Lab+2+-+Force

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Jan 9, 2024

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Laboratory 2 R1 Name ______________________________________ Section ___________ Date ________________ Reading Quiz Grade __________________________ (5 points) A. Preliminary Exercises: Answer the questions below prior to beginning your data collection. Show all work in the space provided. 1. In the table below, the sampling frequency is represented by f s and the sampling period the time between successive samples is given by T s . Fill in the blanks in the table. (5 point) f s (Hz) T s (s) T s (ms) a. 10 b. 0.05 c. 1,000 d. 20 e. 20,000 f. 10 g. 25 h. 0.5 i. 1,800 j. 0.2 Notes f s - sample rate measured in hertz (Hz), where 1 sample per second is 1 Hz. T s is the time interval between samples, measured either in seconds (s), or in milliseconds (ms).
Laboratory 2 R2 2. Compute the frictional force F f applied to the skater shown in the free-body diagram below, if the skater is gliding along the ice and the coefficient dynamic friction between the skate blades and the ice is D = 0.06. (3 points) 3. (a) Compute the mass of the skater from the previous question. (b) Find the net vertical force applied to the skater. Give both the magnitude of the net vertical force and its direction. (c) What is the direction acceleration, up, down, or equal to zero? (3 points) W = 485 N F f N = 545 N
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Laboratory 2 R3 4. (a) Compute the horizontal and vertical components of the 592 N ground reaction force applied to the tip of the crutch shown below. Note that this force is acting at an angle of 35° with respect to the vertical direction. ( 1.5 points ) (b) Do the same for the 120 N force applied by the hand to the crutch handle. Note that this force is acting at an angle of 30° with respect to the horizontal direction. ( 1.5 points ) (c) Finally, find the vector sum of the four external forces applied to the crutch, including the 10 N weight of the crutch and the 440 N force applied by the axilla (armpit). Give both the magnitude of the net force vector and the angle at which the net force vector acts. Make sure to specify whether this angle is with the horizontal or the vertical. (2 points)
Laboratory 2 R4 B. Barbell Lifting Force and Acceleration Instructions for these computations can be found in the introduction. Show work where appropriate partial credit may be awarded. What is the time between samples for these force measurements, in ms ? _____________ ms (1 point) Average sum of upward forces (R + L) during static period: _____________ N (1 point) Weight of barbell (calculated from force data): _____________ N (1 point) Mass of barbell: _____________ kg (1 point) Draw a free body diagram of the barbell below. Show the two hand forces separately on your diagram. (Remember that a free body diagram shows only the body and the external forces that act on it) (3 points) Write the expanded . Give the formula in the the form acceleration = [ fill in right-hand side ]. (Hint: start with equation 2b. Using the forces known to be acting on the barbell, expand upon equation 2b to be more specific to the given scenario. Then, solve for acceleration.) (2 points)
Laboratory 2 R5 RMSD between estimated acc. of the COM and measured acceleration: ________________ m s -2 (3 points) Graphs: Plot the total vertical force applied to the barbell by the hands versus time. Print out your graph and include it with your report. Be sure to label the axes to indicate what is graphed and the units used. (5 points) Plot your estimated vertical acceleration and the measured vertical acceleration versus time (both curves on the same graph). Print out your graph and include it with your report. Be sure to label the axes to indicate what is graphed and the units used. (5 points) Analysis & Discussion 1) Examine your graphs of total hand force and measured acceleration. In the middle of the lift, when the acceleration is near zero , what can be said about the vertical velocity of the barbell is it increasing, decreasing, or constant? (2 points) 2) Again referring to the middle of the lift, when the acceleration is near zero , what can be said about the which is greater, or are they the same? (2 points) 3) Lifting this light barbell is unlikely to cause injury but, based on your force graph, when do you think an injury is most likely during the lift of a heavier barbell at the beginning, the middle, or the end? Justify your answer with specific reference to force magnitude. (2 points) 4) Describe what happens to the vertical hand force as the weight slows approaching the end of the lift, and (3 points) 5) In our experimental set-up, great care was taken to place the accelerometer as close to the center of mass of the barbell as possible. One reason that the RMSD you computed might be different from zero is that the accelerometer might not have been placed exactly . Why might this produce a difference between the measured acceleration and the acceleration computed from the hand forces? (Hint: must every point on the barbell always have the same vertical acceleration? When spinning with arms outstretched, do your fingertips have the same velocity as your COM?) (3 points)
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Laboratory 2 R6 C. Ground Reaction Force During Deep-Knee Bends Instructions for these computations can be found in the introduction. Show work where appropriate partial credit may be awarded. What is the time between samples for these force measurements, in ms ? _____________ ms (1 point) Average vertical GRF during static period: ______________ N (1 point) Weight of subject (calculated from force plate data): ______________ N (1 point) Mass of subject: ______________ kg (1 point) Graphs: Plot the vertical GRF versus time. Print out your graph and include it with your report. Be sure to label the axes to indicate what is graphed and the units used. (5 points) Plot your estimated vertical acceleration versus time. Print out your graph and include it with your report. Be sure to label the axes to indicate what is graphed and the units used. (5 points)
Laboratory 2 R7 Analysis & Discussion 1) Your graph of vertical GRF should show 4 distinct force peaks for each of the three deep-knee bend cycles. Explain why the force goes above the body weight or below the body weight during each of these four peaks. Make reference to aw (Eq. 3) and your estimates of acceleration in each of your four answers. a. First Peak (2 points) : b. Second Peak (2 points) : c. Third Peak (2 points) : d. Fourth Peak (2 points) : 2) There are times during the deep-knee bend cycles when the vertical GRF drops below the body weight but remains positive. If the subject performed the deep-knee bends differently, would it be possible to record a negative vertical GRF? Explain why or why not. (3 points)
Laboratory 2 R8 D. Factors Affecting Friction Forces Observations: Enter numbers between 1-10 in the boxes below. Enter a 1 for the condition in which your perceived pulling force was lowest and a 10 for the condition in which your perceived pulling force was highest. Fill the rest of the boxes according to the scale established by these two ratings. You may use numbers more than once in the table. (3 points) -Soled Shoe Smoother-Soled Shoe Surface Dry; Shoe Empty Surface Dry; Shoe Weighted Surface Wet; Shoe Empty Analysis & Discussion 1) Draw a free body diagram of one of the shoes below as it is being pulled across the surface. (Remember that a free body diagram shows only the body and the external forces that act on it.) (3 points) 2) In these trials, you attempted to drag the shoe across the surface with a constant velocity. If you were successful at this, what does it say about the pulling force relative to the friction force in Eq. 4? Is the pulling force greater than, less than, or equal to the friction force and why? (2 points)
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Laboratory 2 R9 Recall: According to the Coulomb friction model, the tangential force between two bodies in sliding contact is given by F d = d N (6) where d is the coefficient of dynamic friction, a unitless number that depends on the materials and degree of lubrication, and N is the normal contact force. 3) With reference to Eq. 6, explain why the friction forces would be expected to be different in each of following comparisons Make reference in your answer to differences in normal force, coefficient of dynamic friction, or both. a. Grippy vs. Smooth Soles in the Dry/Empty Condition (2 points) b. Weighted vs. Empty Conditions for the Smooth-Soled Shoes (2 points) c. Dry vs. Wet Conditions for the Grippy-Soled Shoes (2 points)
Laboratory 2 R10 4) In these trials, you tried to pull horizontally on the shoe. If instead you had pulled at a slightly upward angle of say 10° above the horizontal with the same force magnitude, how would this change have affected the normal and friction forces (increase, decrease, no change)? Assume that you still caused the shoe to move horizontally across the surface with constant velocity without lifting up off the surface at all. (Hint: start by drawing a free body diagram for this scenario Then, decompose your pulling force into horizontal and vertical components. Apply Newto nd Law.) (6 points)

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