• Jan 9th 2009, 10:10 AM
mattty
The distance, d (in metres), which an object travels. is given by the equation $d=ut-8t^2$ where u is the initial velocity (in metres per second) and t is the time in seconds it has been travelling. Show that there is only one value for t when $u^2 =32d$

I'm having real trouble solving this equation for t using any algebraic method that I've learnt for quadratic equations.
I can't figure out how to put it into the quadratic formula simply for the fact that i can't get any of the variables by themself on either side of the equation to set as x.

The steps I've tried so far in order to rearrange for t are:
$d=ut-8t^2$
$d+8t^2=ut$
divide both sides by t
$d/t+8t=u$
take d/t away from both sides
$8t=u-d/t$
and here is where i lose track, because i figure out that i can't rearrange for t at all. i've considered converting d/t into V (final velocity) but i can't see that helping much.
Any suggestions would be much appreciated.
• Jan 9th 2009, 10:59 AM
Constatine11
Quote:

Originally Posted by mattty
The distance, d (in metres), which an object travels. is given by the equation $d=ut-8t^2$ where u is the initial velocity (in metres per second) and t is the time in seconds it has been travelling. Show that there is only one value for t when $u^2 =32d$

.

Rearrange the expression for $d$:

$
8t^2-ut+d=0
$

This is an equation relating $t,\ u$ and $d$ .

If for some particular $d$ and $u$ we wish to solve for $t$ we will use the quadratic formula. Now the discriminant for the quadratic is:

$
D=u^2-32d
$

and it is well known that we have no real roots for $t$ if $D<0$, two real roots if $D>0$, and exactly one real root if $D=0$.

The last condition above is the one applicable here.

.
• Jan 9th 2009, 08:35 PM
mattty
Many thanks for this, apparently maths at 3am isn't my strong point :D