Activity 3 Tasks on Law of Conservation of Energy (Experiment, Weekly Assessment)
(120 minutes =80 minutes action + 25 minutes report-writing + 15 minutes sharing of info) Engineering processes involve the consumption and conversion conversion of energy. A washing machine converts electrical energy into the spinning action of the the washer. A laptop or a mobile phone phone converts stored chemical energy in a battery battery into electrical energy that powers them. In the process of conversion, energy may be lost through heat, sound and light, which does not directly contribute towards the intended function of the device. The amount of energy yielded yielded for a device to perform its function is usually less than the energy consumed. We shall use a drop test (Fig. 1) to illustrate illustrate energy loss and conversion efficiency. Experiments are never exact. exact.
Due to the limitations of test equipment and environmental
conditions, data collected may display deviations for the same experiment. Due to experimental deviations, the engineer must make a value judgement so as to establish a rational relationship between the variables to be studied. In the drop test experiment, you shall use a rather rudimentary video capture device – your mobile phone, and then use the captured video to plot the time profile of displacement as the balls drops from a height. To view the video and analyse it, you may (again) use a rather rudimentary video editing software, which is available available in most Windows platform – the Windows MovieMaker. MovieMaker. First, know what your mobile phone phone offers in terms of frame speed speed (frames per second or fps). Typical 1
phones offer a 30 fps , but you may check your own phone’s phone’s specs. The MovieMaker software allows one to advance video images in frames (instead of in seconds), thereby allowing finer capture of the motion.
Figure 1: Drop test for three types of balls.
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Calculate the time required for the balls to drop through to the bottom (ignore air resistance).
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Based on your mobile phone’s fps, how many frames would you have captured?
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Any digital controller and analyser digitize data in discrete time-intervals. time-intervals. This is known as the sampling rate. rate. Based on your fps, what is the sampling rate of your mobile phone? How
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Hint: Some mobile phones have a “slow motion” capture option, which multiplies the fps by compromising on the image resolution.
the sampling rate is related to the number of images that you may capture for the entire drop? •
Estimate the velocity of the ball right before impact with the aluminium plate (again, ignore air resistance).
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Write the kinematic equations for displacement vs time, and velocity vs time.
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Secure your mobile phone on the metal stand. Ensure that the phone is of an upright orientation (while you may also use a “landscape” orientation, but due to the geometry of the test rig, it actually compromises the resolution).
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Place the mobile stand at least 50 cm away from the test rig. This is to minimise optical distortion due to extremely close proximity. 2
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Place the ball on the launch pad .
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Activate the video recording on your phone.
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Do a quick release of the ball by pulling on the slider on the launch pad.
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Repeat the ball drop for at least five times.
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Edit the video to isolate separate videos for each ball drop test. Distribute each test to one member of the team.
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Each team member would then have to analyse the video, using any video editing software (those that allows progress in frames are preferred). A good and convenient option is the Windows MovieMaker.
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Plot the displacement–time graph for the ball as it drops. Taking care of translating your time-scale to real time if you have selected slow-motion capture in your mobile phone. 3
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Plot the graph either using a spreadsheet, or python. Use dots for each data point, DO NOT plot a continuous line (Do you know why?).
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Select an appropriate mathematical function to perform a best fit on your data points . Why did you choose that function?
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Each studio group is to split into three sub-groups. Each sub-group is to drop one type of ball only. Hint: You may do a sanity check on your experimental data, by comparing the time observed from the experiment, and the time from your calculations at the beginning of this activity. 4 Hint: What is the relationship between displacement and time? 3
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A representative from each group will then consolidate all data from all team mates on a single plot. Again, use points not lines. Once the plot is consolidated, repeat the best fit through the points.
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Use your best-fit function; compute the velocity of the ball as it drops. Plot the velocity of the ball with drop displacement.
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Plot the total energy (potential + kinetic) with drop displacement, express energy in units of specific energy (J/kg).
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Are there any losses during the drop? If there are, what do you think are the sources of energy loss?
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If there are losses, plot the drop efficiency vs displacement. Define efficiency as:
η drop
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=
Total energy (PE+KE) measured during drop Total energy (PE + KE) assuming no losses
Can you draw an analogy of this simple physical system, to say an electromechanical motor? What are the inputs, outputs and hence efficiency of both the ball dropping, and the motor? What are the respective sources of losses?
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What kind of error did you think you incurred on the displacement and velocity data? Are there any other factors that may affect the accuracy of data capture? List them.
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What could be done to this test to improve the accuracy of data capture?
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Write an individual lab report on your findings.
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Share your findings with other groups after you have submitted your report.
