# Homework Help: Acceleration/veloctiy/displacement/time question

1. Sep 3, 2009

### phongbui

Ok i just started gr 11 physics, and im wondering how would you tackle this problem

In a long distance race, Michael is running at 3.8 m/s and is 75 m behind robert, who is running at a constant velocity of 4.2 m/s. If Michael accelerates at 0.15 m/s2, how long will it take him to catch Robert?

Im assuming in this question you need 2 equations to find the unknown, I don't know how to make an equation with robert being ahead 75m.. :(

Possible formulas that could be used are

V = d/t

d = v1 x t + 1/2(a)t2 <-- (time squared)

a = vf - vi / t <--- final velocity - intial velocity

d = vi + vf /2 (t)

Please keep things simple ^^ as ive just started gr 11 physics :)

Thanks for the help

2. Sep 3, 2009

### rl.bhat

Welcome to PF.
You need two equations to solve this problem.
In time t Robert move a distance x. The in the same time what is the distance moved by Michael when he meets Robert?

3. Sep 3, 2009

### phongbui

hmm... so the first equation would be t = d / 4.2m/s

than the 2nd part... would be d = (3.8m/s)(t) + (1/2)(0.15m/s2)(t2)

is this right so far?

4. Sep 3, 2009

### rl.bhat

The first one is
d = 4.2 m/s*t
And in the second one the displacement is d + 75 m.

5. Sep 3, 2009

### phongbui

Ohhh... Thanks very much