SMS491/EDW742 - BUOYANCY LAB. Class activity (all groups):

Transcription

1 SMS491/EDW742 - BUOYANCY LAB Class activity (all groups): Using the foil and tape construct a vessel that will hold the most pennies and still float. (Pennies that cause the boat to sink do not count). You do not need to use all the material but you cannot use any additional supplies. The vessel that carries more pennies wins! Materials: A piece of aluminum foil (10X10 cm) Scotch tape (30X1.9 cm) Pennies (50) A container with water What considerations went into the design of your boat? What was the most successful design?

2 The concept of buoyancy When an is fully immersed in a fluid the amount of water it displaces is equal to its own volume. There are two forces acting upon it: 1. An upward force the buoyancy force 2. A downward force- the gravity force The buoyancy force arises from an imbalance in the pressure exerted on the by the fluid; because pressure increases with depth, the bottom of the immersed experiences a higher pressure compared to its top. This force is equal to the mass of the volume of water displaced (Archimedes principle). From Newton s second law of motion the two forces can be written as: F and F buoyancy gravity = m = m fluid g = ρ g = ρ fluid V V displaced g g Where m fluid and m is the mass of the fluid displaced and the, respectively, g is the gravitational acceleration constant, ρ fluid and ρ are the densities of the fluid and the body respectively and V displaced and V are the volumes of the displaced water and body, respectively (when the body is fully immersed V displaced = V and recall m = ρv). The difference between the forces will determine if the body will sink, rise or remain neutrally buoyant Δ F = F gravity F buoyancy = V g ( ρ ρ fluid ) When

3 ΔF > 0 the body will sink ΔF < 0 the body will rise ΔF = 0 the body will remain neutrally buoyant (ρ objec = ρ fluid ) Station 1: Materials: A plastic boat A weight or large rock A container with water Lab tape Materials for Station 1 You have a large rock on a boat that is floating in a pond. If you throw the rock into the water and it sinks, what will happen to the water level of the pond (will it increase, decrease, or remain the same?)- predict, observe (use the tape supplied to you to mark the water level before and after you let the rock sink) and explain.

4 EXPLANATION: When the rock is floating on the boat there is no net force acting on it F buoyancy = F gravity and hence m g = ρ V g = ρ fluid V displaced g Since ρ < ρ fluid (it is floating), the volume displaced by the rock floating on a boat must by larger than the volume displaced by it when it is fully immersed in water

5 and therefore the water level of the pond will go down when you throw the rock and it sinks to the bottom. Station 2: Materials: Archimedes ball Syringe A piece of tubing Scale Caliper A container with water A container with an unknown solution Materials for Station 2 1. You are designing a neutrally buoyant float (plastic ball). You want it to remain under water such that only the green stopper is just above the water. Calculate the amount of ballast water needed for the float. Check your prediction, using the ball and syringe provided to you. 2. Now test your float in an unknown liquid. What does your observation suggest about the density of the other fluid compared to that of water?

6 EXPLANATION: To approach this problem you first need to calculate the density of the ball which turns out to be 0.7 g/cm 3 and is hence smaller than the density of water (0.7 g/cm 3 ). Since the volume of the ball remains constant the only way to make this ball neutrally buoyant is to add mass by pulling out air (with the syringe) and replacing it with water. The mass of the ball is g and its volume is 176 cm 3. Thus 54 g (which equals 54 ml of tap water) have to be added to make its density equal to that of water (i.e., neutrally buoyant).

7 When the ball is placed in the unknown solution more volume of liquid is required to make it neutrally buoyant indicating that the unknown solution (water + corn syrup) is denser than water. Station 3: Materials: Convection set-up (science kit.com) Food-coloring (2 colors). Ice Materials for Station 3 1. Fill the apparatus with water. 2. What direction of flow would you expect for the water in the apparatus if the left column of the apparatus would contain hot water and the right column would contain cold water? 3. Put ice in one basin and hot tap water in the other. Add drops of dye to the two columns (different color to each column) and observe whether they circulation you observe agree with your prediction. 4. What if you warm (cool) only one column of the apparatus? Try it. EXPLANATION:

8 When one column contains hot water and the other contains cold water a pressure gradient forms within the apparatus, causing water to flow from high pressure (cold water) to low pressure (warm water; remember that pressure is proportional to density at a given depth). Since the cold water is denser it will move along the lower connecting tube (to the left) and the hot water will move along the upper connecting tube (to the right). If you cool or warm only one column you ll see the same effect though it may not appear as dramatic because the pressure gradient will be smaller. Station 4 (time permitting). Galileo's thermometer. Can you explain how this thermometer works (each glass ball has constant volume and mass of colored fluid within it) How does its buoyancy change? Test it by putting it in a tub with hot water first, then in a tub with cold water. EXPLANATION: Since the balls inside the thermometer have constant volume (glass is highly incompressible) and mass their density remains constant. What changes is the density of the surrounding fluid as a result of heating or cooling. The change in relative density between the balls and the surrounding fluid causes them to rise and sink in the column and rearrange according to their densities. The temperature is read from the metal disk attached to each ball; the lowest glass ball on the top set indicates the temperature.

9 Homework: Journal prompt: what ocean-earth system processes can be explained using the convection demo you used in the lab? Pick one of these concepts and develop a lesson plan that contains background information on the concepts and a class activity that demonstrates convection, using the following materials 2 Empty plastic vials (can use pill vials or film canisters) with two holes drilled on the top Food-coloring (2 colors) Tub with water Ice Screws Lab tape Materials for homework assignment

10 Karp-Boss, Weller and Boss, 2007 This document was last edited on 2/7/2007