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Homework Help: Equation to find density of metal

  1. Nov 4, 2005 #1
    i need help please what si unit do i use to messure the mass of a coin i believe that you would have to use grams but i believe i could be mistaken
    also how woul i calculate the weight of a lump of metal of mass 3.4 x 10 to the power of -six
    I forgot what is the equation to find density of metal and weight of metal under water
    i do not want the answer just a step in the right direction
    Last edited: Nov 4, 2005
  2. jcsd
  3. Nov 4, 2005 #2
    The SI unit for mass is the Kilogram, so using grams is ok.

    Weigh is a force, Newtons 2nd law states; Force = Mass x Acceleration.

    The mass of the object will not change if placed in water. Similarly the acceleration due to gravity does not change in water, therfore it`s wieght is unchanged if on land or in water.

    Q. What is the definition for density?
  4. Nov 4, 2005 #3


    User Avatar
    Gold Member

    The SI unit for mass is kg.
    The weight of and object is proportional to the mass and the acceleration of gravity.

    EDIT: Just saw zanazzi78's reply.
  5. Nov 4, 2005 #4
    10 to the power -6 would be what in normal digits
  6. Nov 4, 2005 #5
    not sure what you mean but ...

    1 x 10 ^ -6 = 0.000001
  7. Nov 4, 2005 #6
    kk thanks i have got a test and are finding out how to do these types of question how would i do this one pleae

    A 2g mass falls 10m. Calculate the kinetic energy of the mass after falling this distance. Gravitational acceleration = 10 m/s ^ 2
  8. Nov 4, 2005 #7
    Do you know the equation for Kinetic energy?
    Do you know the relationship between Potential energy and kinetic energy?
  9. Nov 4, 2005 #8
    The equation for Kinetic energy is

    K.E = 1/2 Mv ^ Kinetic energy +1/2 Mass x Velocity

    Potenial enery and kinetic energy forgot what they had in common
    Last edited: Nov 4, 2005
  10. Nov 4, 2005 #9
    [tex] K.E. = \frac{mv^2}{2}[/tex]
    P.E. = mass x gravity x height
    Q. How are P.E. and K.E. related?
    this will help you work out the K.E. of the system!
    Last edited: Nov 4, 2005
  11. Nov 4, 2005 #10
    i think i have got the answer is it 5.67893 ami right if i am wrong can you show the steps u did to do it
  12. Nov 4, 2005 #11
    How did you get that value?

    if you tell my what you did, i`ll show you where you went wrong.

    What are the units for Kinetic Energy?
    Last edited: Nov 4, 2005
  13. Nov 4, 2005 #12
    the units for kinetic energy are K.E?

    how u find velocity i think i know what to do now thanks
    Last edited: Nov 4, 2005
  14. Nov 4, 2005 #13
    Mass is mesured in Kg
    Time in seconds,
    Energy in ...

    Your answer is wrong. PF forbids people from giving answers. If you show your workings i can help you.
  15. Nov 4, 2005 #14
    i done it twice i don't know if either one is correct

    First attempt

    K.E = ? Mass = 2

    Gravitational acceleration = 10

    so i worked out i need to find velocity so the formula for velocity is V = M x A
    V = 2 x 10 = 20 so velocity is 20 now i have done that i need to do
    K.E = MV ^2 all divided by 2 so thats 40 divide by 2 equal 20 is this right?

    Second atempt

    I think Kinetic Energy = 1/2 Mass x Velocity squared.

    Therefore: 1/2 x 0.002 x Velocity squared.

    We need to work out velocity.

    V = U squared + 2 accel x distance
    V= 0 + 2 x 10 x 10
    V = 200 but that doesnt at all sound right.

    Therfore = 1/2 0.002 x 200 squared
    = 40
  16. Nov 4, 2005 #15
    When you read a question think "What information do i have? how can i use this info?" If a question gives you a height then you need to use it some where, teachers don`t give you info in a question to confuse you!

    Fact; "The lost in potential energy is equal to the gain in Kinetic energy"

    The question gives you the height of the falling object! - 10m

    It gives you mass 2g or 0.002kg

    It gives you acceleration due to gravity - 10m/s^2

    PE= m.g.h

    Therefore you know all the values on the right hand side!:rolleyes:

    But for the K.E expression you only know the mass :confused:

    The question is designed to test your knowlede of the relationship between K.E. and P.E.

    so you work out the P.E. of the object when it is 10m above the ground and then state that once it has fallen 10m, then the very same, amount of energy it "transfered" into Kinetic energy.

    so we now have ...

    P.E. = 0.002 kg x 10 m x 10m/s^2
    = 0.2 ?, you never answered my question "what is energy measured in?"
    so i`ll ask it again, what are the units here? You will loose marks if you don`t get the units right!
  17. Nov 4, 2005 #16
    Energy is messured in joules
  18. Nov 4, 2005 #17
    so what did i get wrong?
    Last edited: Nov 4, 2005
  19. Nov 4, 2005 #18
    only your approach to the question:biggrin:

    You made the assumption that since the question was asking about Kinetic energy you needed to use [tex]\frac{mv^2}{2}[/tex] you don`t. The question gave you all the info you needed to work out the potential energy.

    you first attemp was allmost right you need to use si units i.e. kg inorded to get teh correct value of joules, there is a big dirffrence between 20 and 0.2!

    hope this helped.
  20. Nov 4, 2005 #19
    yes thank you for all your help you really helped me thanks
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