Practical4

1 Practical 4 Physics 131 Fall 2018 I. Testing Experiment a. Set up an experiment as shown in the figure on the right. The object on the left is a 100 g mass, and the object on the right is a 100 g mass with a washer on top of it. Measure and record the mass of the both 100 g masses (they might not be exactly 100 g) and the mass of the washer using the scale at the front of the classroom. b. Hold the left object on a table; the right one is about 40 cm above the table. Use your knowledge of Newton’s laws and kinematics to predict how long it would take the object on the right to move through a distance of 40 cm if you let go of the object on the table. c. On your whiteboard, write • Your measurements of the masses and the distance [Data] • The assumptions or simplifications you made in generating your prediction [Intro/Assumptions] • The diagrams and mathematical representations you used to generate your prediction. Check if your diagrams and mathematical representations are consistent. [Representations] d. After you have made your prediction, work with your group members to perform the experiment and record the time [Data]. Decide how many times to repeat the experiment (how many data points to record). Discuss : Did the outcome match the prediction? Consider the assumptions you made about the motion of the objects, the pulleys, and uncertainties in the data. Summarize your conclusions on your white board. [Analysis/Conclusions] e. Call over your TA, present your results and justify your conclusions. II. Observational Experiment Equipment available: a specially prepared wooden block that has the same materials on two surfaces that have different areas, and different materials on two other surfaces that have the same areas; force sensor; string; the 100 g masses from Activity I. (Link to force sensor manual: https://practicals.physics.utoronto.ca/practicals/6forcesens/ ) a. Work with your group members to design an experiment to investigate what physical quantities affect the maximum static friction force component of the force that the surface exerts on the object pulled across it. Describe your experiment on the whiteboard with a sketch. What quantities will you vary? Remember not to vary more than one quantity at a time. Make a table to record your data. [Intro] b. Conduct the experiment and record the data. [Data] c. Discuss : What patterns did you find? Talk to representatives of other groups. Did they find the same patterns? How did your assumptions affect your results? [Analysis/Conclusions]

2 III. Application Experiment Your group is tasked by a floor company to determine the coefficient of static friction between your shoe and a wooden plank in two different ways. You may use the following equipment: a shoe, a spring scale or force probe, a wooden plank, a rod and clamp to prop up the wooden plank, and a meter stick. a. Devise the first method using the spring scale or force probe as your only measuring instrument. On your white board, include a sketch of your proposed method, a force diagram, and a clear mathematical description that can be used to get a quantitative answer to the problem. Then take your measurements and record your results. b. Estimate the uncertainty in your measured value of μ s from part a . Consider both the resolution of the reading of the force sensor and how much your force measurement varies across multiple trials. c. Devise a second method using the meter stick as your only measuring instrument. On your white board, include a sketch of your proposed method, a force diagram, and a clear mathematical description that can be used to get a quantitative answer to the problem. Then take your measurements, record your results and estimate the uncertainty in your measured value of μ s . d. Estimate the uncertainty in your measured value of μ s from part c . Consider both the resolution of the reading of the meter stick and how much your measured values change across multiple trials. e. Compare the outcome of the two methods (part a and part c ). Do your two measurements agree within your estimated uncertainties? Explain . Discuss what assumptions you made to implement each mathematical method and how these assumptions might impact the results you found. f. Call over your TA to present your two experimental designs and assumptions [intro/assumptions], your diagrams and measurements [data/representations], and your analysis and conclusions, including your estimates of the uncertainty in your results. IV. Observe and Explain Imagine that you have two low friction carts to carry out the following three experiments. You fix a vertical board on cart A and a battery-operated fan on cart B. As the fan blades on cart B rotate, they exert a constant force on the air. Experiment 1: You put cart B on the track and switch on the fan. While the cart is moving to the right, you record this video: https://mediaplayer.pearsoncmg.com/assets/_frames.true/sci-phys-egv2e-alg-4-4-16a

4 V. Diagram Jeopardy Unlabeled force diagrams for objects moving on an inclined surface are shown below. For each case (on your white board), draw the force diagram, sketch and describe in words a process for which the diagram might represent the forces that other objects exert on an object of interest, and label the force arrows on the force diagram. (Make sure to identify the system in your sketch.) Force diagram (label the force arrows). Sketch a situation consistent with the diagram. Describe a process consistent with the diagram. a. b. Call over your TA to share your descriptions [intro], your sketches and labeled diagrams [representations], and your reasoning in coming up with the process [analysis/conclusions]. VI. Equation Jeopardy Work with your group to envision one physical process that the equations below might describe (there are many possibilities). Assume that . The object is on an incline. (Note: Here N stands for the unit Newton and N stands for the normal force.) On your whiteboard, construct a sketch of a process the equations might describe and write in words a problem for which the equations might be used. Identify the system and draw a free-body diagram that would be consistent with the situation. Identify the key assumptions you would use to solve the problem. g = 10 N/kg x : a x = [0 + ( − (100 kg) × (10 N/kg) × cos20 ° )]/ (100 kg) y : 0 = [ N + ( − (100 kg) × (10 N/kg) × sin20 ° )]/ (100 kg) 0 − (16 m/s) = a x t

5 If possible, compare what you came up with to the ideas of another group. Evaluate whether their picture and words are consistent with the given equations. Call over your TA to introduce your problem and state your assumptions [intro/assumptions], present your sketch and diagram [representations], and explain the reasoning you used to envision the problem [analysis/conclusions]. VII. Represent and Reason You throw a tennis ball over a net. Draw an arrow or arrows representing its instantaneous velocity and acceleration and the force or forces exerted on the ball by other objects when at the four positions: (1) after it leaves your hand but before it reaches the top of its flight, (2) right at the top, (3) falling back down, and (4) landing (at the moment it touches the ground). Identify any key assumptions you made. Check your diagrams for consistency with Newton’s laws and be prepared to justify your answers. Call over your TA to share your assumptions [intro/assumptions], your sketch and arrows representing velocity, acceleration, and forces [representations], and the reasoning you used [analysis/conclusions].