Introductory kinematics word problem

1. Jul 4, 2009

shaools

1. The problem statement, all variables and given/known data

A competitor is aiming to complete a 1500m wheel-chair race in less than 4.0 minutes. After moving at a constant speed for exactly 3.5 minutes, there were still 240m to go. What must his acceleration before the remaining distance if he were to finish the race on time?

Given:
d1= 1260m [forward]
d2= 240m [forward]
t1= 210s
t2= 30s

Required: v1, v2 possibly?, and a

2. Relevant equations

v1 = d/t
and d = v1*(t+a(t)^2)/2 ?

3. The attempt at a solution

v1=1260m/210s
v1=6m/s

d = v1*(t+a(t)^2)/2
a= (2d/t^2)-v1*t
a= (2*240m/30s^2) - 6m/s*30s
a= something horribly incorrect

the answer at the back of the book is 0.13m/s^2. im guessing that im not approaching the second part of the question correctly. i tried it again with an equation using v2, but im still getting weird answers.

also, im sorry i dont know how to use latex :/

2. Jul 4, 2009

tiny-tim

Welcome to PF!

Hi shaools! Welcome to PF!

(don't bother about LaTeX for something like this … the SUB and SUP tags are fine, and they save a bit of server space also! )
hmm … you're messy with brackets, which leads you to make mistakes

that middle line is wrong.

3. Jul 4, 2009

shaools

wow.

i managed to screw up the initial equation twice in that post. there's no way to edit posts is there?

i meant to post this equation for displacement and acceleration:

http://id.mind.net/~zona/mstm/physics/mechanics/kinematics/EquationsForAcceleratedMotion/Origins/Displacement/Image78.gif" [Broken]

soo...

d = v1*t + (a*t^2)/2
a = 2d/(t^2) - v1*t

and then when i plug my values in, i get funky answers.
i think i might just be using the wrong equation, or i might be plugging in incorrect values. im really not sure.

thank you for checking tho!

Last edited by a moderator: May 4, 2017
4. Jul 4, 2009

tiny-tim

Nope … still wrong

Write it out step by step … don't try to do any of it in your head!

5. Jul 4, 2009

shaools

ah, bested by bedmas

lol let me attempt this once more...

a= [2(d - vt)]/ t^2 ?

6. Jul 4, 2009

shaools

a = [2(240m - 6m/s*30s)] / 30^2
a = 0.13m/s^2

yup :).

thank you mr. fish !

7. Jul 4, 2009

tiny-tim

:tongue2: blubblelubbleglubblelubblephlrrrrrrp! :tongue2: