I read a newspaper article which discussed effects of the Moon to human lives. The article was totally uncritical but it had one interesting claim about physics. The writes had consulted a physicist who said that there are measurable changes in gravitational force exerted by the Moon on the Earthly objects as relative position between the Moon and the Earth vary. According to this physicist a person could detect these changes in his weight using a very precise balance.
Occasionally some clever students ask me if the gravitational pull of the Moon should be included in free-body diagrams. The newspaper article implies that it could be done. I don't agree.
The gravitational pull of the Moon cannot be measured on the Earth using a balance; precision of the balance has nothing to do with it. The Earth is in free fall toward the common center of mass in the Earth-Moon system. For instance, weight of an object (by weight I mean a balance reading) inside a satellite is very close to zero if the satellite is in free fall (or free float) and the gravitational effects due to satellite itself are negligible. OTOH changes in the gravitational force due to the Moon on the Earth can be observed in tidal effects.
Do you agree with my analysis? Have I missed something?
Regards,
Antti Savinainen
Kuopion Lyseo High School/IB
Finland
Homepage: <kotisivu.mtv3.fi/physics/>