In this picture, a ball is thrown in the air. "G", or acceleration due to gravity, is a given that tells us how fast the ball should be moving while in the air. As the ball is thrown in the air, it accelerates at the constant -9.8 meters per second per second. It slows down, reaches its apex, then comes back down accelerating at -9.8 meters per second squared. If the ball were to be thrown so high that it would take five seconds to come back down, it would accelerate to a scary -49.0 meters per second squared!