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Homework Help: Quick physics (motion) question:

  1. Jun 5, 2006 #1

    I'm new to the forums. I just have a quick question. Anyway, I got a question/scenario:

    Let's say there's a car traveling at 65 mph....if i was to throw a baseball at 60 mph outside in the same direction the car is going, what would be the speed of the baseball? Would it actually travel faster then the car moving at 65 mph? Just a general question. If you can direct me to some law's this problem poses please do so.

    Thank you
  2. jcsd
  3. Jun 5, 2006 #2

    Doc Al

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    If you were able to throw a baseball at 60 mph with respect to the car, then its speed with respect to the earth would be 65+60 mph.

    This is called addition of velocities under Galilean relativity (as opposed to Einsteinian relativity, which must be used when speeds are much higher).

    Welcome to the forums, by the way!
  4. Jun 5, 2006 #3
    Well, I'm still very new to the physics field so I can't give you the equations/laws involved but the short answer to your question is "yes"

    Part of the reason is that the ball already has the same momentum (speed) as the car. Any additional momentum added to the ball will increase its speed so it will go faster than the car. Now if you threw the ball at 60MPH faster than the car, it will almost immediately slow down due to gravity, friction with the air, lack of mass to maintain velocity over distance, etc. The only way that ball would continue to go at 125MPH would be if you were in a weightless vacuum (aka space)

    If anything I have said is wrong, somebody please correct me. I am also interested in any equations dealing with this.
  5. Jun 5, 2006 #4


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    I'm sorry sniper but I have some problems with your reply that I sould point out for the OP's sake.

    I'm afraid the ball does not have the same momentum as the car. You imply that momentum and speed are the same, they are not. Momentum is the product of an objects mass and velocity; a car is much more massive that a ball, therefore if they are traveling at the same velocity they will have very different momentums.

    You also say that

    This also has eroneous implications. You imply here that a very massive object will not acclerate when acted upon by an unbalanced force. What would happen if you through a bowling ball out of the window? Would it keep going forever?
    Last edited: Jun 5, 2006
  6. Jun 5, 2006 #5

    Thank you, I knew I had some of the terminology wrong but I couldn't think of the right word at the moment.
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