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Thread: Length Dilation and Projectile Motion Questions

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    Length Dilation and Projectile Motion Questions

    Hey so this is from a past paper and I just can't figure out how to do it.

    19) The rest length of a train is 200m and the rest length of a railway platform is 160m. The train rushes past the platform so fast that , when observed in the patform's frame of reference, the train and the platform are the same length.

    How fast is the train moving?

    a) 0.6c -> is the answer
    b) 0.75c
    c) 0.8c
    d) 1.25c

    20) A ball is launched horizontally from a cliff with an intial velocity of u m/s. After two seconds, the ball's velocity is in the direction 45 degrees from the horizontal. What is the magnitude of the velocity in m/s at 2 seconds?
    a) u
    b) 1.5u
    c) 19.6
    d) 27.7 -> is the answer

    I've tried drawing them out, rearranging formulas bt i'm just not getting the answer.

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    Re: Length Dilation and Projectile Motion Questions

    For q19 just use the formula:
    Lv = L0*sqrt(1-v^2 /c^2)
    Sub Lv = 160 and then sub in the options for v and you find whichever one gets you L0 (200m) which is v = 0.6c.
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    Re: Length Dilation and Projectile Motion Questions

    Oh thanks you for question 19), i forgot that u can just use the answers from multiple choice and sub it in!

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    Re: Length Dilation and Projectile Motion Questions

    For q20:

    let V be the speed at 2 seconds
    let down be the positive direction

    Vy = Vcos45

    Sub this into Vy = Uy + at, Uy = 0 because there is no vertical velocity at the start
    Vcos45 = 9.8(2)
    V = 27.7 (D)

    So I guess the initial information of u m/s is a distractor
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    Re: Length Dilation and Projectile Motion Questions

    It is launched horizontally, so initially you can consider it to be at it's turning point. Draw a diagram it helps visualise it
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