# What can you predict given acceleration and initial velocity

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1. Oct 3, 2016

### GiantSheeps

1. The problem statement, all variables and given/known data
If you know the acceleration of a car and its initial velocity, you can predict which of the following?
A. The direction of the car's initial velocity
B. The magnitude of the car's final velocity
C. The displacement of the car
D. All of the above

2. Relevant equations
I said A, which was marked incorrect

3. The attempt at a solution
This was a question on a physics test I took recently, and I said the answer was A because both final velocity and displacement are dependent upon time, which we do not know.

However, I was marked wrong and my physics teacher says that the answer was D.

I feel very strongly that my answer is the correct one, and after typing the question into google I saw that a similar question is frequently asked on tests and quizzes and homeworks, except it includes time interval as well

Ex:
If you know the acceleration of a car, its initial velocity, and the time interval, which of the following can you predict?

...with all the same options. I understand why this answer would be all of the above, but I don't see how you could possibly determine final velocity and displacement given only intial velocity aand acceleration.

So am I off base here? If so, how could you determine time given only acceleration and initial velocity? It looks to me as if she may have just missed that key bit about time interval when copying the question....

2. Oct 3, 2016

### billy_joule

I would agree with you.

Maybe ask your teacher to show you how B & C could be done and she will see her error.

3. Oct 3, 2016

### David Lewis

Velocity is a vector quantity, so if you know the initial velocity then you would know both initial speed and direction. Answer A is correct.

The car’s final speed = vinitial + acceleration x time. You can represent time in this formula as an algebraic variable. Then you have a general solution that works no matter how long the acceleration lasts. Answer B is correct if general solutions are allowed.