If the object floats, the buoyant force will be equal to its

lucynemes86

1. Homework Statement

If an object floats, the buoyant force will be equal to its:

a. Mass c. Specific Gravity

b. Density d. Weight

2. Homework Equations

3. The Attempt at a Solution
Is it weight?

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Doc Al

Mentor
What does it mean to float? What must be the net force on a floating object? What forces act on a floating object?

(Use those questions to describe the reasoning behind your answer.)

Bystander

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What are the units?

lucynemes86

I got no units. Nothing. Just wondering when an object floats, is it because the buoyant force is equal to its mass, specific gravity, density or weight?

Doc Al

Mentor
I got no units. Nothing. Just wondering when an object floats, is it because the buoyant force is equal to its mass, specific gravity, density or weight?
Well, which of those possible answers has the same units as buoyant force? (That's Bystander's tip!)

lucynemes86

Well, which of those possible answers has the same units as buoyant force? (That's Bystander's tip!)
Weight? (Newton)

Doc Al

Mentor
Weight? (Newton)
Right.

But it sounds like you need to learn more about buoyant force. This particular question can be answered without knowing anything (except units), since only one of the choices is even a force. But they won't all be like that.

lucynemes86

Right.

But it sounds like you need to learn more about buoyant force. This particular question can be answered without knowing anything (except units), since only one of the choices is even a force. But they won't all be like that.
Thanks. I'm just new to physics. I had physics in high school over a decade ago but I don't remember much, even though I did well. I'm taking a class in college now (getting another degree) and I'm just having trouble with simple questions! :/

lucynemes86

Right.

But it sounds like you need to learn more about buoyant force. This particular question can be answered without knowing anything (except units), since only one of the choices is even a force. But they won't all be like that.
Also, a completely submerged body displaces a volume of liquid equal to its weight too, right?

Doc Al

Mentor
Also, a completely submerged body displaces a volume of liquid equal to its weight too, right?
Are you asking if the weight of the liquid displaced equals the weight of the object? Not in general.

haruspex

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2018 Award
I got no units. Nothing. Just wondering when an object floats, is it because the buoyant force is equal to its mass, specific gravity, density or weight?
I think ByStander meant dimensionality, not units. If you know the units that will tell you the dimensionality, but not the other way around.
Force, mass, density all have different dimensionality. It only makes sense to add items or equate items of the same dimensionality.
Weight is a force.

• Doc Al

phinds

Gold Member
Also, a completely submerged body displaces a volume of liquid equal to its weight too, right?
Think of a large Styrofoam beach ball. It would displace an amount of water that was quite heavy. Is the beach ball that heavy?

haruspex

Homework Helper
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2018 Award
Also, a completely submerged body displaces a volume of liquid equal to its weight too, right?
Only if the only other force acting on the body is gravity, i.e. it is floating with neutral buoyancy.
Otherwise, all you can say is that it displaces its own volume.
If it is tethered to the bottom then the buoyant force is also having to balance the tension in the tether.
If it is resting on the bottom then the buoyant force is assisted by the normal force.

lucynemes86

Think of a large Styrofoam beach ball. It would displace an amount of water that was quite heavy. Is the beach ball that heavy?
Right, not quite heavy... So, it displaces a volume of liquid equal to its density or buoyant force? Maybe to its own volume?

lucynemes86

Only if the only other force acting on the body is gravity, i.e. it is floating with neutral buoyancy.
Otherwise, all you can say is that it displaces its own volume.
If it is tethered to the bottom then the buoyant force is also having to balance the tension in the tether.
If it is resting on the bottom then the buoyant force is assisted by the normal force.
Thank you!!

phinds

Gold Member
Right, not quite heavy... So, it displaces a volume of liquid equal to its density or buoyant force? Maybe to its own volume?
Which one do you think? How could it be density? Do you understand the use of units in figuring out this kind of thing?

haruspex

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Gold Member
2018 Award
Thank you!!
But note that was for a completely submerged body. A floating ball of styrofoam will not be completely submerged. What will the displaced volume correspond to in this case?
One of your answers in post #14 was sort of correct, that the weight of the water displaced is equal to the buoyant force, but that is true whether the object is floating, resting on the bottom, or tethered. In the specific case of a floating body, what other force is the buoyant force equal to?

lucynemes86

Which one do you think? How could it be density? Do you understand the use of units in figuring out this kind of thing?
Volume. Sorry. I'm not a genius. Had physics over a decade ago and don't remember much.

phinds

Gold Member
Volume. Sorry. I'm not a genius. Had physics over a decade ago and don't remember much.
OK, well you didn't answer my question about units so let me be specific. NEVER give an answer to a physics question unless you are confident that you have the units right. That catches LOTS of the mistakes that we all make when we are starting out.

lucynemes86

But note that was for a completely submerged body. A floating ball of styrofoam will not be completely submerged. What will the displaced volume correspond to in this case?
One of your answers in post #14 was sort of correct, that the weight of the water displaced is equal to the buoyant force, but that is true whether the object is floating, resting on the bottom, or tethered. In the specific case of a floating body, what other force is the buoyant force equal to?
Thanks again...

In case of a floating object buoyant force equals weight right?

Doc Al

Mentor
In case of a floating object buoyant force equals weight right?
Right.

haruspex

Homework Helper
Gold Member
2018 Award
Thanks again...

In case of a floating object buoyant force equals weight right?
Yes.

"If the object floats, the buoyant force will be equal to its"

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