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Homework Help: Wierd question involving time and acceleration

  1. Jan 14, 2009 #1
    1. The problem statement, all variables and given/known data

    A pedestrian is jogging on a path and sees a boat coming towards him. He's moving at 10 km/hr and the boat is moving at 20km/hr, how quickly does the jogger approach the boat?

    2. Relevant equations

    I'm guessing these:
    Vf = vi + a * t
    D = vi*t +1/2 a * t^2
    Vf^2 = Vi^2 +2a * D

    3. The attempt at a solution
    well i converted the speeds to m/s
    Jogger:2.77 m/s
    Boat:5.55 m/s
    other than that I've got no idea
    I know we need a time and a distance and maybe acceleration?
  2. jcsd
  3. Jan 14, 2009 #2
    1. Draw a diagram with all the information you have from the problem. That should always be your first port of call.
    2. Consider the information you're given (velocities) and the phrase in the question "how quickly does the jogger approach the boat?" With no other data than those two velocities, what quantity are you limited to being able to find? Do you know what 'relative velocity' means?
  4. Jan 14, 2009 #3
    no we have not discussed relative velocity
  5. Jan 14, 2009 #4
    look at the two velocities that are given to you. your only given velocities, no other information is given. not even distance or time.

    relative velocity is when you compare an objects velocity to a relative object.

    so for example, a poles velocity relative to the earth is 0 m/s, whereas your velocity relative to the earth could be 2 m/s. look at those two velocities, ask yourself "would they be coming at each other at a faster speed or slower?".
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