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Acceleration vs accelerator

  1. Apr 2, 2013 #1
    Will a car continue to accelerate after the accelerator is removed? I know this doesn't seem to make sense, but everytime I let go of the accelerator, the pointer still shows an acceleration though the value is decreasing. I'm just confused about whether it's a decreasing acceleration or constant deceleration.

    Any ideas? :shy:
     
  2. jcsd
  3. Apr 2, 2013 #2
    You have to remember that the speedo is not showing acceleration. The car can still be gaining speed even if the car is decelerating in the direction it is traveling.


    Damo
     
  4. Apr 2, 2013 #3
    the car wont get accelaration after you let go of accelaration since there is no force to give more accelaration in this case ,insted what you see is constant deceleration.
     
  5. Apr 2, 2013 #4
    By definition deceleration is negative acceleration ie. the velocity is changing in the opposite direction to the motion. (velocity is decreasing)... So your statement can't be right. Or am i missing something?

    Did you mean to say, "The car can still be moving forward even if the car is decelerating" ?
    OR
    "The car can still be gaining speed even if the cars acceleration is decreasing" ?
     
  6. Apr 2, 2013 #5

    ZapperZ

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    You did not miss anything. The car cannot be gaining speed in the direction of travel if it is decelerating.

    I would like the OP to clearly explain what he/she is observing here, because that is where this all rests upon. This statement

    "... everytime I let go of the accelerator, the pointer still shows an acceleration though the value is decreasing ... "

    needs to be examined carefully, because there is a strong possibility that the OP is misinterpreting something, or he/she could be going downhill when this happened. We all have let go of our gas pedal and on a flat road, I have never seen the vehicle I'm driving continue to speed up. So this observation does not match what we all have seen previously, without having to understand any physics yet.

    Zz.
     
  7. Apr 2, 2013 #6

    CWatters

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    Which pointer? If you mean the pointer on the speedometer then what do you mean by "still shows an acceleration"? Are you saying the speed continues to increase ?

    It might help to know that the accelerator pedal on a car does not behave like an ideal accelerator. The pedal is called an accelerator pedal but its behavior is much more complicated.

    What's the difference...

    Ideal Accelerator: The more you press the faster is should accelerate. Suppose you want to go at a constant 50mph? Constant velocity = zero acceleration so once you going 50mph you would need to release the accelerator totally. If you wanted to accelerate to 70mph you would press it again and when 70mph is reached you would again release it totally. How far you press the pedal would determine how long it took to get from 50 to 70mph.

    Car accelerator: When pressed the car will accelerate. The harder you press the faster it will accelerate. So far this is the same as an ideal accelerator pedal. However your car cannot accelerate indefinitly. As it gets faster the rate of acceleration reduces until the car stops accelerating and travels at a constant speed. You have to maintain pressure on the pedal in order to maintain that speed. So once upto speed the pedal is really behaving like a speed pedal NOT an accelerator pedal.
     
  8. Apr 2, 2013 #7

    sophiecentaur

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    Sounds like you could have a sticky accelerator pedal. No accelerator, no acceleration - no gain in speed.
     
  9. Apr 2, 2013 #8
    There's a scale on my dashboard with numbers 0, 1, 2, 3... (not more than 8 i think). When I press the accelerator, the pointer will go up to around 2 to 3 immediately. After I removed the pedal, the pointer will slowly come down. It takes some distance for it to decrease from 3 to 2.

    Not sure if it's because my car is too old? :blushing:
     
  10. Apr 2, 2013 #9
    The theory is simple: no fuel, no power, no acceleration.

    In practice there are mechanical delays. In older cars rapid acceleration or high power might open additional carburetor jets....there was also an acceleration pump used....this would dump additional fuel into the intake manifold and that would take a few moments to be consumed....

    Sometimes these extra jets are even provided via a second carb...I had a Corvette with two four bbl carbs with a removable linkage between carbs for operation of one...or both. By manual manipulating the fuel control linkage I could dump raw fuel into both carbs....you could see it flow!!

