posted by barby .
an object accelerates (cahnges 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) a rocket fired straigt up, when the engine thrust is equal to the air resistance
e) an airplane in horizontal flight when the thrust of the engine is equal to the drag of the air
e) is the answer. A plane in horizontal flight has lift force equal to weight in the vertical direction. If drag equals engine thrust, then is no acceleration in the horizontal direction, either.