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Falling rock

  1. Jan 30, 2017 #1
    1. The problem statement, all variables and given/known data
    Rock is falling from 10meters 18km/h speed , what is speed of rock just before it touches the ground

    2. Relevant equations


    3. The attempt at a solution
    So in meters its like 5m/s and h=10m so t=2s if h=d , v=t*d=2s*10m=20m/s but i think i somehow should include gravity too so im stuck
     
  2. jcsd
  3. Jan 30, 2017 #2

    mfb

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    Staff: Mentor

    Why?
    The speed won't stay constant.
    This formula assumes the rock starts at rest. It does not.

    Don't blindly put values into formulas. Think about what is happening and then which formula could be useful.

    Conservation of energy is the easiest approach here, using the kinematic formulas (correctly) is possible as well.
     
  4. Jan 30, 2017 #3
    How could i use kinetic energy when i have no mass and no energy only height and velocity
     
  5. Jan 30, 2017 #4

    mfb

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    The mass will drop out of the calculations.

    You can also start with "energy per mass" everywhere, doesn't matter.
     
  6. Jan 30, 2017 #5
    Mfb is correct.

    If my assumptions are correct, you are still in the early stages of learning Physics where mass can often be ignored, mainly just for practice.

    Jump into the earlier chapters of your book (if you aren't there already) and skim through any sections that focus on "Particle Under Constant Acceleration."

    Just keep in mind that this early stage of physics requires tons of assumptions and ignorance. Don't let it fluster you (read a head and practice with other variables if you are inclined).
     
  7. Jan 30, 2017 #6
    I was just watching the Particle Under Constant Acceleration , but there's given time , and acceleration , could you atleast tell me what formulas should i use
     
  8. Jan 30, 2017 #7
    As much as I hate doing this, because this is how I was treated, you should find it yourself.

    The location of the formula was given, along with the general name of it, and you already know you don't have to worry about mass. I opened my book right up to chapter 2, skimmed, found the name and equation, and posted on this thread. Go for it. It's easy.
     
  9. Jan 30, 2017 #8

    mfb

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    That is the point of your (!) homework problem.

    Multiplying or adding numbers is something your calculator can do. Figuring out which numbers to multiply or add is the actual physics.
     
  10. Jan 30, 2017 #9
    If my calculations were correct should i get 15,15m/s?
     
  11. Jan 30, 2017 #10
    My calculations were 14.66, but I did them off the top of my head.

    Your answer makes sense but always check it again.
     
  12. Jan 30, 2017 #11
    Maybe that's because i took 10m/s^2 and you 9.8m/s^2 this is possible too
     
  13. Jan 30, 2017 #12
    That is probably the most likely case.

    And just as a heads up, physics is wholly about logical problem solving a **** ton of research. If the research isn't your thing, get better at it or trying a different field (not attempting to deture you).
     
  14. Jan 30, 2017 #13

    gneill

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    We won't confirm possible guesses. We don't know w
    You should become familiar with what are known as the SUVAT equations (look it up!). For kinematic problems such as this you will almost always find that one of the SUVAT equations suits the situation by involving the right mix of variables (the givens and the unknown).
     
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