Constant Velocity Implies Zero Net Force

Steps:

• Identify motion: Block slides down incline at constant velocity, so acceleration is zero along the board.
• Resolve forces: Gravity component down the board is mg sin θ; normal force perpendicular; kinetic friction opposes motion, so up the board with magnitude μ N = μ mg cos θ.
• Apply Newton's second law: For constant velocity, net force along board is zero, so friction up equals mg sin θ down.
• Direction check: Friction acts up the board to oppose downward slide; resultant force is zero.

Why B is correct:

• Newton's first law states that constant velocity requires zero resultant force; friction up the board balances the gravitational component down.

Why the others are wrong:

• A: Friction opposes motion (up, not down); resultant zero, not down.
• C: Friction up, not down; resultant zero is right but friction direction wrong.
• D: Resultant down would cause acceleration, violating constant velocity.

Final answer: B