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Acceleration of a car and jet plane problem

  1. Nov 7, 2018 at 2:11 PM #1
    As we know, POWER is what accelerate a car, FORCE itself doesn't accelerate a car, if FORCE cousing movement it change into work, and work over time = POWER.

    The FORCE from jet engine pushes a plane forward. But as we know (or I'am wrong) FORCE don't push enything, only POWER couses acceleration. This is why I'am confused

    Engine power from the car goes to the gearbox etc.... and finally to the wheels.
    The wheels begin to rotate, and this rotation (acceleration) is cousing by TORQUE at the wheels or POWER at the weels (i think power)?

    What is correct and what is wrong with what I write?
     
  2. jcsd
  3. Nov 7, 2018 at 2:24 PM #2

    berkeman

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    But F = ma...
     
  4. Nov 7, 2018 at 2:28 PM #3
    So F = ma works in jet engine but why we dont use it at car engine? - and we combine "ma with distance over time" to calculate power ?
     
  5. Nov 7, 2018 at 5:42 PM #4

    jack action

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    The force causes the acceleration (##F = ma##);
    The power determines the speed you can achieve with the given force (##P = Fv##).

    So, if you want to keep a constant acceleration, as the velocity increases, you will have to increase the power (##P=mav##).

    Usually, you have a limited amount of power (i.e. a limited mass fuel rate to burn), so the available acceleration will decrease as the velocity increases.
     
  6. Nov 7, 2018 at 6:51 PM #5

    berkeman

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    I think it's more accurate to look at the momentum of the exhaust from the jet or rocket to calculate the "thrust". What links have you been reading about how jets and rockets work? Have you read about Specific Impulse yet? :smile:
     
  7. Nov 7, 2018 at 9:55 PM #6

    Dale

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    As was pointed out by @berkeman and @jack action this is incorrect. It is the force that causes the acceleration by F=ma, not the power. For instance, when accelerating from rest the acceleration is non zero and the force is non zero, but the power is zero. If it were power that caused acceleration then nothing at rest could ever start moving.
     
  8. Nov 8, 2018 at 2:20 AM #7
    It depends on the actual problem you want to solve that what tool is practical. If you do it right then calculations with power might represent a shortcut when force will depend on speed, for example (for most vehicles, it does and it is quite complicated, actually). Yet, it is still force what causes acceleration - by calculating with power you just skipping some complicated math.
     
  9. Nov 8, 2018 at 3:31 AM #8


    At 20:56 this guy talk about what we feel when we accelerate a car. He said that power cause acceleration not moment of force (torque)... Goshh I got so little brain to understand this ...:)
     
  10. Nov 8, 2018 at 4:40 AM #9

    CWatters

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    What you feel is the reaction force from the seat due to acceleration. What causes that accelerstion is the net force or torque acting on the car.

    He goes further to say that it's the power that determines the torque so you are really feeling the power. That's true but don't confuse power with maximium power. Petrol engines typically have discrete gear ratios so you cannot always operate the petrol engine at the right rpm to deliver max power. So torque isn't always at a maximum.

    Things are slightly different for an electric motor. They can generate close to max power over the whole of the speed range including low speeds where more of that power is available to accelerate the car (less needed to overcome drag). So an electric car can sometimes accelerate faster than a petrol car even though its maximum power output is lower.
     
  11. Nov 8, 2018 at 6:55 AM #10

    Dale

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    The guy is wrong. This is why we use the professional scientific literature as our primary source here
     
    Last edited: Nov 8, 2018 at 7:06 AM
  12. Nov 8, 2018 at 8:57 AM #11

    jack action

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    We can only feel forces. Our brains can interpret those as accelerations.

    The reason why power is considered the source of acceleration is because power is actually the source of the force. It is fairly simple to state ##a=\frac{F}{m}## and end it there. But having an acceleration necessarily means the velocity increases. As the velocity increases, we need more power to maintain the same force (##F=\frac{P}{v}##). Otherwise - keeping the same power input - the force will drop, therefore the acceleration will decrease as well.

    This is why we can state «We need power to accelerate» (##a=\frac{P}{mv}##) even though it is the force that causes acceleration. With half the power, you will get half the acceleration as the velocity increases because you will only produce half the force (comparing the motion at the same velocity). When in motion, we need power to get a force.
     
  13. Nov 8, 2018 at 9:02 AM #12

    Dale

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    Not if v=0.

    You need force to accelerate, that is it. The force is provided by the torque, so it is the instantaneous torque that provides acceleration.

    The problem is that people look at engine specifications and think that the max torque or the max power is the instantaneous torque or power. It is the instantaneous torque that gives acceleration at any point, but the instantaneous torque may be less than the max torque and the instantaneous torque may be limited by the max power.
     
    Last edited: Nov 8, 2018 at 9:12 AM
  14. Nov 8, 2018 at 9:13 AM #13

    jack action

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    Yes, but if you constantly stay at zero then you're not accelerating. As soon as you'll reach v=0+, power will become relevant.
     
  15. Nov 8, 2018 at 9:23 AM #14

    Dale

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    Sure, but it directly disproves the claim that you "need power to accelerate". You do not because acceleration can be nonzero while power is zero.

    Again, instantaneous torque is the only parameter that is directly related to the instantaneous acceleration at all times.
     
  16. Nov 8, 2018 at 9:31 AM #15

    jack action

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    The claim is that you need power to get a force while in motion. And you need a force to get an acceleration.

    When comparing vehicles in motion, I can assure you that the one with the greatest power can always accelerate faster than the other one. In this context [NoteToSelf]always state the context[/NoteToSelf], you need power to accelerate.
     
  17. Nov 8, 2018 at 9:40 AM #16

    russ_watters

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    While I agree more with Dale, let me try to spin the statement in a way that creates agreement:
    We need linearly increasing power to continue the same rate of acceleration (not including losses) as speed increases.

    To say that in car terms: almost any car will jump off a starting block at at a high acceleration. I had a 92 horsepower coup that I could spin the tires on if I let off the clutch to fast. But in order to keep accelerating rapidly after the initial jump, you need high horsepower. Indeed, the purpose of the gearing is partly to enable constant power acceleration (except for a Tesla, which can do constant torque acceleration).
     
    Last edited: Nov 8, 2018 at 1:59 PM
  18. Nov 8, 2018 at 10:50 AM #17

    Dale

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    I am ok with that, because if you want to keep accelerating then you need to maintain a high torque and the faster you go the more power is required for the same torque. It is still the instantaneous torque that causes the acceleration, but the instantaneous torque is limited by the maximum power (at high speeds) rather than the maximum torque (which is the torque limit at low speeds).
     
  19. Nov 8, 2018 at 12:07 PM #18

    Dale

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    The claim from the video is “power is what accelerates the car so it is always the power that you feel”. That claim is wrong.
     
  20. Nov 8, 2018 at 12:12 PM #19

    jack action

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    Agreed. Full disclosure: I didn't watch the video.
     
  21. Nov 8, 2018 at 12:18 PM #20
    So " always power you feel" is wrong. It should be like this:
    Engine power generate a force which accelerate (and force you feel) a car because F = ma.
    More power = more force pushing car forward.

    Is that correct?
     
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