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Car travelling at constant speed (embarassed to ask)

  1. Jul 17, 2008 #1

    If a car provided 10 N force and friction force is 10 N wouldn't the forces cancel out and be zero and nothing happens. If so why is that we say car travels at constant speed. If you can explain this to be I'll be very greatful. Thanks!!
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  3. Jul 17, 2008 #2


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    This is just Newton's first law. An object at rest stays at rest and an object in motion stays in motion with constant speed in the same direction unless acted upon by an unbalanced force. Since the two forces cancel out (i.e. are balanced) the car tends to stay in motion at a constant speed and direction.
  4. Jul 17, 2008 #3
    Constant speed just basically means that the car's acceleration is not changing resulting in an absolute value of velocity that stays fixed.

    In your example above, if you don't apply enough force to get the car to move, then yes, nothing will happen.

    In order to apply that force, you need some sort of acceleration. Once you have accelerated enough to overcome the force of friction and get your car moving, you can keep the car at a constant speed by keeping a constant amount of throttle/gas to the engine.

    If you weren't to keep a constant throttle, friction would eventually slow the car down until it stops again, but that would mean you would have a negative acceleration, resulting in a non-constant speed.

    As you can see, just because your acceleration is 0 doesn't mean your speed is 0.
  5. Jul 17, 2008 #4
    Thanks for your help:smile:. So is the car moving forward with a constant force of 10 N now.vWouldn't the car stop because net force is zero?
  6. Jul 17, 2008 #5
    If you stopped applying a constant force of 10N, then the car would stop because you'd have friction working against you. If the net force is 0, then the motion remains constant.

    Basically if you have an imbalance, something is going to speed up or slow down (via F = ma). You don't experience this if the forces sum to 0, which is why your velocity would be constant.
  7. Jul 17, 2008 #6
    Thanks a lot for your help BryanP :smile:. I understand the friction case and but I'm still bit confused with the motion. So if the car engine provides a contious force of 10 n and friction constantly provides 10 n why would the car move. I know you are right but in my head I'm still bit confused. Do you have any other analogies or something to explain this.
  8. Jul 17, 2008 #7


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    No it wouldn't. The car needs a net force to slow it down. When the car is slowing it is accelerating and since Newton's second law states [itex]F=ma[/itex], when there is no net force there is no acceleration.
  9. Jul 17, 2008 #8
    If you are providing the car with 10N and the force of friction requires you to exceed 10N to move the car, then if you're providing the same amount of force of friction, you will not move (assuming this car was NOT moving to begin with).

    In order to move the car you NEED to provide more force than the opposing force in your situation (which is friction).

    Once you overcome that force, lets say you apply 20N of force to get the car to move, you will have some velocity (without doing the actual calculation, lets say your velocity is 100 km/h).

    Now lets say you step on the pedal just enough to keep the car pushing with a force that matches the force of friction, then the car will stay at your speed of 100 km/h.

    If you don't put ENOUGH acceleration, meaning the car is pushing less than 10N in this case, the car will start to slow down. If you don't supply any gas at all, the force of friction will just take over until your car reaches a halt.

    If you put more gas, causing a net force of over 10N, your car will speed up... meaning it will be going faster than 100 km/h.

    As you can see, if you balance the forces, your car will stay in constant speed. You could be in motion or at rest. If the car is going 0 km/h when you're applying the force equal to the friction, then you will remain at 0. If you're going 100 km/h while you're applying the force equal to friction, then you will remain at 100 km/h until you let friction slow you down, or if you provide more acceleration to the car.
    Last edited: Jul 17, 2008
  10. Jul 17, 2008 #9


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    You seem to have forgotten Newton's First Law. An object will either remain stationary or will continue moving with a constant velocity until it is acted upon by a net force. Since the car has no net force, it will continue moving with a constant velocity since it is moving already.

  11. Jul 17, 2008 #10
    I appreciate your lengthy respone :smile:. Thanks a lot it did clear up many of my problems. Why is that to start the car we need more than 10 N. Is it because the car's intial speed is 0 km/h and if we provide 10 n back the net force would be zero (zero kmh) . Or I think I'm getting it now so even though there is no apparent change because the forces are balanced the car would still travel at constant speed. Thanks again for your help. It was very useful:smile:
  12. Jul 17, 2008 #11
    Imbalance of net forces is what causes something to accelerate/decelerate and move or not.

    As ZapperZ said, you could be stationary or already in motion. If the forces balance, then your velocity will remain constant.
  13. Jul 17, 2008 #12


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    So far the answers have been referig to Newtonian physics, I wonder if thinking in terms of relativity would prove illuminating? It has been stated that, according to Newton's laws, an object at rest tends to stay at rest and an object in motion tends to stay in motion. But from a relativistic viewpoint, the object in constant motion is at rest. For example, when you are in the car travelling at constant speed, you are, to all practicle purposes, sitting still. Unless you look out the window or feel the bumps in the road, you can't tell you're moving. A person sitting in the seat next to you gets niether closer nor farther away. From your "frame of reference," you are sitting still.

    This might help you get a better understanding of your own statement here:
    Yes, if the force from the engine and the force from resistance due to friction are the same, the car's speed will remain constant, and that includes a speed of Zero. So, if the car at rest has a resistance of 10n, and you apply 10n of force, the car's speed remains constant at Zero. If the car at 50kph has a resistance of 10n (wouldn't be true, but if it were...), and you are applying 10n of force, the speed remains constant at 50kph.

    Did I belabor that too much? Anyhow, sounds like you've got it.
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