Results 1 to 3 of 3

Math Help - Exponential decay?

  1. #1
    Banned
    Joined
    May 2009
    Posts
    1

    Exponential decay?

    Hi i've been stuck on a question for ages and don't know if i have even got it right or not, anyone able to help?

    A radioactive mass M(g) in a lump of material is given by M=20e^-0.0015t where t is the time in seconds since the first observation

    i) what is the initial size of the mass?

    I have 20g down for this since t=0

    ii) what is the mass after 1 hour?

    i have 0.0903g since t=3600

    iii) the half life of a radioactive substance is the time it takes to decay to half its mass. What is the half life of this material?

    I can only get this by the trial and error approach and would like help if anyone can tell me how to work it out.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by tuzzzan View Post
    Hi i've been stuck on a question for ages and don't know if i have even got it right or not, anyone able to help?

    A radioactive mass M(g) in a lump of material is given by M=20e^-0.0015t where t is the time in seconds since the first observation

    i) what is the initial size of the mass?

    I have 20g down for this since t=0

    ii) what is the mass after 1 hour?

    i have 0.0903g since t=3600

    iii) the half life of a radioactive substance is the time it takes to decay to half its mass. What is the half life of this material?

    I can only get this by the trial and error approach and would like help if anyone can tell me how to work it out.
    If t_{1/2} is the half life we have:

    10=20 ~e^{-0.0015 t_{1/2}}

    so:

    e^{-0.0015 t_{1/2}}=0.5

    Take logs:

    -0.0015 t_{1/2}=\ln(0.5)

    etc.

    CB
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    May 2009
    Posts
    3
    Ahhhh thank you so very much, I understand now
    Cheers
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Exponential Decay
    Posted in the Math Topics Forum
    Replies: 6
    Last Post: April 15th 2013, 12:19 AM
  2. exponential decay
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: July 22nd 2010, 11:03 PM
  3. Decay rates in exponential decay
    Posted in the Math Topics Forum
    Replies: 2
    Last Post: December 21st 2009, 06:20 AM
  4. Exponential Decay
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: October 11th 2008, 06:48 AM
  5. Exponential decay
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: March 31st 2008, 08:33 AM

Search Tags


/mathhelpforum @mathhelpforum