posted by Rob .
An object accelerates (changes its speed) only if the forces acting on it in one direction are greater than the forces in the opposite direction, All of the following objects will accelerate EXCEPT?
A) a ball striking a wall, in which the force of the ball on the wall is equal to the force of the wall on the ball
B) a gas balloon in which the buoyant force is greater than its weight and air resistance
C) a man in a parachute when the air resistance is less than his weight
D) an airplane in horizontal flight when the thrust of the engine is equal to the drag of the air
D) will result in unaccelerated motion in a straight horizontal direction