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A powered toy car moving at a constant speed

  1. Oct 21, 2015 #1
    Okay, so please help settle a dispute.

    A toy car is moving at a constant speed of 1 m/s. The car is powered by a battery.

    Now, when the car is going at a constant speed, there is obviously no net force on the car.

    The dispute comes when trying to draw a FBD.

    Is there friction (traction) between the ground and the wheels?

    If so, what force is balancing out the traction to keep the car from accelerating? At that speed, air resistance is negligible, correct?

    If not, then why will the car only move when powered on? As soon as power stops, the car decelerates which indicates where was a negative force on the car being countered by traction.
  2. jcsd
  3. Oct 21, 2015 #2


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    Whether air resistance is negligible or not depends on how accurate you want to be. What other retarding influences can you think of? For instance, does the car vibrate at all as it rolls across the ground? Where does the corresponding energy come from?

    Edit: Additionally, the fact that the car decelerates when you remove power does not demonstrate that there was forward traction while the car was moving. It could, alternately, demonstrate that there is retarding traction while the car is slowing down.
  4. Oct 21, 2015 #3
    So if I were to take this toy car and

    What would demonstrate the lack of or existence of traction?

    If the car were to roll at a constant speed onto a near frictionless surface in a vacuum, moving normal to the interface, if there was no traction then it should keep rolling at the same speed on this new surface?

    Or, if the car were to roll at a constant speed onto a road that is free to move, if there was traction, then the road would move backwards as the car moved forward? If there was no traction while moving at a constant speed, the car should just keep moving as normal.
  5. Oct 21, 2015 #4


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    I thought the dispute was about a free body diagram. You do not have to run an experiment to draw a free body diagram.
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