Wanted: leson plan about student in trash can

[krhyou]

I have heard of a density, bouancy, or volume lesson in which a child submerges him/herself in a very large water filled trash can, the amount of displaced fluid is measured, and then I forgot the rest. Can't remember if this was to find just volume, or if mass was involved and density was found. Can anyone help me out? I teach middle school. Thank you!

[Unregistered]

The amount of water displaced has the same mass as the child.

Consider Newton's second law, as applied to the child standing on a weight scale. With N being the normal force (which is what a weight scale actually measures), m the mass of the child, and g = +9.8, we get ...

N(in air) - mg = ma = 0 (the child is not accelerating)

N(in air) = mg

When immersed in the water (d and V are the density and Volume of the child)...

N(in water) - mg + dVg = ma = 0,

N(in water) = mg - dVg.

Therefore,

dVg = N(in air) - N(in water),

d = [N(in air) - N(in water)]/(Vg)

So what does this all mean? Well, if you have the "weight" scale reading when the child is in air and in water, you can subtract the two, divide by the child's volume and g, and find the density of the child. This will indicate the fat percentage of the child if you look it up on a chart.

I suggest visiting physicsforums.com as well.

[krhyou]

That's a bit more complicated than what I had in mind. I know that the volume of the child is equal to the volume of the fluid displaced. I can then convert the child's weight in pounds to kilograms, and then divide to find density. Is there a written lesson plan for this? It should be fun to do----when the weather warms up! I'm in CA , and it is ONLY 70 degrees F here today!!! Thanks,,, hoping to hear from some of you!

[ploughlane]

http://www.rockyhill.org/news/Density/human_density.htm

I didn't do this experiment but my 6th grade class and I used the data to calculate the density of a child and then compared this value to other densities we had measured.

My class found it very amusing!

[krhyou]

thanks for your encouragement! I hope my students enjoy it! Any worksheets you can pass along? Krhyou

[Unregistered]

I made a boo-boo. The density in the equation refers to the fluid (1000 kg/m^3 for water). Instead of using the equation to find d, you use it to find V, the volume of the boy. From that you can find the density of the boy.

I know that the volume of the child is equal to the volume of the fluid displaced.

Yes, if the child is completely submerged. Measuring the amount of volume is sometimes a little tricky, which is why I offered the alternative derivation.

[ploughlane]

Check your yahoo email account!

[krhyou]

http://www.rockyhill.org/news/Density/human_density.htm

I didn't do this experiment but my 6th grade class and I used the data to calculate the density of a child and then compared this value to other densities we had measured.

My class found it very amusing!
Thank you so much, Ploughlane! Just opened the attachment, and it is exactly what I have been looking for! This is a great resource. I just joined, and will have to keep using it. I am so happy to have the lesson plan. You're the best! Karen

[krhyou]

I made a boo-boo. The density in the equation refers to the fluid (1000 kg/m^3 for water). Instead of using the equation to find d, you use it to find V, the volume of the boy. From that you can find the density of the boy.



Yes, if the child is completely submerged. Measuring the amount of volume is sometimes a little tricky, which is why I offered the alternative derivation.
Thank you for your efforts to help me. What do you teach? I got a great lesson plan for the activity if you are interested, I'll be happy to pass it along to you. Let me know. Karen