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Homework Help: Relative Velocity of a pickup truck

  1. Sep 15, 2008 #1
    1. The problem statement, all variables and given/known data
    A person riding in the back of a pickup truck traveling at 70 km/h on a straight, level road throws a ball with a speed of 15 km/h relative to the truck in the direction opposite to the truck's motion. What is the velociy of the ball (a) relative to a stationary observer by the side of the road, and (b) relative to the driver of car moving in the same direction as the truck at a speed of 90 km/h?

    2. Relevant equations
    VBT = 15 km/h
    VBG = VBT + VTG
    VBG = 15 km/h + 70 km/h

    Trying to find the velocity of the ball with respect to the ground (observer)

    I’m not sure if this is correct but the answer I got was 85 km/h

    3. The attempt at a solution

    My notes in class said add the velocity of the truck with respect to the ground plus the velocity of the ball with respect to the truck to get (a). I'm not sure where to start with (b)?
  2. jcsd
  3. Sep 16, 2008 #2
    You don't really need any formula's to solve this. Just think about it logically for a second.

    Before the ball is thrown in the direction opposite of the truck it is moving at the same speed as the truck right?

    When the ball is thrown, if we pretend the truck is not moving, then the ball will look like it is traveling 15 km/h backwards by an observer in the truck.

    But the truck is moving! So if the ball is 15 mph backwards from a truck moving 70 km/hr, two concidences logically follow.

    1.) The ball has to be moving slower than the truck. This means that the ball cannot be moving 85 km/h, unfortunately.

    2.) Relative to the stationary observer (the observer by the side of the road) the ball will be moving 70km/h - 15km/h

    You must remember that velocity is a vector that has both a magnitude and direction, you accounted for the magnitude fine, but you switched the direction of the ball.

    Does this make sense?
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