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course phy 213
quicklab9#$&*
course phy 231
Brief rotating strap and magnetsUses toy car, magnets, rubber bands, rulers
See also Pictures related to Straps and Toy Cars
Toy car and magnets
Uses toy car, magnets, rubber bands, rulers
Using one magnet to repel the other, which is attached to the car, use magnet separations 8 cm, 5 cm and 3 cm to propel the car. Measure how far the car then coasts for each separation. Sketch a graph of coasting distance vs. separation.
Use your graph to predict coasting distances corresponding to separations of 4 cm and 6 cm.
Give your data and your estimates:
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At 8 cm separation, the car drifted 12cm
At 5cm separation, the car drifted 28 cm
At 3cm separation, the car drifted 54 cm
I fiddled with changing e^x, and I got 150e^(-.325x) to fit my points pretty well. And using this graph for 4 and 6 cm displacement, I got:
For 4 cm separation estimation: 41cm
For 6 cm: 21.5 cm
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Because of the geometry of the magnets, the force function is fairly complicated. You won't find a simple function to model it. The exponential function does as well as most of the available options, but the force function is not exponential in nature.
Analysis:
Now assume that the car and magnet have mass 80 grams, and that the coefficient of rolling friction is .03.
What is the force of friction?
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That would be the normal force times .03 which acts in the direction opposite to movement. So the force of friction os (.03)(9.81)(.08)=.023 N
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For each trial, how much work is therefore done against friction as the car coasts to rest?
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Work is force times displacement in the direction of the force, so for the first trial the work would be (.023)(.12)= .0028 J
2nd trial would be N(.28)=.0064 J
3rd trial would be N(.54)= .0124 J
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We can assume that all the potential energy in the magnets, before release, ends up being dissipated against friction after the car is released.
For each trial, how much potential energy do we infer the system has at the instant of release?
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Since it is all dissipated by friction, then the potential energy would be equal to the work. So for each trial the energy would be .0028 J, .0064 J, and .0124 J, respectively
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Sketch a graph of PE vs. the separation of the magnets. You will have three points.
Find the average slope between the first and second point, and between the second and third point. Be sure to detail your reasoning of the first slope, including all the quantities used to find the slope, with units at every step.
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For points 1 and 2, using the formula for slope which is (y2-y1)/(x2-x1), I get(0064J-.0028J)/ (5cm-8cm) =-3/2500 J/cm
Points 2 and 3: (.0124-.0064)/(3-5) = -3/1000 J/cm
@& -3 / 2500 J / cm = -.0012 J / cm
-3 / 1000 J / cm = - .003 J / cm.
If you just give the numbers as fractions, the reader has to do calculations to compare them. While it's very good to give the fractions, the final results need to be easily comparable by the reader.
Either the form given above, or scientific notation (e.g., -1.2 * 10^-3 J / cm) would be easily comparable.
If more than three zeros are involved the scientific notation is preferable.
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What do you think your slopes represent?
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Rate at which energy dissipates with distance of the magnets
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How much potential energy did the magnet system gain between your first separation and your second?
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.0036 J (.0064-.0028)
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How far are the magnets displaced between your first separation and your second?
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First separation: 3 cm,
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What therefore is the average magnetic force between these separations?
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First separation: 2500/3 J/cm(3/.0036)
If .0036 Joules of work are done by the magnetic force through a distance of 3 cm, then I believe the average magnetic force is around .1 Newton.
See if you can confirm that this is about right, and in the process obtain a more accurate result.
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Using the same reasoning, find the average magnetic force between the second and third separation.
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.006 J(.0124-.0064)
That's the change in potential energy, not the aveage force.
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At what point after its release do you think the car reached its maximum velocity? What could you easily measure to obtain a reasonable prediction of the point at which this should occur?
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I think the car obtained its maximum velocity as soon as the magnetic force was less than the frictional force. I could find this position by finding the closest point at the magnet where the car can rest.
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Good data. See my note, and the appended instructions for analysis.
@& Much of your work is good, but you did not correctly answer the questions about the average magnetic forces.
See my notes, which should help clarify the situation, and please submit a revision.*@