PHYS1002 Physics (Fundamental) Lab Session, e.g. 2FND Date Group number/ Data set 7FND 25/3/2021 8 Lab 4: Buoyancy The logbook consists of three parts: Part A: Measuring Densities Part B: Buoyant Force Design Task Completion Experiment Part A – Measuring Densities Aim: To determine the density of an ‘iceberg’ (actually a metal sphere). This part should not take longer than 60 min. Logbook 4.1 When objects have simple geometric shapes we can often use basic mathematical formulae to calculate their volume. However, in the real world this is not always the case. Detail a step by step method on how you would find the density of an irregularly shaped object? First, balance is used to measure the mass of irregular objects. In the second step, prepare the container with volume mark and pour enough water into the object. In the third step, the irregular object is immersed in the second step, and the height of the object before and after being placed in the container is checked. After calculation, the volume of the irregular object can be found. Finally, because we know that the density of something is mass divided by "d=m/v" volume, we get the density value of irregular objects. Logbook 4.2 Watch the lab video to answer this section. For reference, here are the timestamps: 0:11 Checkpoint 1 summary 0:42 Hydrometer 1:16 The Titanic Fluid In the video, you saw an “iceberg” being placed in some liquids. What does the iceberg do in each of the liquids? Based on these observations predict a ranking for the densities of the `Titanic fluid’, fresh water and iceberg. 1 PHYS1002 Physics (Fundamental) When the density of an object is greater than that of a liquid and gravity is greater than buoyancy, the object sinks. When the density of an object is equal to the density of a liquid and gravity is equal to buoyancy, the object floats. When the liquid discharged from the object is small, the density of the object is smaller and the density of the liquid is higher. It can be seen from the video that when the iceberg is in the water, the ball sinks, and in the blue liquid, the iceberg is exposed to the water without sinking to the bottom, but reveals a small part of its volume. Therefore, it can be concluded that the density of blue liquid is greater than that of water, and the density of iceberg is greater than that of water but less than that of blue liquid 2 PHYS1002 Physics (Fundamental) Logbook 4.3 Open your dataset and navigate to sheet C1 on the bottom left corner. The mass of the “iceberg” is measured using a beam balance and is provided in cell C15 (Note a beam balance is a device for measuring mass by balancing weights at two ends). The container used to measure the water volume displaced has an interval of 20 mL. Calculate the volume displaced by the iceberg and the uncertainties. Fill out cells B10, D10, A15 and B15 then copy your answers to these tables: Initial Volume (mL) Uncertainty (mL) 1400 Volume Displaced (mL) 500 Final Volume (mL) 10 Uncertainty (mL) Uncertainty (mL) 1900 Mass of IceBerg (g) 10 10 Uncertainty (g) 552.2 0.05 Logbook 4.4 Figure 1 depicts the hydrometers used in the data collection. Figure 1: Hydrometers Explain: • What the hydrometer measures and in what units and • Why the hydrometer scale increases downward. Discuss with others and make an educated guess. Hydrometer use to measure the density of liquid. The unit is kg/m^3. For the reason that since the density of fluid is large, the hydrometer will not sink down; and as the density of fluid is lower, the hydrometer will sink. So, the hydrometer scale increases downward. 3 PHYS1002 Physics (Fundamental) Logbook 4.5 Knowing the volume and mass of the iceberg you found in Logbook 3.3, calculate the density with the uncertainty, fill in cells B3, C2, C3, and C4 in the Excel spreadsheet, C1. (Refer to the Uncertainties Supplement from your first lab on how to propagate uncertainties) The intervals on the hydrometer are 5 kg/m3 apart. Copy your answer into this table: Density (kg/m3) 1000 1104 1180 Material Water Iceberg Titanic fluid Uncertainty (kg/m3) 2.50 22.2 2.50 Does this agree with your earlier prediction when ranking the densities? Yes, Part B – Buoyant Force Aim: To measure the buoyant force acting on an `iceberg’, in water and `Titanic fluid’. This part should not take longer than 40 min. For reference, the video timestamp is 1:51 Checkpoint 2 summary. Planning & Predicting: In this experiment, you will measure the buoyant force acting on an `iceberg’, as shown in Figure 2. Consider the case of the iceberg hanging from a spring balance as shown in the Figure. In Figure 2a the iceberg is ready to be immersed in fresh water; in Figure 2b it is partly immersed in the water; in Figure 2c it is fully immersed but not in contact with the bottom of the beaker; and in Figure 2d it is resting on the bottom of the beaker. Figure 2: An iceberg hanging from a spring balance is lowered gently into a container of water. 