Finding Initial Speed: A Basic Physics Problem Solved

[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 learned to solve other problems.

Homework Statement

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

Homework 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.

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.

 

[Doc Al]

If I multiply the time by acceleration (6x2.5ms-2), I get the change in speed(15ms-1) right?

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"

 

[Greenjacket]

Thanks Doc!