1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Bus question

  1. May 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A bus travels 650km in 3 hours less than the other bus whose speed is 15 km/h slower than the speed of the first bus.

    A. Assemble the equation.
    B. Solve the equation and state the speeds of the buses.

    2. Relevant equations

    3. The attempt at a solution
    V = d / t
    Velocity of the first bus = Velocity of the second bus - 15; V1 = 650 / t2 - 3
    Time of the first bus = Time of the second bus - 3; V1 - 15 = 650 / t1 + 3

    I don't really know.
  2. jcsd
  3. May 28, 2013 #2
    I'm not even sure If I can use physics equations in a mathematics exam
  4. May 28, 2013 #3


    User Avatar
    Science Advisor
    Homework Helper

    Of course you can use v=d/t. I think you are just getting confused with all the variables. Just pick one. Let t be the time of the faster bus. Now write the velocity relation just using the variable t.
  5. May 28, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Pls use enough parentheses to avoid confusion.
    You'll do better if you write the second equation using the same two unknowns.
  6. May 28, 2013 #5

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Either use a single t, or else use the two of them properly, as in:
    [tex] V_1 = 650/t_1\\
    V_1 - 15 = 650/t_2\\
    t_2 = t_1 + 3
    You will have three unknowns ##V_1, t_1, t_2## and three equations, so you ought to be able to solve. Your blunder was to put ##t_2-3## in the first equation and ##t_1 + 3## in the second equation, but never specifying what is the relation between ##t_1## and ##t_2##.

    BTW: if you do insist on putting the +3 and -3 in your two equation, you need to write them properly. What you wrote means, literally,
    [tex] V_1 = \frac{650}{t_2} - 3.[/tex] If you mean
    [tex] V_1 = \frac{650}{t_2 - 3}[/tex] then you need to use parentheses, like this:
    V1 = 650/(t2-3). Do you see the difference?
  7. May 29, 2013 #6
    While the advice above is reasonable, I think we're skipping steps here.

    There are FOUR equations in FOUR unknowns to start with. The four unknowns are ##t_1, v_1, t_2, v_2## for the time and velocity of the two buses on the 650km journey.

    One of the equations is
    $$t_1v_1 = 650$$
    ... find the other three equations.

    Then we can reduce the system to TWO equations in TWO unknowns by substitution.

    Then we will be able to reduce to a quadratic equation in one unknown.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted