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Homework Help: Simple lab with two blocks and pulley

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data
    I have 2 blocks. M1 is resting on a horizontal plane and attached to a string that goes over a pulley to M2. The plane is like an air hockey table to simulate very little friction. I attached spark tape to M1 so I could measure the time it took and distance traveled. Now, reality and the numbers I got from my equations do not jive:

    Distance traveled on plane(X1)=.8462m
    Time to travel(X1)=1.5s
    actual acceleration =.38m/s^2
    predicted acceleration=.9347m/s^2

    Distance traveled on plane(X1)=.7018m
    Time to travel(X1)=0.9s
    actual acceleration =.87m/s^2
    predicted acceleration=2.0329m/s^2

    Distance traveled on plane(X1)=.6797m
    Time to travel(X1)=1.2s
    actual acceleration =.47m/s^2
    predicted acceleration=1.4338m/s^2

    2. Relevant equations
    I used a = m2g/(m1+m2) to predict acceleration

    3. The attempt at a solution
    It must be that friction is a factor in this but I do not know how to calculate friction yet. I think it is the normal force of M1 times some constant but I'm not sure how to get it from there. It should be simple but the method escapes me. For instance in test 2 and 3 the set up is identical except I added .1kg to M1 so that I could find some friction value.

    thanks for the help
  2. jcsd
  3. Mar 1, 2012 #2
    I did some further calculations to find the difference between my predicted acceleration and the actual and found a lost of nearly 60% for each. The 2nd and 3rd trial had only the .1kg difference and the difference between the two was about 10% lost acceleration. Meaning trial 2 lost 57% and 3 lost 67%. Could I say that the friction coefficient is 0.01 (10%) then?

    Having said that, this still does not explain where the other lost acceleration went.
  4. Mar 1, 2012 #3
    can anyone at least tell me if the equation I used to calculate acceleration for this is correct?

    I am pretty stuck on this. Right now I'm assuming a reduction of 10% per .1kg. Given that I figured that M1 reduced my acceleration by about 19%. So I'm still missing about 30%. My caliper and scale errors don't even add up to .5%.
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