4 PHYS1002 Physics (Fundamental) Logbook 4.6 For each situation identify which of the following forces are acting and their direction (up or down). (a) weight force (W) (b) buoyant force of the water (B) (c) pull of the spring balance (P) (d) contact force of the bottom of the beaker on the iceberg (C). Copy the diagrams into your Logbook. On each diagram draw arrows (refer to the theory box below) showing the vertical forces acting on the iceberg. Label the arrows clearly. Insert screenshot of free body diagrams (or picture of hand-drawn free body diagrams) 5 PHYS1002 Physics (Fundamental) Logbook 4.7 Do you expect the buoyant force on the iceberg to be larger in Titanic fluid or in freshwater? Explain your reasoning by comparing the buoyant force with the weight of the iceberg. Because the iceberg well float on both freshwater and titanic fluid, so the buoyancy force is the same. Logbook 4.8 Now navigate to Sheet 2 (C2) on the bottom right corner. Fill in cells B10, A12, B12, B16, A18, B18, B22, A24, B24, A27, B27, A32, and B32. Note the spring balance has intervals of 0.5 N. Copy your results here for each of the cases presented in Figure 2. a) Iceberg suspended above water Spring Balance Reading (N) Uncertainty (N) 5.4 0.3 Weight (N) Uncertainty (N) 5.4 0.3 b) Iceberg partially submerged in water Spring Balance Reading (N) Uncertainty (N) 1.4 0.3 Buoyant Force (N) Uncertainty (N) 4 0.3 6 PHYS1002 Physics (Fundamental) c) Iceberg fully submerged in water Spring Balance Reading (N) Uncertainty (N) 0.5 0.3 Buoyant Force (N) Uncertainty (N) 4.9 0.3 d) Iceberg fully submerged and in contact with the base Contact Force (N) Uncertainty (N) 0.5 0.3 Calculation of Buoyant force from the measurement of the displaced volume Buoyant Force (N) Uncertainty (N) 4.9 0.3 Logbook 4.9 When the iceberg is in air it has a buoyant force acting upon it (it is submerged in air). Why can we ignore this? The buoyant force provide by air is so small that we can ignore it. DESIGN TASK Logbook 4.10 Fill out this Lab Practical Design Task which includes specifying equipment one needs to use and how you are going to do it, including what may lead to uncertainties. This will be good practice for your Lab test later in semester. Task: Design an experiment to find out how adding salt to water affects density of the water. Equipment: include sketch 2 glasses bottle 2 eggs, salt. 7 PHYS1002 Physics (Fundamental) Procedure: can include diagram In the first step, we put an egg into the first glass with warm water, then observe and record the position of the egg after falling into the water. In the second step, we pour a large amount of salt into the second glass with warm water and mix well. In the third step, we put another egg into the second cup of water with salt, and then observe and record the position of the egg after falling into the water. Sources of uncertainty: What do you have to be careful about to get a ‘proper’ value? the angle of seed -Since two eggs of similar size are used as the experiment equipment, it is impossible to guarantee whether the density is the same. Then, the density of salt water may affect the result Discussion: the density of salt water is greater than density of water As can be seen from the picture after the third step, the egg sinks to the bottom of the water cup without salt. In the salted water cup, the egg floats in the water cup. Therefore, it can be concluded that the density of salt water is greater than density of water. 8 PHYS1002 Physics (Fundamental) References: Identify who contributed to this design and reference other sources of information: The idea of the experiment: Little bins for little hands. (2019, September 12). Salt Water Density Experiment For Kids. https://littlebinsforlittlehands.com/simple-salt-waterdensity-science-experiment-saturday-science/ Figure 1:African Natural Hairstyles.Retrieved from: https://www.pinterest.com/pin/218635756883232767/ Figure 2:How an egg beat Kylie Jenner at her own Instagram game. Retrieved from: https://www.theguardian.com/technology/shortcuts/2019/jan/14/kylie-jenner-theegg-instagram-likes Figure 3:Out of the salt shaker Retrieved from: https://www.tailracecommunitychurch.com.au/out-of-the-salt-shaker/ Step 2&3: Little bins for little hands. (2019, September 12). Salt Water Density Experiment For Kids. https://littlebinsforlittlehands.com/simple-salt-water-densityscience-experiment-saturday-science/ COMPLETION From the work performed in this lab about buoyancy, explain how a ship like the titanic which weighs about 46 kT can float in water? 9 PHYS1002 Physics (Fundamental) the Titanic has a huge surface area at the bottom, and there will be some unique space at the bottom of every ship, in order to store some air to make the boat lighter, so that the boat can float on water. On the other hand, sea water’s density is larger than water, so with those features, Titanic can float in water. Floating in the ocean is different to floating in freshwater. Why would it be easier to float in the ocean? Ocean’s water has huge percentage of salt, so sea water’s density is larger than freshwater. When you are done with all your tasks inform the tutor and submit your logbook, your logbook is due four days after the end of your lab session and will be marked by a tutor to check your participation. 10 PHYS1002 Physics (Fundamental) 11 PHYS1002 Physics (Fundamental) 12
