Lab 5: Circular Motion Physics 193 Fall 2006 Page 1 of 5 Lab 5: Circular Motion LEARNING GOALS OF THIS LAB: 1) Use your knowledge of Newton’s laws and circular motion to make a prediction about the outcome of an experiment. 2) Apply your knowledge of Newton’s laws to solve a practical problem. 3) Use two independent methods to determine a quantity experimentally. 4) Evaluate simplifying assumptions that you use in your mathematical procedure. I: Testing experiment: Circular motion, Pendulum The goal of this experiment is to decide whether we can trust Newton’s laws when an object is in circular motion. Available equipment: a scale, a pendulum, a spring scale. a) Attach the pendulum bob to the spring scale. Let it hang freely and notice the reading of the spring scale. Explain the reading using a free body diagram for the bob. What force does the spring scale read? Explain. b) Now imagine that you pull the bob to the right and let it swing (do not perform the experiment yet). Use your knowledge of Newton’s second law applied to circular motion to draw a velocity subtraction diagram and a free body diagram for the bob at the bottom of the swing. Make sure that the diagrams are consistent with each other. c) Now use the diagrams to predict whether the scale will read the same, more or less when the bob is at the bottom of the swing compared to when it is at rest. What are your assumptions? d) Explain to Saalih why you need to make a prediction before performing the experiment. e) Now perform the experiment and record the result. Did it match the prediction? If not, should you modify the idea under test or your application of this idea to this particular situation, or your assumptions? f) Why was this experiment called a testing experiment? What physics idea were you testing? g) How are an explanation or an idea different from a prediction? II: Application experiments: Circular motion, Conical pendulum Self-assessment G1, G2, G3, G5, D5 Experiment A conical pendulum is different from a regular pendulum that you used in experiment I. Instead of moving back and forth in a vertical plane, its bob moves in a horizontal plane following a circular path. The string follows a path of an inverted cone. Before you read on, set up a conical pendulum on your table and then call the instructor to check your set up.Lab 5: Circular Motion Physics 193 Fall 2006 Page 2 of 5 Design two independent experiments to determine the magnitude of the net force exerted on a conical pendulum bob by other objects as the bob of the pendulum moves in a circular path. Available equipment: A ring stand, a heavy bob at the end of a string, a meter stick, a spring scale, a stopwatch, a poster paper, a marker. Include in your report for each experiment: a) Describe your experimental design. Include a labeled diagram. b) Devise a procedure that you will use to determine the net force. c) List the pieces of knowledge that you are going to apply to find the net force on the bob. d) List the physical quantities you will measure. Briefly (but specifically) describe how you will measure them. For example, if you are measuring the length of the pendulum string, mention which point on the bob you consider – the top of the bob, the center of the bob, or the bottom of the bob. e) List additional assumptions that you made. Explain how each assumption can affect the result. f) List sources of experimental uncertainty, and possible methods to minimize them. Evaluate how experimental uncertainties can affect the data. g) Perform each experiment and record the data. Make sure you take steps to minimize experimental uncertainties. h) Determine the net force based on your procedure and measurements. Use the weakest link rule to determine the uncertainty in the outcome. i) After you have performed both experiments, compare the two values you obtained. Decide if these values are different or not. If they are different, discuss possible reasons for the difference (think of the effects of your assumptions). j) In one of the experiments you probably tried to get the bob to move in circle. However you noticed that the path that the ball followed was not an exact circle – it looked more like an ellipse. Compare the net force exerted on the bob at different points of the trajectory (try to make the comparison quantitative). Now think whether the assumption that the path is circular makes you overestimate or underestimate the average net force. III. Why did we do this lab? a) Explain how your understanding of physics or scientific abilities is different now compared to before the lab. b) What would you improve in this lab?Lab 5: Circular Motion Physics 193 Fall 2006 Page 3 of 5 IV. Homework This homework will help you to understand the different stages of the construction of scientific knowledge. Read the following passage focusing your attention on the scientific procedures and scientific abilities. Please complete before coming to lab 6. Ignaz Philipp Semmelweis studied medicine at the University of Vienna where in 1844, at the age of 25 he received the degree of Doctor of Medicine. Later in the same year he qualified for the degree of Master of Midwifery, and from that time forward devoted the remainder of his life to the science and practice of Obstetrics. Upon receiving his Master's degree he at once applied for the position of Assistant in the Lying-In Division of the huge Vienna General Hospital, and was eventually appointed to that post. The General Hospital's Lying-In Division was the largest of its kind in the world. It was also one of the most deadly due to prevalence among its post-partum patients of what was known as "the endemic puerperal fever of Vienna." The sensitive and deeply humanitarian Semmelweis was appalled by the death rate from puerperal fever in the Lying-In Division, and searching for the cause and control of this pitiless disease became his life's work. For a laboratory he had the First and Second Obstetrical Clinics, each averaging about 3000 deliveries per year. When he tabulated the deliveries and deaths by month and year in each of the Clinics for the six-year period from 1841 to 1846, he found that the First Clinic, where medical students were trained, had a death rate from puerperal fever of 9.9%; whereas, the death rate in the Second Clinic where midwives did the deliveries was 3.3% - only one-third that of the First Clinic. It would be too chilling