Answer: An external force.
- A The force of gravity
- B An external force
- C The force of friction
- D Its own inertia
Correct answer: B. An external force
Explanation: By Newton's first law, a body maintains uniform velocity unless an external unbalanced force acts on it.
Free-body diagram of a block on an incline: weight (mg) resolves into mg sinθ along the slope and mg cosθ into the slope, balanced by the normal force N and friction f.
Concept context
Newton's three laws, friction, circular motion, and free body diagrams.