Re: Simple physics
The weight of any object has been clearly defined to be the product of the mass(m) and the gravity acceleration (g). It won't change whichever state the object is at, staying, moving, accelerating, ... even thought it DOES vary slightly for different places on the earth.
Let's forget about the reletivity effect. According to what you said, I think what you are asking is the force that is moving the object right at the crashing. You know the force is a vector and, therefore, you can't simply add the two terms mg and ma together, especially in the situation stated in your question. When the car craches into a wall, the direction of a is along the horizontal direction while the g in the vertical direction. Therefore, at the crashing, your weight does not change, but you have another force along the horizontal direction rushing you into the wall.