Newton's First Law for Constant Velocity

Steps:

• Constant speed in a straight line means constant velocity (no acceleration).
• Newton's First Law states that zero net force causes constant velocity.
• Resultant force is the net force on the object.
• Thus, resultant force must be zero.

Why D is correct:

• Newton's First Law: An object in motion stays in motion at constant velocity unless acted on by a net force, so resultant force equals zero.

Why the others are wrong:

• A: Weight acts downward (mg); resultant force is the horizontal net force, which is zero.
• B: Resistive force equals the driving force for constant speed, making their resultant zero, not equal to resistive force alone.
• C: Mass times velocity is momentum (p = mv), not force (F = ma).

Final answer: D