By: Miss Casie J. Varner
What is Density?
Density consists of how an object's volume and mass are related. Volume
consists of the how much space the object fills and mass is how much matter the object is made of. Density is calculated by dividing the object's mass by it's volume (D=M/V). Thus, density can be altered by changing an object's mass (adding to it or taking away from it) or by changing its volume (increasing or decreasing the amount of space taken up by the object).
What is the Principle of Buoyancy?
The principle of buoyancy is related to water's property of "uplift". This is the upward push of water upon an object that is placed on it. Gravity counters this effect by pulling the object down while the force of buoyancy is pushing the object up. If an object's density is more than that of the liquid, then it will sink (force of gravity will overcome force of buoyancy). If an object's density is less than that of the liquid, then it will float (force of buoyancy will overcome force of gravity).
When scientists want to know if a substance or object will sink or float, they check its density. The density of a substance is the weight of a standard amount of the substance. The amount that scientists usually use is the cubic centimeter or cm3. A cubic centimeter is a little cube that is one centimeter long, 1 centimeter wide and 1 centimeter deep.
If a cubic centimeter of a substance weighs more than a cubic centimeter of water, the substance will sink. If a cubic centimeter of a substance weighs less than a cubic centimeter of water, the substance will float.
The Float or Sink – What do you Think!? Activity will give you the chance to experiment with certain objects to see whether or not they will float in water without having to actually place them in water to see if they actually do float or sink.
Materials You Will Need:
- Pencil
- Masking tape
- Metric ruler
- Two tea light candles
- Magic Dough/Clay
- Water
- Cups
- Paper Towel
Procedure:
- Tape each end of the pencil down to the desk.(Fig1) Roll two small pieces of tape so that the sticky side is out.
- Stick each piece of tape to the opposite end of the ruler. Remove both candles from their metal containers. Place an empty metal container on each piece of tape. Lay the ruler on the pencil so that it is as balanced as possible.(Fig2)
- Look at the number on the ruler that is directly over the center of the pencil. Write this number down as your balance point.
My balance point is ________________.
- Carefully place one of the candles back into its metal container on one end of the ruler. Make sure the same balance point is directly over the center of the pencil.
- Slowly and carefully, add water to the container on the other end of the ruler until the container is full. Use a paper towel to clean up any spillage.
- Which is heavier, the wax or an equal amount of water?
Do you think wax will float or sink in water? Why? Try it and see by filling up your
bowl half way with water and then place the wax in the bowl of water.(Fig3)
- Reset your ruler on the balance point with an empty container on each end. This time, instead of putting wax in one of the containers, put Magic Dough/Clay in the container until it is filled. Place your ruler on the balance point and slowly add water to the empty container at the other end.(Fig4) Use a paper towel to clean up any spillage.
- Which is heavier, the Magic Dough/Clay or an equal amount of water?
Do you think Magic Dough/Clay will float or sink in water? Why? Try it and see by
placing it in the bowl of water.(Fig5)
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Estimated Time around 20 min.
Challenge: Instead of using water, try using cooking oil or vegetable oil. Will the objects that sank in the water still sink now that it is in the oil? Will the objects that floated in the water still float now that it is in the oil?
Useful Hints: Make sure that your balance point stays the same throughout the entire activity. If the balance point changes then your experiment will not be done precise. When you look to see which object is heavier, make sure that you have the same balance point. If your ruler is showing you a different balance point then you need to move the ruler so that your original balance point is on the pencil. Having your original balance point will give you the right answer as to whether your object will float or sink.
Useful Links:Link