Answer: No external force acts on the system.
- A No external force acts on the system
- B Speed is constant
- C All forces are balanced internally
- D Friction is absent
Correct answer: A. No external force acts on the system
Explanation: Conservation of momentum: total momentum is constant when the net external force on the system is zero.
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.