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Homework Help: Very simple question (basic kinematics)

  1. Dec 18, 2014 #1
    1. The problem statement, all variables and given/known data

    I'm taking my first basic physics course and I came across this simple question (which I got the wrong answer apparently). The question was this: what is the velocity and acceleration of an object which has reached it's maximum height after being thrown directly upwards?

    2. Relevant equations

    Well, you don't really need equations for this, unless you want to mathematically prove your answer. In that case:

    Acceleration = (Velocity - Initial Velocity) / Time
    Time = Distance / Average Velocity
    Velocity = Initial Velocity + Acceleration(Time)
    Distance = Initial Position + Average Velocity(Time)

    These are very basic Kinematic equations.

    3. The attempt at a solution

    What I answered was that the velocity is 0 (which was correct) and that the acceleration was also 0 (which was incorrect). I went by the test a little too fast and realized my answer was most likely wrong after comparing answers with other students. The mistake I made was assuming that since it was at 0 velocity, there couldn't possibly be any acceleration whatsoever, but I did not take into account that acceleration = gravity. Since gravity is always influencing an object, there must be a rate of acceleration (-9,8 m/s). But my question is how could there possibly be an acceleration for an object which has a velocity of 0?
  2. jcsd
  3. Dec 18, 2014 #2
    The velocity is zero only at one instant of time. A second earlier it was positive (the ball was moving upward), and, a second later, it was negative (the ball was moving downward). So the velocity was decreasing the whole 2 seconds of time.

    P.S., You chose a really great avatar. I thought that the Tyson Cosmos series was wonderful, and I enjoyed it immensely.

  4. Dec 19, 2014 #3
    I see. So gravity is a constant influence on the movement of objects, regardless of their velocity. Then could you say that an object lying on the floor with no movement also has an acceleration? Since gravity is applying a downward force on it. But my question has been answered, thank you very much.

    P.S., Haha thanks, I loved the show more than any other I've ever watched.
  5. Dec 19, 2014 #4

    No. If the object is lying on the floor, there is another force acting on it besides gravity. It is the upward force of the floor on the object. This upward force cancels the downward force of gravity, so the object is not accelerating.

  6. Dec 19, 2014 #5
    Now I understand. Thank you for your help!
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