EXPERIMENT 8 BUOYANT FORCES

Transcription

1 EXPERIMENT 8 BUOYANT FORCES INTRODUCTION: The purpose of this experiment is to determine buoyant forces on submerged solid objects, and to investigate the dependence of buoyant forces on volumes and masses of submerged objects BACKGROUND: When a solid objects submerged in a fluid (gas or liquid), an upward force is exerted by the fluid on the object. This force is called the buoyant force (B). The magnitude of the buoyant force always equals the weight of the fluid displaced by the object (Archimede s Principle). In other words, B = ρ f Vg (1) where, ρ f = Density of fluids (mass/unit volume of fluid) V = Volume of the solid object g = Gravitational acceleration (9.81 m/sec²) Let us examine the external forces acting on an object submerged in a fluid (see figure 1). The object is supported by a string attached to a balance. Assuming the system is in equilibrium, then: B = mg T 1 (Eq. 2) where T 1 = Tension in the string (weight of the object when submerged). If the same object is weighed in air and assuming no buoyant force due to air: mg = T 2 (Eq. 3) From equation 2 and 3 one can find that, B = T 2 T 1 = (weight of the object in air) (weight of the object in fluid) T 2 T 1 Spring balance Suspended sphere (Eq. 4) mg mg B Container filled with liquid

2 Figure 1: Setup and Analysis of Buoyant Forces The apparatus shown in figure 1 will be used for this experiment. The spring balance provided has a special hook attached to the bottom. Masses to be measured should be attached to this hook, and their weight should be read from the spring balance scale. THE EXPERIMENT: 1 Experimental Apparatus: The apparatus for this experiment consists of: Spring balance, masses to be measured, and beaker, Verniercalliper 2 Experimental Procedure: A. Measurements for Different Spheres With the Same Volume: Spheres of different masses but of equal volumes are provided for the purpose of this study. Prepare a table to record your results as you proceed. Measure the dimensions of these spheres and be sure that their volumes are almost the same. Use a vernier for these measurements. Attach one of the spheres to the spring balance. Measure the mass of the sphere in air. Continue to measure the masses of other spheres in air. Submerge one sphere in liquid measure its mass while in the liquid. Do the same for other spheres and find their masses while in liquid. Use equation # 4 to determine the buoyant force in each case. Record your results and include units. B. Measurements with Different Masses: Another set of spheres of different volumes and masses are provided for the purpose of this part of the experiment. Again, prepare another table to record the results of this part and proceed as follows: Use a vernier to measure the dimensions of all masses ( except those that were used in part A). Find the volume of each mass. Measure the masses of the spheres in air. Submerge one sphere in liquid measure its mass while in the liquid. Do the same for other spheres and find their masses while in liquid. Use equation # 4 to determine the buoyant force in each case. Record your results and include units. Use graph paper to plot buoyant force versus volume of submerged object for each liquid. DATA ANALYSIS:

3 Now with all these data at hand, you should be able to answer the following questions: What conclusion could by reach on the basis of the results of part A Do you think that the above conclusion would be reached if you use another liquid? Explain. Using the results of part B (graph), determine the density of water.

4 PHY 1400 LABORATORY REPORT EXPERIMENT 8 BUOYANT FORCES NAME:. DATE:. SECTION:. THIS PAGE NEEDS TO BE DONE AT HOME BEFORE COMING TO THE LAB. SESSION 1. EXPERIMENTAL PURPOSE: State the purpose of the experiment.( 5 points ) 2. EXPERIMENTAL PROCEDURES AND APPARATUS: ) Briefly outline the apparatus used and the general procedures adopted. (5 points

5 3. RESULTS AND ANALYSIS A. Measurements for Different Spheres With the Same Volume: Data (25 points ) Data Table I: Object Measured Length radius Volume (cm 3 ) air (g) fluid (g) Buoyant force (N) Calculations Show your work for one of the objects measured (5 points)

6 B. Measurements with Different Masses: Data (25 points ) Data Table II: Mass Measured Length radius Volume (cm 3 ) air (g) fluid (g) Buoyant force (N) Calculations Show your work for one of the objects measured (5 points) GRAPH (15 points) QUESTIONS (15 points)