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Propelling force on a vehicle

  1. Feb 28, 2012 #1
    Hi all.
    I didn't get it cleared although i tried searching the internet.
    1) Firstly, how does a vehicle gets off...i mean how the torque is converted into propelling force. I don't want to know the mechanism, gears etc...i want to know the physics...
    2) Another question is that whether static friction force is itself the propelling force in this case? And how the friction force varies from the start. Like at t=0, friction force is totally static. Then how does it proceed on.
    3) Also what is the condition when wheels start slipping?
    4) If I were to design an engine how would I decide what's the torque of engine to propel a vehicle of say mass 'm' and acceleration 'a'
    Just for simplification i assume that at low speeds the drag force is negligible.
    5) Lastly, what's the difference between the terms traction, tractive force and tractive effort?
    I Googled but everyone has different things to say about it!! And is tractive force equal to [mass x acceleration] of the vehicle? or is it equal to the static friction force. I'm totally confused over this topic. Please, help required.
    Thanks in advance.
  2. jcsd
  3. Feb 29, 2012 #2
    Hey guys...no reply??
    Atleast suggest something!!
  4. Feb 29, 2012 #3


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    Draw yourself a FBD of a tire subject to a weight force down, a normal force upward from the pavement, a friction force from the pavement, a reaction force on the axle, and a torque on the axle. Then write the equations of motion for the tire. This should give you an idea about where the force driving the car forward comes from.
  5. Feb 29, 2012 #4

    jack action

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    The propelling force is the friction force between the tire and the ground. The relationship between the torque and the friction force is Wheel Torque = Friction Force X Tire Radius.

    The friction force induces an acceleration to the vehicle, which increases its velocity and then initiate motion. I'm not sure if this answers your question.

    See Sources >> Friction coefficient at the bottom of this page.

    See Theory >> Longitudinal acceleration >> Accelerating at the bottom of this page. The general case is equation (5b) where you could replace the friction force (Ft) with Wheel Torque / Tire Radius as seen previously. So, assuming no aerodynamics forces or rolling resistance:

    Vehicle Acceleration = Wheel Torque / Tire Radius / Vehicle Mass

    All of these terms are related to the tire friction force.
  6. Mar 2, 2012 #5
    Thanks all!! I think i got it.
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