What Factors Determine Whether Objects Float or Sink?


Personal Lab 5

Background

Swimming is a skill many of us learn at an early age, and an activity many do to maintain good health. An important part of learning to swim is to "learn how to float" in the water. What causes us to float? Why must we learn how to float? Have you ever swum in the ocean, or the Great Salt Lake in Utah or the Dead Sea in the Middle East? Did you find it easier or more difficult to float in salt water?

What happens to a rock when it is dropped in water? When a block of wood is dropped in water what happens? What conditions determine whether an object floats or sinks? How can a ship constructed largely of metal float?

Purpose

1. To investigate the conditions under which an object floats or sinks.

2. To identify the factors that determine whether objects float or sink.

3. To understand why a ship can float.

Materials

Spring Scale waterpennies
paper clip, large (1) plastic beaker, 400 ml  35-mm film canisters (2)
spoonstring, 5" lengthscissors

Procedure

1. Remove the lid from one film canister. Poke holes in opposite sides of the canister near the top. Draw the two ends of the string through the holes to serve as a handle about 2.5 inches in length, so that the canister can be suspended from the Spring Scale. (Note: You could use the paper clip as a handle for your film canister instead).

2. Fill the other film canister with water and place it in the freezing compartment of your refrigerator for several hours. If you have no refrigerator available, ask your lab instructor to freeze a canister of water for you. You may perform the "ice" component of this experiment during your regularly scheduled Tuesday or Wednesday lab.

3. Use your Spring Scale to find the net force (in Newtons) acting on the film canister when suspended in air: 1) empty, 2) filled with water, 2) filled with ice, 3) filled with water plus various numbers of pennies (say, 5,10, etc. pennies). Create a Data Table and record your observations in your lab book. Your Data Table should have the following headings:

Contents
of Film
Canister		 Net
Force
in air
(N)		Net
Force
in
water
(N)		Bouy-
ant
Force
(N)		Volume
before
Object in
water
(ml)		Volume
while
Object in
water
(ml)		 Volume
of water
displaced
(ml)
empty        
water        
ice        
5 pennies        
10 pennies        
etc...        

4. Fill your 400 ml (milliliter) plastic beaker with a volume of water equal to 300 ml (3/4 full). (1 ml is one thousandth of a liter. Two liters of liquid make roughly a quart.) Be sure to read the ml scale on your beaker with your eyes horizontal with the beaker’s scale to insure an accurate measurement. Record the volume of water in the beaker in units of ml. (Use the spoon to retrieve objects from the beaker of water.)

5. Use your Spring Scale to find: a) the net force (in Newtons) acting on the film canister when suspended in water (not touching the sides or bottom of the plastic beaker): 1) empty, 2) filled with water, 3) filled with ice, 4) filled with water plus the same various numbers of pennies as before. Record your observations in the Data Table in your lab book.

6. Examine your data for patterns in the volumes, forces, etc.

Questions

1. What patterns, if any, did you notice in your data? Describe all trends or patterns you noticed in the volumes, differences in volumes, forces, differences in forces, etc.

2. From the patterns in 1), can you make a general statement about the factors which determined whether objects float or sink?

3. What did you observe about the floating or sinking properties for the canister when filled with ice? Was anything different from just the plain water-filled canister? If so, describe your observations.

4. For each experiment use the information in your Data Table to find the net upward force (i.e. the "buoyant force") that the water exerted on the canister.

5. Do you notice any patterns in the buoyant forces you found in 4) that you could use to predict whether an object will float or sink? If so, make a statement that gives a condition for an object to float.

6. Why can a ship made largely of metal float in water? Explain your reasoning.

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