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Homework Help: Torque needed to accelerate rail-mounted vehicle

  1. Feb 20, 2017 #1
    <Moved from a technical forum. Therefore no template.>

    I am currently analysing the forces acting on a moving vehicle. The vehicle is mounted on a rail with wheels on both upper and lower side of the rail with a spring increasing the reaction force, and thereby the friction. The vehicle should be designed to reach an acceleration, a, of 9 m/s2.

    The vehicle have a mass, m, of 5 kg, and a cross-section area, A, of 0,04 m2. The wheel have a radius, r, of 0,106 m.

    Until now I have used the following equations:

    Fnet = Ft + Fd + Fr
    Fnet: Total longitudinal force required to reach the acceleration
    Ft = Traction force from the driving wheel
    Fd = drag force
    Fr = rolling resistance

    From this I get Fnet = 57,56 N
    Do I need to include the friction force in this equation?

    From this I have used two different formulas to calculate the Torque required to reach the needed acceleration: T = Fnet*r and T = I*alpha.
    I: moment of inertia of the wheel
    alpha: angular velocity

    I know the second equation is wrong, since only parameters for the driving wheel is included, but i have no idea how to use this for the whole vehicle.
    With the first equation I get a required torque of approximately 6,10 Nm. I do not believe this is correct, and I think I am missing out on something.

    I've been searching the forum for this problem, but I have not yet found an answer to this specific problem.
    Last edited by a moderator: Feb 20, 2017
  2. jcsd
  3. Feb 20, 2017 #2


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    The only figures I see for inputs are 5 kg, 9 m/s2, 0,04 m2 and 0,106 m. Starting from those inputs, how did you deduce 57,56 N?
  4. Feb 20, 2017 #3
    Im sorry for the limited explanation. I used the different equation for drag force, rolling force and F = ma for the traction force. The reason why I didnt't explain it more thouroughly is that it is alot of parameters and coefficients. This is not really the part I am questioning. What I want to know is which different forces i need to take into account, and how to calculate the required torque.
    Hope this clarify your question in some way.
  5. Feb 20, 2017 #4


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    If Fnet is 57.56 N and the cart's mass is 5 kg then the resulting acceleration will be 11.5 m/s2. So there is definitely something amiss.
  6. Feb 20, 2017 #5
    I think you are mixing up Fnet and Ft. Ft = ma = 5*9 = 45 N. From this we also need to include the drag force due to air resistance which is given by Fd = 1/2*p*v2*Cd*A and rolling resistance F = Crr*N. Then all the forces is summed together to get Fnet. But these forces are not the problem. I'm trying to figure out which other forces that will have an impact on the system.
  7. Feb 20, 2017 #6


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    Perhaps you can explain your sign conventions and the source of each force.

    Fnet would ordinarily be the total external force on the cart. Fnet = ma
    Ft would naturally be the traction force on the cart.
    Fd would naturally be the drag force on the cart.
    Fr being rolling resistance would normally manifest as a torque rather than as a force.
  8. Feb 21, 2017 #7
    Fnet is the total external longitudinal force. It includes the needed traction force (m*a = 5*9 = 45 N ) from the wheels, and I also need to compensate for the counterforces in the other direction to still achieve the acceleration of 9 m/s2.

    In my understanding, torque and force goes hand in hand. To find the needed torque from the driving wheel, all forces need to be added toghether. That means that the torque would need to be represented as a force. This is no problem as I have the diameter of the wheel.

    I think my question might have been a bit missleading. If we forget the numbers for now, which forces/torques would have an impact on the movement of the car?
  9. Feb 21, 2017 #8


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    You compensate for the counterforces by increasing Ft. The result vector sum of all forces is an Fnet that is exactly large enough to produce the required acceleration.
    The problem is that rolling resistance manifests as a torque. If you are already tracking Ft as a force, there is no place left to put Fr as a force. It is better modeled as if it were friction at the axle (as if the bearings were sticky).
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