# Basic Initial Speed Problem

• Greenjacket
In summary: You've summarized the conversation between a student and their physics teacher regarding solving problems involving initial speed and acceleration. The student was unsure of how to rearrange equations and the teacher provided helpful tips and resources. It was determined that the equation a=V-U/t could be used to solve the problem and the student was reminded of the formula v_f = v_i + \Delta v. The teacher also provided a link to a summary of kinematic formulas for further reference. In summary, the student was given guidance on how to approach the problem and provided with helpful resources to aid in their understanding.

#### 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.

Greenjacket said:
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"

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Thanks Doc!