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A point mass moves in the x-y plane under a constant acceleration vector that is directed perpendicular to the x-axis (i.e., along the y-direction only). Which of the following quantities does NOT change with time?

1. Only the y-component of the velocity vector
2. Only the x-component of the velocity vector
3. Only the y-component of the acceleration vector
4. Only the x-component of the acceleration vector

Correct answer: Only the x-component of the velocity vector

Solution

With acceleration only in the y-direction, aₓ = 0 (constant) and a_y = constant. The x-velocity remains unchanged (vₓ = const), while the y-velocity changes. Both components of acceleration are constant (aₓ=0, a_y=a). The question asks what does NOT change — the x-component of velocity does not change. Note that both x and y acceleration components are also constant, but the option asking about 'only x-component of acceleration' is also technically correct for not changing. The most standard intended answer is that vₓ remains constant.

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