# Homework Help: Calculate the buoyant force of a liquid

1. Sep 21, 2009

### semc

Hey guys during my lab i am required to calculate the buoyant force of a liquid so i have to submerge the object in the liquid and take the reading on the balance. after that i have to lower the object to the bottom of the beaker and take the reading again. How are the data going to help in calculating the buoyant force?

2. Sep 21, 2009

### willem2

You put a beaker of liquid on one of the scales of a balance, and weigh the beaker

1. while an object suspended from a string is completely submerged in the liquid
2. with the same object on the bottom of the beaker, no longer suspended from the string.

If the buoyant force of a liquid pushes an object up, the object must also push down on the liquid.

What are all the forces acting on the beaker + liquid in the first weighing? whats the difference in the second?

3. Sep 21, 2009

### semc

the difference at the bottom is that the object has no upward force right?so the difference is the buoyant force?

4. Sep 22, 2009

### willem2

No, the buoyant force is still the same as long as the object is submerged.
the difference is what the tension on the string was.

I'm not sure why the experiment is done in this way. the easiest way to get the
buoyant force would be to weigh the beaker with the object submerged in
it haning from a string and weigh just the beaker and liquid.
The difference between those gets you the buoyant force.

5. Sep 22, 2009

### semc

Sorry i really need help on this one can anyone tell me why the reading on the balance changes when the bead is initially lowered into the fluid? The balance measure the normal force acting on the beaker right? So when the bead is submerge in the fluid the weight of the bead adds to the weight of the fluid so the normal force is larger so the reading on the balance increases right? So why is it that when the bead touches the bottom of the beaker the reading increases?