Speed in still water/ throwing object horizontally.

  1. 1. The problem statement, all variables and given/known data
    A stone is thrown horizontally with an initial speed of 10 m/s from the edge of a cliff. 4.3 s later the stone hits the ground. What is the height of the cliff?

    A swimmer heading directly across a river 200 m wide reaches the opposite bank in 6 min 40 s. She is swept downstream 480 m. How fast can she swim in still water?

    2. Relevant equations
    The problem here is that I don't understand why some equations are used sometimes...

    3. The attempt at a solution
    For example, for the first question, I just calculated 43 meters, since the speed is 10 meters per second. I am aware that the speed accelerate continuously, but I don't understand which equation to use.

    For the second question, it's the same thing. I found 0.5 m/s.

    Thanks for explaining where I am not thinking correctly. I know that my answers are not correct (well, pretty sure)..
     
  2. jcsd
  3. Hootenanny

    Hootenanny 9,681
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    You need to consider components of the velocities and accelerations, then apply the kinematic equations that I'm sure you know. For example, in the first case, you should split the speed [and acceleration] into two separate values, one for the vertical direction and one for the horizontal direction. Then you apply the kinematic equations to each direction separately.
     
  4. Thanks!

    But in the second question, the water is still... Do the equations still apply?...
     
  5. Hootenanny

    Hootenanny 9,681
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    Not a problem!

    Yes, in the second case the equations still apply. The idea behind the question is to find the component of her velocity which is perpendicular to the banks (and hence the velocity of the water), when she is swept downstream. This will be her speed in still water.
     
  6. Thank you :)
     
  7. Hootenanny

    Hootenanny 9,681
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    Pleasure :smile:
     
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