# How to determine velocity from speed? *edit?

I got in a brief argument yesterday jumped the gun and said "Acceleration = Change in speed/ time in which it occurred"

Now today because is said "speed" Not velocity I'm losing the argument. What I need is

What information is required (besides objects speed and direction) is required to calculate velocity and in turn calculate acceleration.

Assuming all variables are obtainable is velocity just speed and direction? Just assuming its an object in motion what would I need to know about that object to calculate velocity. And say it was changing directions what effect on velocity would that have?

### 3 Answers

- Anonymous9 years agoFavourite answer
ok, I'm gonna take a stab at it but I'm having a hard time understanding your question:

The reason acceleration is not the change in speed per unit time is because of the speed versus velocity distinction. Speed is the distance traveled in a certain amount of time. Velocity is the displacement from an initial position in a certain amount of time.

For example:

A rectangular track has dimensions of 100 meters by 50 meters. If you were to run around this track in 10 minutes, you would travel a total distance of 100+50+100+50 meters. This means that your average speed was 30 meters per minute. However, your displacement is zero when you return to your original position. While you have moved, you have not made it anywhere. Your average velocity would be zero.

The key point here is that velocity is a physics term that describes both rate of change in somethings position per unit time, AND the direction of that change. Importantly, acceleration is DEFINED as the rate of change in velocity per unit time. Acceleration, therefore, implies direction since velocity has a direction (they are both vectors).

In order to determine something's velocity, you just need to know how many units of distance are traveled in a single line each second/minute/unit time. If something is changing directions continuously, its velocity, by definition, is also changing with each change in direction.

- Andrew SmithLv 79 years ago
Velocity is just speed with a direction.

But CHANGE in velocity requires you to subtract V2 - V1 AS A VECTOR.

This is a completely different answer from S2-S1 ( where S is the speed alone)

I cannot show how to add or subtract vectors here.

But I can give one example.

Initial speed is 30 m/s North.

Final speed is 30 m/s East.

Time to change is 10 seconds

S2 - s1 = 0

Change in speed / change in time = 0

V2 - V1 = 42 m/s South East.

Acceleration = change in velocity / change in time

= 42/10 = 4.2 m/s south east.

Note how different the two answers are.

Look up vectors for more information.

- math fanLv 49 years ago
If you have direction (being a unit vector in the direction of motion) call it U. And speed is v. Then velocity is simply vU (which is a vector. If direction is changing velocity is changing. The change in velocity also depends on the change of speed.