Physics
Physics
Problem
A block rests on a rough horizontal surface. A student’s free-body diagram shows four arrows that all start at the block’s center: (1) gravity downward, (2) normal force upward, (3) kinetic friction to the left, and (4) an upward “supporting force exerted by the block on itself.” Which revision would make the diagram most consistent with accepted conventions? a) Reverse the kinetic-friction arrow so that it points to the right, opposite the block’s impending motion. b) Remove the “supporting force exerted by the block on itself,” because an object cannot apply a force to itself. c) Replace the single gravity arrow with two component arrows: one vertical and one horizontal. d) Shift the normal-force arrow so that it originates at the contact surface rather than at the center dot.Intuitive explanation
The key insight is recognizing that a free-body diagram shows external forces acting on the object, not internal forces or forces the object exerts on other things. Gravity, normal force, and friction are all legitimate external forces, but a "supporting force exerted by the block on itself" is neither external nor does it belong on a diagram of forces on the block—that's an internal property or a misunderstanding of Newton's third law. Remove arrow (4) and you'll have a proper diagram that clearly shows all real external interactions.
Intuitive explanation
The key insight is recognizing that a free-body diagram shows external forces acting on the object, not internal forces or forces the object exerts on other things. Gravity, normal force, and friction are all legitimate external forces, but a "supporting force exerted by the block on itself" is neither external nor does it belong on a diagram of forces on the block—that's an internal property or a misunderstanding of Newton's third law. Remove arrow (4) and you'll have a proper diagram that clearly shows all real external interactions.