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Physics help for two questions please!

  1. Jun 24, 2013 #1
    Hi so the first question involves a velocity time graph and finding the average velocity between time 0 and time 6 s. it says use any method but I only know how to find the area under. I get how to find the time between each interval. For example between time 0 and 1.5, you would use length x width since it is a rectangle. I can figure out the answer for each interval but am stuck on time 5-6 s as I don't quite understand the shape. Any help would be great :)


    http://m.flickr.com/photos/97943984@N07/9121926127/lightbox/

    Next question.

    In a circus performance, a money is strapped to a sled and both are given an initial speed of 4 m/s up a 20 degree incline track. The combined mass of monkey and sled is 20 kg and the coefficient of kinetic friction between sled and incline is .20.
    A) how far up the include do the monkey and sled move before coming to a stop?
    B) how much time passes from the time the monkey-sled starts going up the ramp until it comes to a stop?

    For a, I get that you have to find the force of friction and eventually get to the point of finding acceleration so you can use the vf2 = vi2 + 2ad but when I did it, I didn't get the right answer for d. My textbook told me the answer should be 1.5 m but I am only getting 1.3 and am getting 1.5 for the acceleration so I'm not sure what I'm doing wrong? I found the vector by doing cos and sin of 20 and multiplying it by the force of gravity and was also able to find the force of friction.
     
  2. jcsd
  3. Jun 24, 2013 #2

    Simon Bridge

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    Welcome to PF;

    1. the average velocity is the total displacement divided by the time. The displacement is the area under the v-t graph. Shade the area under the graph between t=5s and t=6s - that is the area you need to find. You may have to break it up into sub-areas ... note: your link does not come out for me.

    2. Did you draw a free-body diagram?
    Did you sketch a v-t graph for the monkey+cart?
    We need to see your working.
     
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