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I What is the effect of centripetal force on velocity?

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  1. Sep 13, 2016 #1
    If a car is being driven with a constant force (the angle of the pedal is kept constant) and it goes around a curve, will the velocity change? If so, is this due to centripetal force?
     
  2. jcsd
  3. Sep 13, 2016 #2
    Of course, it will change. If it did not change then you will go in a straight line and not around a curve.
     
  4. Sep 13, 2016 #3
    Don't you mean that the velocity changes because you are going around a curve?
    Could you clarify why it is changing, given that the forward force is the same? Why does going around a curve exert a counter force?
     
  5. Sep 13, 2016 #4

    jbriggs444

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    Ummm, because there is a sideways force?
     
  6. Sep 13, 2016 #5
    What is the source of this sideways force?
    People, it might be obvious to you, but it is not to me, which is why I am asking the question, so a more in depth answer would be much appreciated.
     
  7. Sep 13, 2016 #6

    CWatters

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    Velocity has components speed and direction. Going around a corner means the direction is changing so the velocity is changing. The speed need not be changing.

    Forum rules discourage us just giving people the answers because they learn more when they figure things out for themselves. Have a think about the forces that act on a car. Have you tried to push one sideways? Why would that be hard? Why do you have to slow down in wintery conditions?
     
  8. Sep 14, 2016 #7
    Ok, I thought velocity and speed were the same thing. So, if a car is being driven with a constant force and it goes around a curve, does that mean the speed also remains constant?
    Because it is heavy?
    Because the road is slippery?
     
  9. Sep 14, 2016 #8

    jbriggs444

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    The idea that "velocity" and "speed" are different is drilled into one's head constantly in first year physics. As others have said, "speed" is how fast you are going without regard to direction. By contrast, "velocity" takes direction into account. 50 miles per hour forward is a different "velocity" than 40 miles per hour forward and 30 miles per hour leftward even though both are exactly the same "speed".

    In an ideal car -- perfect wheels with no rolling resistance and no sideways slip, and no air resistance, there would be no reason to put your foot on the accelerator at all. You could coast at constant speed forever. Since your car requires force to maintain a constant speed, we need to consider what makes your car less than ideal.

    So.. how about you tell us? Why does your car slow down if you do not keep your foot on the gas?
     
  10. Sep 14, 2016 #9

    CWatters

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    So what is the car relying on to turn a corner?
     
  11. Sep 14, 2016 #10

    A.T.

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    What does this mean in terms of forces?
     
  12. Sep 14, 2016 #11
    Well, I'm not a physicist, and I'm hoping this forum is open to those physicsally challenged!

    So, if you add a leftward force (wind, say) of 30 mph to a car that was going 40 mph it would then be going 50 mph? (computed by Pythagoras I guess)

    I feel you want me to say friction. But is that a force? Air resistance? Both?
     
  13. Sep 14, 2016 #12
    Turning the wheel? Is that exerting a sideways force? Does that effect the speed?
     
  14. Sep 14, 2016 #13

    jbriggs444

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    Yes, that's Pythagoras in action.

    But you you cannot directly equate a leftward force with a leftward addition to velocity. There is a relationship via quantities known as momentum or impulse. A leftward force applied for a specified time interval will produce an impulse which is the product of the two. The leftward impulse divided by the mass of the car will tell you how much its leftward velocity changes as a result.
     
  15. Sep 14, 2016 #14
    Just to clarify, are you answering yes to the question as I'd thought the answer would be no. Are you saying that if the forward speed of a car is 40 mph, which is then exposed to a leftward force (wind, say) of 30 mph, the car would then speed up (over time) to go at a forward speed of 50 mph?

    I would have thought that the speed would still be 40 mph, but it would be pushed to the right to change direction, if the steering wheel was not turned.

    If the steering wheel was turned, into the wind, the speed and direction of the car would remain the same. Yes?
     
  16. Sep 14, 2016 #15

    jbriggs444

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    No, as I said, the relationship between force and speed is not direct. The relationship between wind speed, then force, then car speed is even less direct. And if you do not turn the steering wheel, a lateral force will (ideally) have zero effect on the vehicle.

    If you turn the steering wheel, the car turns. When you normalize the wheel, the car retains its current heading. The wind does not enter in.

    Edit: If you want to imagine an airplane, then wind velocity adds as a vector to airplane velocity to get ground velocity, but that's not the subject matter here.
     
    Last edited: Sep 14, 2016
  17. Sep 14, 2016 #16

    A.T.

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  18. Sep 14, 2016 #17
    Let's come back to speed later, but first can we clarify this point.

    This doesn't seem right to me. If there is a lateral force on an object surely that object will move laterally. If it is wind then the car will be moved laterally (maybe only slightly), so it would be necessary to turn the steering wheel into the lateral force to maintain the heading. Generally we don't notice the effects of wind except when we go across a windy bridge and we have to make an abrupt correction with the steering wheel to maintain our heading.

    Similarly, if a car is being driven along a straight road but one which slopes from left to right the force of gravity would draw the car to the right. However, by turning the steering wheel to the left the straight heading of the car is maintained. In essence we would be applying an upward force to compensate for the downward force of gravity. Do you not agree?
     
  19. Sep 14, 2016 #18
  20. Sep 14, 2016 #19

    A.T.

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    The fundamental force behind friction is electromagnetic.
     
  21. Sep 14, 2016 #20

    jbriggs444

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    No, I do not agree. The rightward turning of the steering wheel creates a rightward angle of the front wheels on the car which tends to cause the car to progressively turn farther and farther right. This is desirable if it maintains the desired course while the vehicle "crabs" left under the influence of a leftward wind. However, if one maintains the steering in this position (ignoring caster effects on the steering mechanism) the car will continue to turn further right than is desired. Ideally you want a one-time rightward steering input to counter a continuing leftward force.

    In addition, I used the word "ideally" in my earlier posting. Ringing in non-ideal effects is not appropriate.
     
    Last edited: Sep 14, 2016
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