SAT Subject Physics Practice Question 623: Answer and Explanation
4. A batter hits a long fly ball. Neglecting air resistance, the baseball's horizontal component of speed is constant because it
A. is not acted upon by any forces.
B. is not acted upon by gravitational sources.
C. is not acted upon by any vertical forces.
D. is not acted upon by any horizontal forces.
E. the net forces acting on the baseball are zero.
Correct Answer: D
The correct answer is (D). According to Newton's first law, an object in motion remains in motion and an object at rest remains at rest unless a net force acts on the object. The ball is in free fall. Even though it is accelerating on the y axis (up and down), nothing is affecting the ball on the x axis (horizontal)