1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to find the mass

  1. Nov 29, 2012 #1
    it reads, '' two identical objects move with speeds of 5m/s and 25m/s. what is the ratio of their kinetic energy



    so i think i need to use the equation ke=1/2 mv^2
    this is how far i got ke=12.5*m and ke=312.5*m
     
  2. jcsd
  3. Nov 29, 2012 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Hello chris.hart. Welcome to PF !

    What is the ratio of those two numbers?

    How does that ratio compare to the ratio of the two speeds?
     
  4. Nov 29, 2012 #3

    i cant know what the ratio is without finishing the arithmetic? or am i missing something
     
  5. Nov 30, 2012 #4
    Before using any numbers, write out the ratio using symbols and show us what you get.
     
  6. Nov 30, 2012 #5
    ratio of their KE somehow means that you need to compare them. How does the first one compare to the second.
     
  7. Nov 30, 2012 #6

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I get that (312.5m)/(12.5m) = 25.0 .

    That's the ratio of the kinetic energy of the faster object to the the kinetic energy of the slower object.
     
  8. Nov 30, 2012 #7
    thanks guys but i figured out my problem.
    id use the formula pe=ke which is the equivalent of mgh * .5mv^2
    so i can just simply eliminate the mass on the both sides and solve the equation
     
  9. Nov 30, 2012 #8
    Hmmm potential energy and gravity have nothing to do with this problem.

    A ratio of two quantities is just one divided by the other.
     
    Last edited: Nov 30, 2012
  10. Dec 1, 2012 #9
    Since the objects are identical they have the same mass Ek1/Ek2=½mv1^2/½mv2^2=v1^2/v2^2...
     
    Last edited: Dec 1, 2012
  11. Dec 1, 2012 #10

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Those velocities should be squared .
     
  12. Dec 1, 2012 #11
    You're right.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook