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What should I title my lab?

  1. Sep 10, 2016 #1
    1. The problem statement, all variables and given/known data
    We recently conducted a physics lab in class where we designed an experiment to "prove" the kinematic equations. We used photogates, ramp and a car.

    2. Relevant equations
    We have to show v = u+at and the other kinematic equations are true with our collected data.

    3. The attempt at a solution
    I want to name this lab "Proving kinematic equations", but according to my teacher this would not be correct as the kinematic equations are as good as proven already, we are just showing they work on the lab.
     
  2. jcsd
  3. Sep 10, 2016 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    You could title it "Checking the Kinematic Equations", or something similar. BTW, if your car wheels have any mass compared to the mass of the car, you should take that into account or your results will not match the expected values closely. Do you know why that is? What additional equation should you include in your analysis to account for the mass of the wheels?
     
  4. Sep 10, 2016 #3

    mfb

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    2016 Award

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    "Testing kinematic equations"?

    Formulas like v=u+at for constant acceleration are mathematical truths - they directly follow from the definition of the acceleration. Experiments cannot prove anything, they can only disprove things.
     
  5. Sep 10, 2016 #4
    What do you mean? As far as I know the only essential variables are initial velocity, final velocity, acceleration and displacement. A little more information on the setup.

    I measured the length of the ramp, this value is my displacement.
    Placed a photogate at beginning of ramp, length of car / the time it took the car to pass through photogate = initial velocity
    Placed a photogate at bottom of ramp, length of car / the time it took the car to pass through photogate = final velocity
    I placed the photogates about 5 cm apart at beginning of ramp and measured acceleration between these points. I repeated this at multiple points on the slope to calculate average acceleration throughout the ramp.

    I'm sorry if I'm being stupid/missing obvious error. I'm obviously not very good at physics :(
     
  6. Sep 10, 2016 #5

    mfb

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    Put a frictionless sliding block and a rolling wheel next to each other on a ramp. Which one arrives at the bottom first?

    Your car is basically a combination of sliding block (main part of the car) and wheel (the wheels). If the mass of the wheels is small you can ignore this.
     
  7. Sep 10, 2016 #6
    The car was made of plastic, with the wheels being small and with low mass, so I can probably ignore this. However, just for future reference, a couple questions:-
    1) How would I even mass the wheels of the car (assuming I'm not able to take them off)
    2) How would the mass of the car factor into the equations
    A) v = u + at?
    B) s= ut + .5 * a * t^2
    C) v^2 = u^2 + 2as

    I'm just coming to appreciate how many holes my physics knowledge has. I will be taking the IB Physics SL exam in May, any suggestions on how I can compensate for these holes (reading material, etc?). Thanks for any help you can provide.
     
  8. Sep 10, 2016 #7

    mfb

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    Your calculation of the acceleration on the slope makes the assumption that your wheels are irrelevant, or not rotating. In reality they are rotating, which also needs energy, and you don't account for that rotation at the moment.

    Measuring the mass of the wheels can be tricky, and you probably can neglect them.
     
  9. Sep 10, 2016 #8
    How would I account for rotation?
     
  10. Sep 10, 2016 #9

    mfb

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    I'm sure your textbook has something about rolling objects, but I think that is beyond the scope of this thread.
     
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