SAT Subject Physics Practice Question 9: Answer and Explanation
9. Which one of the following statements is true concerning the motion of an ideal projectile launched at an angle of 45° to the horizontal?
A. The acceleration vector points opposite to the velocity vector on the way up and in the same direction as the velocity vector on the way down.
B. The speed at the top of the trajectory is zero.
C. The object’s total speed remains constant during the entire flight.
D. The horizontal speed decreases on the way up and increases on the way down.
E. The vertical speed decreases on the way up apeed changes. D is false because the horizontal component is constant throughout the motion.
Correct Answer: E
Since the acceleration of the projectile is always downward (because it’s gravitational acceleration), the vertical speed decreases as the projectile rises and increases as the projectile falls. A is false because the acceleration vector is straight down, but the velocity is never straight up or straight down. It would be true of the vertical component of the velocity, but not of the total velocity. B is false because the projectile still has its horizontal velocity at the top of the trajectory. C is false because the vertical component of the velocity changes, even if the horizontal component does not, so the total speed changes. D is false because the horizontal component is constant throughout the motion.