Gravity, force, and buoyancy

Could someone please give me an accurate description of gravity, force, and buoyancy, and the relationships between them?

F=ma, or Fg=G*((m1*m2)/(d*d)) ? Which is used in which situation?

Does buoyancy affect weight, or merely counteract the effect of weight?

Stuff like that would be useful.

Thanks.

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enigma

Staff Emeritus
Gold Member
Both gravity and bouyancy are forces.

Gravity causes all mass in the universe to be attracted to all other mass.

Bouyancy is a force which is caused by the pressure on the lower portion of an object being higher than the portion on the top. The result is a force which points in the opposite direction of gravity.

It doesn't actually change the weight due to gravity, but it lessens the net force acting on a body.

Buoyancy is NOT one of the four forces, correct?

Buoyancy itself does not reduce a thing's weight?

HallsofIvy

Homework Helper
F= GmM/r2 is the (Newtonian) formula for calculating the gravitational force between two objects of mass m and M at distance r apart.

F= ma is the formula for calculating the force (of any kind) necessary to accelerate an object of mass m at acceleration a.

"Bouyancy" is the upward force a liquid imparts on an object in the liquid: it's essentially the force the object has to exert (downward) to push the liquid out of the way. Whether it reduces the weight depends on exactly what you mean by weight. "Weight" normally is the force of gravity on an object- though sometimes, for convenience we take it as the NET downward force- if you do that, then bouyancy does reduce the net force by imparting an upward force.

No, bouyancy is NOT one of the four fundamental forces: gravitational, electro-magnetic, weak nuclear, and strong nuclear forces.

krab

Buoyancy is essentially gravity. A cork in water is forced up by the fact that the water is trying to fall down. The cork is trying to fall down too, but much more weakly than the water, so it loses out and lets the water fall down, thus forcing the cork up.

This is most easily seen by forcing the cork down in water in a smallish container. Forcing the cork under water visibly raises the water level. Thus work is reqired to push the cork down, because you are effectively lifting water by doing this.

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