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Homework Help: Lab on motion and forces

  1. Apr 13, 2016 #1
    I apologize if my thought process is unrefined. I didn't really learn and remember much from high school physics that benefits me right now in college physics.

    Until this point we covered Newton's three laws of motion. And instead of a cotton ball, we used a coffee filter.

    "Our reasoning is this: Each air molecule, on impact, imparts a small force to a falling object. Using the traditional force formula F = ma (Newton, 1687), we see that each collision effectively reduces the gravitational acceleration of any object falling through air by an amount that is inversely proportional to that object’s mass (i.e., a = F ⁄ m ). Thus the light cotton ball is slowed more than a comparably sized (and heavier) rock."
    1. The problem statement, all variables and given/known data
    Since coffee filters fall slower than the rock since it is lighter, then we should modify the formula for distance s fallen in time 5. Instead of s=1/2gt^2,
    the correct model for falling cotton should be s=1/2kt^2 where k is an acceleration smaller than g.
    Confirm or deny this.
    (we suggest that the effect of air resistance is greater on the coffee filter because it is lighter)
    You have a coffe filter, meter stick, stopwatch, motion sensor, and a x/t and v/t graph.
    2. Relevant equations
    F=ma, ay=1/2at^2

    3. The attempt at a solution
    We did this lab today but my partners and I were stumped and likely performed an incomplete or probably unrelated experiment.

    We initially thought we should experiment on the effect of surface area on how fast the filter will fall on the ground. But after talking with my other two partners, we just decided to experiment on the effect of increased mass on the rate of falling.
    What we did was we used the motion sensor to record the position and velocity over time of initially one coffee filter on a fixed height. Then we added a coffee filter on the first filter to increase the mass. We did this one more time so the last trial was recording the x/t and v/t of 3 coffee filters layered on one another.
    What we found was the v/t got more leveled and less steep. From this, we figured that as we increased the mass of an object, it was more able to overcome the force of air resistance as it falls on the ground so "k" approached closer to "g" instead of "k" being an acceleration smaller than "g."
    I feel as if that our experiment didn't properly address the fact that regardless of mass, whether the coffee filter is lighter, it should still fall at the same time as a rock.
    What did we miss and how could we have done this experiment better to address the fact that even though the coffee filter was lighter, s=1/2gt^2 should still apply and that g SHOULD NOT be changed to "k" which is a smaller acceleration than g?
  2. jcsd
  3. Apr 13, 2016 #2


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    The problem is not looking at the result and finding different k. The problem is showing that s=kt^2/2 is not a good description. I suggest you look at your x/t graph, is it a parabola? The same for your v/t graph, is it a linearly increasing function?
  4. Apr 13, 2016 #3
    The x/t graph was a straight line with negative slope(since the filter was released from a higher point) and the v/t graph was also about a straight line with a negative slope from what I can remember when we used the pasco motion detector
  5. Apr 13, 2016 #4
    Just think inspecting the graph of v vs t, can you conclude that the motion is with zero acceleration, constant non-zero acceleration, or varying acceleration. Then think of modelling. There is no question of remebering just inspect the graph and think about it.
  6. Apr 13, 2016 #5
    each varying graph had a certain acceleration but the acceleration in each particular graph was non-zero
  7. Apr 13, 2016 #6
    Good but is it constant or varying can you distinguish between average acceleration and instantaneous acceleration from v-t graph
  8. Apr 13, 2016 #7


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    If you cannot easily upload the results, it might help us commentators if you could give a bit more detail about them. I assume the motion sensor gives 'instantaneous' velocities. Can you post the times, speeds and positions of just the first three measurements for the single paper? If that does not include a time zero, are you adding a time zero datapoint to your graph?
  9. Apr 13, 2016 #8
    would average acceleration be the final velocity-initial velocity divided by tfinal-tinitial? I'm not sure how to know instantaneous acceleration form a v-t graph though by just looking at it
  10. Apr 13, 2016 #9
    We turned in our graphs to our TA and didn't save our data since we were so behind with our lab that we rushed. :( I'm so sorry
  11. Apr 13, 2016 #10


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    How would you estimate velocity at some instant from a distance-time graph?
  12. Apr 13, 2016 #11
    Draw any graph other than straight line and just think about it it is a mathematical skill!
  13. Apr 13, 2016 #12
    i know that if you have a curvy graph like y=x^2, you take the derivative of that to find the slope at that particular x.
    So instantaneous acceleration would be just the slope of the velocity graph if it is approximately linear?
  14. Apr 14, 2016 #13
    i would look at the x/t graph and estimate the slope of that which I noticed was negative
  15. Apr 14, 2016 #14


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    Yes, the slope (even if it is not approximately linear, though that does make it harder to judge).
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