Unit 6: Properties of Matter

Chapter 17: Properties of Matter

Lab 17.3—Buoyancy of Fluids

In this investigation, you will investigate how the shape of an object influences whether it sinks or floats, and you will explore the relationship between the weight of an object and the weight of the water it displaces.

In the previous investigation, you learned that a solid material will float if it is less dense than the liquid in which it is immersed, and sink if it is denser than the liquid. You may have noticed, however, that ships are often made of steel, which is obviously denser than water. So how does a steel boat float? In this investigation, you will experiment with modeling clay to discover how and why boats can be made of materials that are denser than water.

Procedure/Data/Results:

  1. Before molding your stick of clay, find its density. Find its mass using the electronic balance (tare balance weighing paper first). Use the formula Volume = length × width × height to calculate its volume. Then calculate its density.

mass ______(g) volume ______(mL) density ______(g/mL)

  1. Predict whether or not your stick of clay will sink or float when placed in water and tell what led you to that prediction.
  1. You will now see if the clay floats or sinks AND collect the water it displaces at the same time. Using the large displacement tank as an overflow can, fill the tank with water and let the excess water run out the spout. Obtain a small dry beaker, find its mass, then place it under the spout after water has stopped dripping from the spout.

mass of small dry beaker ______(g)

  1. Place your stick of clay in the displacement tank. As soon as the water stops flowing, remove the beaker from under the spout and set it aside. Quickly remove the clay from the tank and set it on paper towel to dry. Did your clay sink or float?
  2. Find the mass of the beaker + the displaced water. ______(g) Now subtract the mass of the beaker to find the mass of just the displaced water. ______(g)
  3. Pour the displaced water into a clean, dry graduated cylinder to find its volume.

______(mL)

  1. Mass and weight measure two different properties of matter. Mass refers to how much matter the object contains. Weight measures the gravitational pull between the object and (in our case) Earth. The gravitational force between a 1-kg object and Earth is 9.81 newtons, and a 1-g object’s weight on Earth is 0.00981 newtons. You can use this information to calculate the weight of your displaced water from the mass which you found above. Calculate the weight of the water displaced. Using your data from Step 5, multiply the mass of water displaced by 0.0098 N/g to find the weight of water displaced in newtons.

weight of water displaced ______(N)

  1. Now find the weight of your stick of clay. Note that you have its mass in grams in Step 1.

weight of stick of clay ______(N)

  1. You know that steel can be fashioned into a shape that floats. Can you do the same thing with clay? For this part of the investigation, you must use ALL of your clay. Mold it into a shape that you believe will float.
  2. When you are ready to test a shape, lower it into a container of water approximately three-fourths full. If the clay sinks, retrieve it immediately and dry it with a paper towel. Avoid mixing water into your clay, or it will get very slimy. When your clay is dry, modify your “boat” and try again.
  3. When you have successfully molded a boat that floats, take it out of the water and dry it with a paper towel. Then, prepare your displacement tank just as you did earlier (overfill it with water and let the excess run out the spout). Be sure to use the same small beaker as before to catch the water displaced; place this beaker under the spout of the displacement tank after water has stopped dripping from the spout. Be sure the small beaker is dry when you begin. Carefully place your boat in the displacement tank. Avoid making waves. When the water stops flowing, move the beaker away from the displacement tank spout. Retrieve your boat and set it aside to dry.
  4. Find the mass of the beaker + the displaced water. ______(g) Now subtract the mass of the beaker to find the mass of just the displaced water. ______(g)
  5. Calculate the weight of the displaced water from its mass. ______(N)
  6. Pour the displaced water into a clean, dry graduated cylinder to find its volume.

______(mL)

  1. When your boat is dry, first measure its mass, then calculate its weight.

mass of boat ______(g) weight of boat ______(N)

  1. Organize your data from above into the table below.

Weight of clay (N) / Volume of displaced water (mL) / Weight of displaced water (N)
When it was in stick form / Step 8: / Step 6: / Step 7:
When it was in boat form / Step 15: / Step 14: / Step 13:
  1. Did the weight of clay change during the investigation? Give a reason for your answer.
  1. Which displaced more water, the stick of clay or the clay boat?
  2. Which weighed more, the stick of clay or the water is displaced?
  3. Which weighed more, the clay boat or the water it displaced?
  4. When you changed the shape of your clay, what happened to the amount of water it displaced?
  1. What is the relationship between the weight of a sunken object and the weight of water it displaces?
  1. What is the relationship between the weight of a floating object and the weight of water it displaces?
  1. If you had a clay boat that weighed 100 newtons, how many newtons of water would it displace?