Physics Homework Question

[Joszer]

This has stumped me and 2 other people who have also tried to solve this problem. My instructor was about to explain it but just left us with we can get it through the equation of Buoyant force Bf=pgv. Here is the actual question from my book.



Quote:
An object has a weight of 8.0N in air. However, it apparently weighs only 4.0N when it is completely submerged in water. What is the density of the object?

I have no idea how your supposed to figure this out, my and my friend have been trying to figure this out for over an hour now and we dunno where to start with it, since all your apparently given is the Buoyant force which comes out to be 4N and that leaves you with two unknowns, p and v.

If anyone can help me that would be great cuz Ive got a test coming up on Monday and something this confusing is sure to be on it.

Thanks in Advance

[Toucan]

Unable to answer your question for you.
But I'm sure you will find an answer here:-

http://www.physicsforums.com/

[Modred]

Hmm, could you explain what p and v are in the equation? G is the downward force of gravity, correct?

[Joszer]

yeah you're right, P is density and V is volume

[RedRagToAnOrc]

This has stumped me and 2 other people who have also tried to solve this problem. My instructor was about to explain it but just left us with we can get it through the equation of Buoyant force Bf=pgv. Here is the actual question from my book.



Quote:
An object has a weight of 8.0N in air. However, it apparently weighs only 4.0N when it is completely submerged in water. What is the density of the object?

I have no idea how your supposed to figure this out, my and my friend have been trying to figure this out for over an hour now and we dunno where to start with it, since all your apparently given is the Buoyant force which comes out to be 4N and that leaves you with two unknowns, p and v.

If anyone can help me that would be great cuz Ive got a test coming up on Monday and something this confusing is sure to be on it.

Thanks in Advance

I'm no Physics genius, being 15, but isn't there any way you could work out one of said unknown values using the difference between the weight in air and weight in water? The fact that it gives you two such values suggests that you should be able to work something out using them.
I don't know, something involving Gravitational Field Strength? Just a shot in the dark.

[FallenLord]

B = pvg, where B is the upward buoyant force (the 'apparent' weight in water), p is the density of water, and v is the volume of the object.

B, p, and g are all known. Solve for the object's volume.

The weight of the object is known, and its volume is now known. Solve for the object's density. (p = w/v/g)

I checked my physics book to make sure that the p in the equation refers to the density of the fluid, since otherwise B would simply be the object's weight. :)

[Modred]

I believe Fallen Lord has it, with perhaps one exception. Density is mass per volume; there's no gravity involved. ;)

[Yoda]

F(wt) = mg

8 = m(9.8)

m = 0.816 kg

I can say that much, but the formula looks like a Year 12 Physics formula, I don't know that one yet.

[Modred]

Yoda, what you posted is a variation of F=ma (force = mass x acceleration), the most basic form of computing force on an object. For buoyant force, an object is suspended in a fluid and the downward force is thus changed. The formula does almost the same thing, but there's one extra part, the density of the fluid.

Just thought I'd make sure and clear that up a little.

[Joszer]

wiat if P is already known....i have to solve for P. it's lame though because B=pvg and p= (m/v) so you get B = (m/v)vg. The V's cancel out and you get B = mg which would be weight. This is confusing me lol. I remeber doing a problem like this in high school but i totally forgot how lol.

[FallenLord]

Equations:
B = p*v*g (where v is the volume of the object being submerged and p is the density of the fluid)
p = m/v
w = m*gKnowns:
B = 4 N
p = 1 kg/L (density of water)
g = 9.81 m/s^2
w = 8 N (weight of object)
B = p*v*g
v = B/p/g
v = (4 N)/(1 kg/L)/(9.81 m/s^2)
v = 4/9.81 L

w = m*g
m = w/g

p = m/v
p = (w/g)/v
p = (8 N)/(9.81 m/s^2)/(4/9.81 L)
p = 2 kg/L

[GrassDragon]

FallenLord has it. Your problem is the buoyant force (B) is equal to the weight of the water displaced by the object. B=pvg, but here the p refers to the density of the water that takes up the same volume as the object. Since you know the buoyant force, you can solve for the volume of the object, and then the density of the object is easily solved using the weight.

[Modred]

Gosh, I feel silly. I was wondering why Fallen_Lord had gravity in his equation for finding density. Density = mass / volume. Force = mass * accleration, in this case modified to read Weight = mass * gravity, so mass is weight / gravity. I feel very silly for not seeing that.