Physics acceleration and velocity question

when does acceleration is positive but velocity is negative?
when does velocity and position are positive and acceleration is negative?

thanks :)

Acceleration is positive and velocity is negative: I'm going to assume you mean the object is travelling backwards by "velocity is negative." Acceleration will be positive when the object's velocity is approaching positive infinity. So if the velocity were to be negative, meaning the object is travelling backwards, then the object must be approaching to either a complete stop or a positive velocity in order to have positive acceleration, and the object's velocity must be increasing at all times in order to maintain a positive acceleration.

Velocity and position are positive and acceleration is negative: Simply the opposite of what I wrote for your first question.

Take a look at the graphs in this web page. They show the Position, Velocity and Acceleration of the object as it moves. http://www.physicsclassroom.com/mmedia/k…

It depends on what referance you are considering.
If you are trying to stop something than accel. and velo. are of opp. signs.