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Physics

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A police radar has an effective range of 1.0 km, and a motorist's radar detector has a range of 1.9 km. The motorist is going 100km/h in a 80km/h zone when the radar detector beeps. At what rate must the motorist decelerate to avoid a speeding ticket?

  • Physics -

    Find: a
    Given:
    s = (1.9-1.0)[km]
    u = 100[km/h]
    v = 80[km/h]
    Using two equations:
    <1>: s = u t + ½ a t²
    <2>: v = u + a t

    Rearrange <2> to get t.
    t = (v-u)/a

    Substitute this into <1> to get a.
    s = u(v-u)/a + (v-u)²/(2a)

    Rearrange
    .: a = (v²-u²)/(2s)

    Calculate using the given values.
    I suggest converting from [km/h²] to [m/s²]

  • Physics -

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