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Object launched from a moving vehicle

  1. Jan 16, 2013 #1
    Got into a discussion today at work with a few co-workers about the physics involved in the below scenario, figured the only way for us to come to a better understanding was to ask someone with more extensive knowledge, so here goes,...

    A flat bed truck is moving along at 60mph, a person is standing on the bed at the rear of the truck and throws a baseball forward ( the direction the truck is moving ) at 60 mph.

    how fast is the ball moving?

    I do understand that forces such as wind resistance factor in. Also that if the speed of the ball is measured from the moving vehicle then its speed is only as fast as its thrown. My question is from a fixed point such as the ground, does the speed of the truck add to the speed of the thrown ball.

    please explain keeping in mind my physics knowledge is not as extensive as most of yours !
  2. jcsd
  3. Jan 17, 2013 #2


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    Staff: Mentor

    If we ignore wind resistance... The ball ends up moving at 60 mph relative to the truck and 120 mph relative to the ground. Throw the ball backwards instead of forwards, and it would be moving at 60 mph (in the other direction) relative to the truck and at zero(!) mph relative to the ground- someone standing at the side of the road would see the ball dropping straight down while the truck zoomed away from it.
  4. Jan 17, 2013 #3


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  5. Jan 17, 2013 #4


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    If you factor in relativity, the velocities don't exactly add, but there are some diminishing returns as you approach the speed of light.
    In particular, 60mph combined with 60mph gives 119.999999999905... mph.
    For typical human scenarios, the discrepancy from the simple sum is extremely small.
  6. Jan 17, 2013 #5
    Not only extremely small, but within the error predicted by quantum mechanics.
  7. Jan 17, 2013 #6


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    Also much smaller then our ability to measure either time or distance.
  8. Jan 17, 2013 #7
    I'm adding this because I think it's an important point to make, too:

    If the person on the truck were to just drop the ball then the ball would have a speed of 0mph relative to the truck and 60mph relative to the ground.
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