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Q & A: homework advice

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Most recent answer: 09/25/2013
Q:
Hi guys, This is for a school project. We had to find the theoretical time it takes for a rounded object to roll down an inclined plane. We did the practical experiment. After many hours of researching the internet, the two formulas that I used were a = g * sin(theta) which was the formula for acceleration (with no friction or inertia etc) and t = SQRT(2d/a) which was the formula for time in the absence of final velocity. So I had a 10 degree inclined plane, the average time it took for a 10g gluestick rolling down it was 1.09 secs. On the theory side of things, a = g * sin(theta) which after substitution is a = 9.8 * sin(10). The answer was a = 1.7. Using the second formula, t = SQRT(2d/a) which after substitution was t = SQRT(2* 41/1.7) Note* 41 was the displacement/distance of the inclined plane and the distance where the gluestick travelled. The answer to that equation was 6.95 which is quite different to the practical time of 1.09 secs. So, Am i using the right formula's? and if not could you please provide me with the proper ones relevant to this experiment as well as a worked example? Thank you very much, Chris. PS, Please dont use any complicated expressions as im only 15 year old and this is way out of my comfort zone, im just interested in physics and such. If you do have to use complicated expressions can you please explain them to me as well? Thank you.
- Chris Adams (age 15)
Los Angeles, California
A:

As we say at several points in our answer guidelines, we don't do school assignments for people. Each day we delete several of them. In this case, your assignment offers a chance to comment on some very useful guidelines for how to work problems, without doing the assignement for you.

You give a lot of numbers (9.8, 1.7, 6.95, 41) which are supposed to stand for physical things. Is that 6.95 apples? Horses? Feet? Gigayears? The first rule for doing any calculation like this is to be very careful to keep track of the units throughout all the steps. If you do that here, you'll find that your answer will be more reasonable, although still wrong.

The other bit of advice is to think about what's going on physically, not to just grab a formula that has some of the right variables in it. In this case, the problem of cylinders rolling down planes is a very standard one worked out in most beginning physics texts, so you should have little trouble finding the right formula.

Mike W.


(published on 09/25/2013)

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