.7kg)(52.9kg) / (28.7)^2)]

4. Oct 15, 2004

### tony873004

This question is phrased weird. If they are both standing on the dance floor, then she will not see Tom by looking up unless there's a mirror on the ceiling, which would be irrelivant to the problem anyway.

She feels an attraction. If the attraction is gravitation, find its magnitude.
This should be 9.81 m/s^2 in the down direction. Despite the massless floor, the Earth that the massless floor is on is still pulling with a magnitude of 9.81 m/s^2.

She will definately feel the Earth's attraction. As far as the gravity exerted by Tom's mass, she definately will not feel this attraction. The most sensitive equipment on Earth would not be able to detect Tom's presence by his gravity field, so Sally certainly can't feel Tom's gravitational attraction. This implies you don't have to compute anything.

5. Oct 16, 2004

### cepheid

Staff Emeritus
No silly...she feels some attraction, not her own weight! We all feel our weight all the time, and we don't classify it as anything out of the ordinary. So why would she suddenly start paying attention to it? The question implies that she feels some gravitational attraction other than her attraction to the Earth (her weight). They want you to compute her gravitational attraction to Tom, just to drive home the point that everything made up of matter in the universe is gravitationally attracted to everything else, but the effects in everyday situations (such as between people) are miniscule. Yeah ok, so she would never actually be able to feel said attraction, but you're being too literal...get into the spirit of the question! Didn't you ever get these silly problems in highschool..."ooh, I think there's some "attraction" between Tom and Sally...can you compute its magnitude?", says the physics teacher with a ridiculous grin on his face, thinking himself terribly witty.

As for the looking up part, they don't mean "straight up"! The question is trying to be melodramatic...she was glancing down distracted for a second (for whatever reason), and when she looked up again, there he was.

A more important question comes from this phrase: "If the attraction is gravitation, find its magnitude." What if it isn't? I think there's actually some chemistry going on here...

Last edited: Oct 16, 2004
6. Oct 16, 2004

### arildno

"As for the looking up part, they don't mean "straight up"! The question is trying to be melodramatic...she was glancing down distracted for a second (for whatever reason), and when she looked up again, there he was."
Isn't the usual sequence:
Suddenly catches sight of TOM, flustered, looks down, then helplessly, is drawn to look at him again??