    So it might be possible for any mechanical linkage to introduce an extremely brief delay during which time additional fuel continues to be provided and burned and some acceleration might be possible....I doubt it would register as increased speed via a speedometer but if it does it would be very brief.

    I suspect modern computer controlled fuel injected engines would have negligible delay.
     
    Last edited: Apr 2, 2013
  11. Apr 2, 2013 #10

    ZapperZ

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    That is not an example of a vehicle that continues to accelerate. Read your original post and see why many of us are scratching our heads by your statements, especially the one I quoted. Can you see why you made a mistake here?

    A vehicle, especially if it is highly efficient and on a smooth road, can coast for quite a while before you notice a drop in speed. This is not unusual. But this is different than saying it continues to accelerate!

    Zz.
     
  12. Apr 2, 2013 #11
    The scale on the dashboard is the Tachometer which measures the Revolutions per minute of the engine. They can increase as the car slows as the gears change. I have never seen a regular car that measures acceleration.
     
  13. Apr 2, 2013 #12
    Also 0 mph to 60 mph in 8 seconds (pretty respectible) is an acceleration of .34g. If your car does 8 that would be impresive.
     
  14. Apr 2, 2013 #13

    Drakkith

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    I'm in agreement with Schicog. I believe the gauge you are reading is the Tachometer which measures the number of revolutions the engine is performing every minute (RPM, usually measured in 1,000's). When you press the accelerator down the engine immediately revs up and starts to apply torque to the drivetrain to increase your velocity. Once you let off the accelerator less fuel is given to the engine, but it generally stays revved up for a few seconds and slowly drops. (How fast depends on the car)
     
  15. Apr 2, 2013 #14

    CWatters

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    coconut62 - What are the units on the display? I suppose it could even be the battery charge current, although I think most of those read to 0 to 30 or 60Amps ?

    Does the meter change if you rev the engine while it's in Neutral/Park?
     
  16. Apr 2, 2013 #15

    A.T.

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    Most cellphones have accelerometers. Get an app that displays it, and do the test again.
     
  17. Apr 2, 2013 #16

    sophiecentaur

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    I think we could be confusing a subjective sensation in the car and the reality of the dynamics.
    If you want to indicate the acceleration, just hang a pair of furry dice or a plastic St Christopher from the drivers mirror (a pendulum) and see which way it hangs. If it is hanging towards the rear of the car then you have positive acceleration and if it hangs forward, you have negative acceleration ('deceleration'). An alternative is to have a nervous aunt in the passenger seat and you just listen for how loud the intakes of breath are.
     
  18. Apr 2, 2013 #17

    Dale

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    I agree with the others that suggest it is the tachometer. Look closely at the gauge, it should say something like "RPM x 1000".

    If you put the car in neutral or park and lightly step on the gas the engine will rev up and the needle should swing to 2 or 3 while speed remains at 0.
     
  19. Apr 3, 2013 #18

    A.T.

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    While the original issue has been probably solved here is something that often causes confusion:
    Acceleration in physics is defined as a vector: dV/dt, and the sign of its components depends on the coordinate choice. So there is no reason why acceleration, that is opposed to velocity, must be negative in general.

    What the quotes above assume is an alterntaive defintion of acceleration: d|V|/dt
     
  20. Apr 3, 2013 #19
    To not confuse the OP I kept it to simple straight line motion. I didn't say it but I implied that the direction of travel is positive and hence negative/decreasing change in velocity was negative acceleration. I could make the velocity negative and the acceleration positive in this example it makes no difference. Obviously if the velocity and acceleration are made up of more than one component then it makes little sense to say negative acceleration.

    In straight line motion if velocity is positive and it is decreasing then we have negative acceleration. Not sure where I am wrong here?
     
  21. Apr 3, 2013 #20

    HallsofIvy

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    I don't believe there is any guage on a car that directly measures acceleration- although I once had a car that, when you accelerated too fast, the "check oil" warning light came on!
     
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