A sled of mass 5.0kg is pulled across a frozen lake by a light rope that makes a 25∘ angle above the horizontal. A student proposes the following two lists of forces.
List X (forces acting ON the sled):
1. Weight of sled pulling downward
2. Normal force from ice pushing upward
3. Tension from rope pulling up and forward
4. “Rope’s reaction force” pulling back on the sled
List Y (partner forces exerted BY the sled):
1. “Sled pushes on Earth” downward
2. “Sled pulls on ice” downward
3. “Sled pulls on rope” down and backward
4. “Sled receives friction force” backward
a. Evaluate List X. Identify any forces that should be removed or corrected, and justify your choices.
b. Evaluate List Y. Identify inaccuracies in the student’s characterization of the partner forces. For each entry that is incorrect or incomplete, provide the correct description, including direction and the object on which the force acts.
c. Explain how establishing a clear system boundary for the sled resolves the student’s confusion about which forces belong on List X versus List Y.