GED Science TestPhysical ScienceMedium
A student uses a spring scale to pull a wooden block across a rough surface at a constant velocity. The scale reads 15 Newtons. If the student then lifts the block slightly while still pulling it at a constant velocity, and the scale now reads 10 Newtons, what is the most likely reason for the change in the scale reading?
- AThe kinetic energy of the block decreased.
- BThe normal force acting on the block decreased.
- CThe coefficient of kinetic friction decreased.
- DThe gravitational force on the block increased.
Show answer & explanationAnswer & explanation
Correct answer: B. The normal force acting on the block decreased.
Friction force is directly proportional to the normal force (the force pressing the surfaces together). By lifting the block slightly, the student reduces the normal force, which in turn reduces the friction force, requiring less pulling force (10 N instead of 15 N) to maintain constant velocity.
Why the other options are wrong
- A. Maintaining constant velocity means kinetic energy is constant, not decreasing.
- C. The coefficient of kinetic friction is a property of the surfaces in contact and does not change when the normal force changes.
- D. Lifting the block slightly does not increase gravitational force; it remains constant unless mass changes.
Friction Force
A force that opposes motion between surfaces in contact.
- Static friction opposes initial motion, kinetic friction opposes ongoing motion.
- Proportional to the normal force (F_friction = μ * F_normal).
- Depends on the coefficient of friction (μ) between the surfaces.
- Acts parallel to the surfaces, opposite to the direction of motion.
Memory trick: Friction's Friend is Normal Force.