# Question on my lab?

So we did a lab this week in my physics class.

We had a car on a horizontal table, that was attached to a string with weights (The weights were over the edge of the table.) We then calculated the experimental acceleration and the calculated acceleration. These numbers were different.

My teacher asked us "What was causing the difference between the calculated accel. and experimental acceleration..."
He also mentioned something to do with Newton's laws.

Do you know the answer/Can help me out?