Sunday, October 23, 2011
HW6 #17
Hey I am having a problem getting #17 to work out on the homework with my numbers. When I tried solving it with other people's numbers the same way, I got the right answer, but mine arent working out. I have an 18 kg child on a playground swing moving with a speed of 4.7 m/s at the lowest point, and the swing is 1 meter long. I need to find the angle the string makes with the vertical. First I tried using conservation of energy with K=U, so .5mv^2=mgh. The m's cancel out and I am trying to solve for h, height above the lowest point. I end up with .5(4.7^2)=9.81h, and when I solve for h, I get 1.12, which doesn't work because it is higher than the length of the string. I also tried solving it by setting cos(theta)=(centripital acceleration)/g, and the acceleration is greater than g so it doesnt work. Please let me know if I am doing something wrong! Thanks!
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2 comments:
nevermind, i used 1-cos(theta) instead of sin, and I ended up with an answer greater than 90 degrees, and it was right
While the numbers worked out for many students, some students got answers that didn't make sense (like Mike's, as posted above), and others got answers that gave a calculator error.
If the numbers are not working out for you please let me know so that I can not count this problem for you. I think that a minority of students is affected, so I will still count this problem for everybody else. I need to know if more people had bad numbers.
Dr. Winters
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