Sunday, February 05, 2006

Will a grape float or sink?

Bouyancy and Density

Will a grape float or sink in water? How about salt water? Learn about bouyancy and Archimedes' principle with this fun experiment.

What you will need:

    1) Two Clear Glasses

    2) Warm water (Hot faucet water will suffice)

    3) Grapes

    4) Salt

    5) spoon

What to do

    1) Fill the glasses

    2) Disolve about three teaspoons of salt in one glass. Be sure to stir the salt into the water. Using warm water improves the disolution. 3) Place one grape in each glass. Do they sink or float?

What Happens

You should see the grape in the fresh water glass sink to the bottom and the slat water grape float.


Recall Archimedes’ principle: A floating object will displace a volume of fluid that has a weight equal to the weight of the object that is floating. If the object weighs more than its own volume in fluid, then it will sink.. In other words, a floating object is less dense than the fluid, but an object that is denser than the fluid will sink and the bouyancy force felt by an object is equal to the weight of the fluid displaced.

The grape is denser than fresh water, so it sinks in the fresh water glass.However, when you disolve salt in water, you increase the water's density to a value greater than the density of the typical grape - so the grape floats in salt water.

Moreover, the level the grape floats at is determined by the density of the water - if the water is very dense, only a very small part of the grape needs to be submerged to generate a sufficient bouyancy force to hold up the grape's weight, but if the water is only slightly denser than the grape, it will take most of the grape's volume to generate the required bouyancy force. Thus, you can use the height of the grape to estimate the amount of salt in the water.

Starting with fresh water, sink the grape in the glass. Slowly disolve a small amount of salt in the glass. Slowly increase the amount of salt until the grape rises to the top. Now, as you add more salt, the grape should rise upwards. There is a maximum amount of salt you can disolve into the water, so you might not be able to lift your grape a long way out of the water, but with care, you should be able to observe the grape's upwards motion.

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