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.
To determine which object will not accelerate, we need to assess whether the net force acting on the object is zero or non-zero.
According to Newton's Second Law of Motion, an object will accelerate when the net force acting on it is non-zero. Net force is the vector sum of all the individual forces acting on an object.
Now, let's consider each scenario:
a) In the case of a ball striking a wall, when the force of the ball on the wall is equal to the force of the wall on the ball, the net force is zero. Therefore, the ball will not accelerate. Thus, this scenario does not involve acceleration.
b) In the case of a gas balloon, when the buoyant force is greater than its weight and air resistance, there will be a net upward force. Hence, the balloon will accelerate upward.
c) In the case of a man in a parachute, if the air resistance is less than his weight, there will be a net downward force. Consequently, the man will accelerate downward.
d) In the case of a rocket fired straight up when the engine thrust is equal to the air resistance, there will be zero net force. Consequently, the rocket will not accelerate.
e) In the case of an airplane in horizontal flight when the thrust of the engine is equal to the drag of the air, there will be zero net force. Consequently, the airplane will not accelerate.
Thus, based on the analysis, the only scenario where the object will not accelerate is (a) when a ball strikes a wall, and the force of the ball on the wall is equal to the force of the wall on the ball.