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B Final velocity confusion

  1. Nov 10, 2018 #1
    I have a question which asks me if an object with a mass of 2250kg starts from rest at the top of a frictionless ramp 10m long and is at an angle of 10 degrees horizontally, what is the velocity of the object the moment it reaches the bottom of the ramp?
    I have been trying to work out this question for hours but it baffles me. It does say It can be calculated using the energy method or the linear motion method but I don't properly understand them.
     
  2. jcsd
  3. Nov 10, 2018 #2

    Dale

    Staff: Mentor

    What is the formula for gravitational potential energy and the formula for work?
     
  4. Nov 10, 2018 #3
    PE = mgh and W = Fd
     
  5. Nov 10, 2018 #4

    Dale

    Staff: Mentor

    Oops. I meant the formula for gravitational PE and kinetic energy. Sorry.
     
  6. Nov 10, 2018 #5
    Kinetic energy is 1/2 m v2 but I don't have velocity
     
  7. Nov 10, 2018 #6

    Dale

    Staff: Mentor

    Right, it is what you want to calculate
     
  8. Nov 10, 2018 #7
    So because GPE = 220725
    v2 = 220725-1/2m?
     
  9. Nov 10, 2018 #8
    my calculations were 220725-½2250 = 219600squared
    root 219600 = 468.6 m/s2
    I feel like I am doing something wrong.
     
  10. Nov 10, 2018 #9

    jtbell

    User Avatar

    Staff: Mentor

    Did you really mean to write 220725 minus 1/2m?

    Kinetic energy is 1/2m times v2, not 1/2m plus v2.

    By the way, most people here will read 1/2m as "one divided by 2m", not "one-half m". Better to write it as (1/2)m or more simply m/2.
     
  11. Nov 10, 2018 #10
    I see what I have done, I got the right answer at last. Thanks for the help :)
     
  12. Nov 10, 2018 #11

    jtbell

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    Staff: Mentor

    Also, it's a good idea to "first do the algebra, then do the arithmetic." In this case, start with (1/2)mv2 = mgh, then rearrange the equation to put v all by itself on the left, and finally plug in the numbers and calculate the result in one go on your calculator. See what happens to m?
     
  13. Nov 10, 2018 #12

    Dale

    Staff: Mentor

    Where are you getting the ##-\frac 1 2 m##? Check your units. Does that make sense? Can you subtract mass from energy?

    Always include your units, what you have written is confusing and including units will reduce the confusion and avoid mistakes like this.
     
  14. Nov 10, 2018 #13

    sophiecentaur

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    Science Advisor
    Gold Member
    2017 Award

    Stick with the algebraic representation and re-arrangement until the very end and then substitute the numbers that you have. That way you will be able to solve any such question.
     
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