Confusing Physics Question

1. Feb 27, 2005

jen333

Just looking over some extra physics questions and I came upon this one, it seems quite easy, but difficult when i actually attempted it:
Just as a traffic light turns green, a waiting car starts off with a constant acceleration of 6.00m/s^2. At the instant the car begins to accelerate, a truck with a constant velocity of 21.0m/s passes in the next lane. How far will the car travel before it overtakes the truck? How fast will the car be travelling when it overtakes the truck?
I attempted this question by changing the car's acceleration to a velocity by multiplying with an amount of time, etc.
I imagine that this question could possibly be done with trial and error,but i'm just wondering if there's a quicker way to attempt this question or a specific formula. Any hints or pointers, plz post!

2. Feb 27, 2005

jen333

problem solved!

I have solved this question, therefore i am no longer using this thread. If there is anyone who wishes to post here, by all means go ahead!
Thx,
jen333

ps) for those who would like to know, the answer is 24m/s.

3. Mar 1, 2005

Atomos

It has been a while, so maybe I am wrong, but this is what I got:
$$0.5at^2=21t$$
$$3t^2=21t$$
$$t^2=7t$$
$$t={0, 7}$$

therefore:
$$v=ta$$
$$v=42$$

Last edited: Mar 1, 2005
4. Mar 1, 2005

dextercioby

Yes,the velocity is 42m/s...It's correct.And the distance is 147m...

Daniel.