Science Buddies: "Ask an Expert"

Hello,
My daughter wants to conduct an experiment on buouyancy. She needs to measure the force in newtons exerted by a metal cube before it is submerged in water and after it is submerged in water. I was looking for a force guage, dynamometer but the ones out there are very expensive. Could you please guide me where I can buy a good force meter at a reasonable price.
Thanks
ami

I haven't researched the cost of force meters however a force meter may not be necessary for your experiment. We know a person's weight on earth is W = F = mg. Where W is weight, F is force, m is mass in kg and g is graviational acceleration which is 9.8m/sec^2. So with a weight scale you can get weight, W in lbs and convert that to force, F in newtons using the following conversion factor 1 pound = 4.448 newtons. So that helps with the the unsubmerged measurement next step is to find a waterproof weight scale!?! Perhaps you can try putting the scale in a big storage size zip lock bag.

Regarding this experiment of my daughter she framed her question as

Would an object weigh the same in water as on land?

Is this a valid research ?
Her variables are
1. Manipulated variable - Weight of the object
2. Responding variable - Amount of water displaced
3. Control - Amount of water in the cup.

I feel that she needs to modify it to see how density of an object affect the amount of water displaced. She is in 5th Grade and at her level the question she chose is very preliminary in my opinion.
Please advice

anuami wrote:Would an object weigh the same in water as on land?

There is an inherit problem with the question. "Would an object totally submerged in water weigh the same as it does in air?" is a question that eliminates some of the problems. The condition: "in water" could be floating or submerged or partially submerged. The condition: "on land" could be completely burried in dirt, resting on dirt, partially submerged in dirt.

By commonly accepted physics definitions, the "mass" of an object will stay the same as long as it stays intact and nothing is added to the object, so the weight (gravitational force acting on a mass) will be the same as long as you are on or near enough to the same planet. If your project is actually trying to verify these commonly accepted physics definitions, then the problem is beyond this grade level as it will involve coming up with extremely accurate measuring devices in an attempt to find differences that nobody else has found.

The area of interest is more likely to be gravitational force less boyancy force and with those kinds of investigation, the density and other properties of the object (shape, absorbency, etc) will affect the result.

Rather than a "force meter", try searching for a "spring scale."

I see some on educatorsoutlet.com with scales in grams and Newtons for less than 3 bucks apiece (before shipping). sciencekit.com has some nicer ones for 21 dollars each.

I'm a homeschool mom and have ordered from both of these companies sucessfully before. There are LOTS of other places to try as well.