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    Hi,

    I know this might sound like a silly question but it's worth 3 marks and so I didn't know whether I'd missed something.

    The question says:
    Car mass = 90kg
    Speed = 30kmh^-1
    Air resistance = 90N
    Forward driving force = 170N

    Part One asks you to work out the acceleration which I got to be 0.08m/s^2

    It then says...
    A student is told that the driving force is kept constant for 5s and is asked to calculate the final speed of the car. Explain why the equation v=u+at cannot be used.

    Would I be right (and complete) in saying that we cannot use this equation because we do not know the magnitude of the resistance force at the new speed?

    Thanks,
    Blake
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    It's not explitly stated in the question but as you increase your speed you'll increase the resistance force. This means that the NET force is non-constant, leading to non constant acceleration. This means you cannot use the suvat equations (including v=u+at) as they are constant accelerations equations
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    You would have to integrate an acceleration expression or differentiate a position expression
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    (Original post by EwanWest)
    It's not explitly stated in the question but as you increase your speed you'll increase the resistance force. This means that the NET force is non-constant, leading to non constant acceleration. This means you cannot use the suvat equations (including v=u+at) as they are constant accelerations equations
    Awesome, I thought so, I didn't think my answer was worth three marks but I didn't include why the resistance force would change, thanks
 
 
 
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