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 folloeing 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 straight 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

To determine which object will not accelerate, you need to analyze the forces acting on each object and compare them.

Let's go through each choice and evaluate the forces:

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. In this case, the forces on the ball are equal but opposite in direction. Therefore, the net force is zero, and the ball will not accelerate.

b) A gas balloon in which the buoyant force is greater than its weight and air resistance. A gas balloon rises because the buoyant force (which is the difference between the weight of the balloon and the weight of the displaced air) is greater than its weight and air resistance. The net force is in the upward direction, causing the balloon to accelerate upward.

c) A man in a parachute when the air resistance is less than his weight. When a person jumps out of an aircraft with a parachute, the air resistance increases significantly. If the air resistance is less than the person's weight, the net force would be downward, causing the person to accelerate downward.

d) A rocket fired straight up, when the engine thrust is equal to the air resistance. If the engine thrust is equal to the air resistance, the net force on the rocket would be zero, resulting in no acceleration.

e) An airplane in horizontal flight when the thrust of the engine is equal to the drag of the air. In this case, if the thrust of the engine is equal to the drag of the air, the net force on the airplane would be zero, and it would not accelerate.

From the analysis above, the only object that would not accelerate is 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.