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Effect of friction on total energy?

  1. Jun 11, 2015 #1
    hey guys,

    I am trying to find the effect of friction on the total Energy of an object
    my object is a small car with 2 motors with a known speed and a constant one (without an acceleration)
    this object will be dragging a certain mass behind it

    I already came out with an equation but not really sure if its working

    The equation :
    1/2 m*v02= 1/2 m*v2+ Ef
    knowing that Ef = X + added mass

  2. jcsd
  3. Jun 11, 2015 #2


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    You speak of a cart with a single constant speed. Then you write an equation with two different variables (v0 and v). This suggests that the speed is not, in fact, expected to be constant.

    What exactly is it that you plan to measure? What is X? What is this mysterious "added mass"?
  4. Jun 11, 2015 #3


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    Welcome to the forum.

    It is not clear what you are trying to work out. If the car has constant speed, then the kinetic energy is constant. So why do you have ##v_0## and ##v##? This confuses me.

    Also: Is the fact that the car has two motors important? What does ##X## refer to? Is the "added mass" the mass of the trailer? And why should ##E_f## be related to the added mass?

    The friction might be causing something to get warmer. Or it might cause the car motor(s) to have to work harder so using up fuel faster. But at constant speed it does not change the energy of the car.
  5. Jun 11, 2015 #4
    Yeah sorry this was a bit unclear
    The experiement will be separated in two steps
    The first is to add a known mass on my cart and measure it speed
    The second part is to drag the same mass behind the cart and again measure it speed
    By doing this I'll be able to see the effect of friction on the speed of the cart

    Thanks for your help and hope this helps a bit
  6. Jun 11, 2015 #5


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    Your expectation is that the cart towing the block will attain a constant speed lower than of the cart when it is not towing a block? You after an equation that predicts this? Well, let's figure out what parameters such an equation should have...

    What do you think is responsible for the limited speed attained by the cart when it is not towing a block?
  7. Jun 11, 2015 #6
    what i'm trying to prove is that with the friction between the floor and the weight will eventually slaw down the cart
  8. Jun 11, 2015 #7


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    Then run the experiment. See whether your prediction is correct.
  9. Jun 11, 2015 #8
    Yeah this is what im planing to do, but i need to compare my experimant with theoritacal values and also to calculate the friction effect on the speed
    And i came up with these equations but not sure if they work

    do you have any opignion about the equation (it will be helpful)

  10. Jun 11, 2015 #9


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    The equations do not work.

    Again, I ask, what do you think is responsible for the limited speed of the unladen cart? Where does that limitation show up in your equations?
  11. Jun 11, 2015 #10
    I think the experiment you are proposing is both complex and confusing.

    You have wheels and motors involved here. Wheels roll. But wheels also slip and slide. Rolling friction is what makes rolling possible. That is, without friction, there would be no rolling. But when wheels slip and slide, then you have kinetic friction. You also have motors, which convert electrical energy into rotational mechanical energy.

    http://www2.hcmuaf.edu.vn/data/phamducdung/thamkhao/truyendong/ly%20thuyet-truyendong/friction-rolling/Rolling%20Motion.htm [Broken]

    When you have pure rolling motion without any sliding, the instantaneous velocity of the point of the wheel in contact with the ground is zero.
    Last edited by a moderator: May 7, 2017
  12. Jun 11, 2015 #11


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    Static friction makes rolling possible. Rolling resistance opposes it.
  13. Jun 11, 2015 #12
    Thanks for the correction.
  14. Jun 11, 2015 #13
    Yeah that is true
    But in my experience I'm ignoring the friction of the wheels

    And for the motor im only using it to have a constant speed throw the experience
  15. Jun 11, 2015 #14
    What you are trying to do (I think) is get a thing moving at some speed. The thing has a force pulling or pushing it, and then it has kinetic friction opposing it. Then, you suddenly through more weight on the thing, while the pulling / pushing force remains the same? The kinetic friction (as I recall) is proportional to the normal force. When you increase the normal force, you increase the kinetic friction. If by increasing the normal force, the kinetic friction exceeds the applied pulling/pushing force, then the thing will decelerate.

    But will the motors cause the pulling force to remain constant, or will the motors cause there to be an adjustment to the pulling force at some point?
  16. Jun 11, 2015 #15
    Yeah you got it right
    My motor should have the same power with or without the weight it won't readjust with the mass
  17. Jun 11, 2015 #16


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    Will the motor generate the same power regardless of the speed at which the cart is moving? Will it produce the same forward force?
  18. Jun 11, 2015 #17
    Yeah it should be doing it regardless of the mass
    Or at least I programmed it to do so
  19. Jun 11, 2015 #18


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    Which is it? Same power or same force? And how do you know?
  20. Jun 11, 2015 #19
    its the same power, the motor is programmed to do x number of cycles in a second
  21. Jun 11, 2015 #20
    x cylces per second is an angular velocity (rev/min or rad/s). Power = Torque x Angular Velocity. What kind of motor are you using? Do you know its torque versus speed characteristics?
  22. Jun 11, 2015 #21
    the motor im using if the nxt lego motor i think their is a way to know the troque versus on the internet
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