PHYSICS
posted by Laila .
A bolt drops from the ceiling of a train car that is accelerating northward at a rate of 3.15 m/s2.
What is the acceleration of the bolt relative to the train car?
I don't understand how to do this problem at all... and why isn't it zero? It seems if the train is going forward and the bolt is going backward at the same acceleration the acceleration should be zero?

acceleration is caused by forces.
After the bolt is released, the only force on it is gravity. Horizontally, acceleration is zero, there is no horizontal force on the bolt. Vertically, gravity is operating on it. 
. The problem statement, all variables and given/known data
A bolt drops from the ceiling of a train car that is accelerating northward at a rate of 3.50 m/s2.
(a) What is the acceleration of the bolt relative to the train car?
(b) What is the acceleration of the bolt relative to the Earth?
3. The attempt at a solution
(a)10.4 m/s2 at 19.7° to the south from the vertical
(b)9.8 m/s2 vertically downward
I don't understand why the acceleration in (a) is not vertically downward since it is relative to the train. As we could assume the train is not moving, then the motion should be vertically downward isn't?