# Homework Help: Basic Initial Speed Problem

1. May 11, 2008

### Greenjacket

[SOLVED] Basic Initial Speed Problem

Hello,
I've just picked up physics in year 10 and I'm keen to get started. So far I've been fine; I've been taught how to find velocity and acceleration of certain objects, but I'm unsure how to re-arrange the equations I've learnt to solve other problems.

1. The problem statement, all variables and given/known data
After 6 seconds of acceleration at 2.5ms-2, a car is moving at 60ms-1. What was the initial speed of the car if the acceleration was:
a. Positive
b. Negative

2. Relevant equations
I think the equation I should use is a=V-U/t (must learn LaTeX...), but I have a feeling I have to re-arrange it somehow.

3. The attempt at a solution
If I multiply the time by acceleration (6x2.5ms-2), I get the change in speed(15ms-1) right? Also, subtracting the initial speed from the final speed gives me, again, the change in speed (which I don't know) right? If I use a trial-and-error method and simply guess the figures, I'd most likely get it wrong. Please help. I'm not expecting anyone to give me the exact answer, but if someone could point me in the right direction it would be greatly appreciated.

2. May 11, 2008

### Staff: Mentor

Right. The magnitude of the change in speed is 15 m/s. If the acceleration is positive, the change is +15 m/s; if negative, -15 m/s.

The final speed is initial speed + change:
$$v_f = v_i + \Delta v$$

You can use that to solve for the initial speed for both case.

A summary of kinematic formulas is given here: https://www.physicsforums.com/showpost.php?p=905663&postcount=2"

Last edited by a moderator: Apr 23, 2017
3. May 11, 2008

Thanks Doc!