A puck slides over smooth ice after a hit. Someone draws its free-body diagram with one extra arrow: a forward force labeled 'the hit'. The hit ended a moment ago.
Is that diagram right? Checking a diagram is a skill of its own.
Three errors cover almost every bad free-body diagram: an extra force that no object exerts, a force that belongs on another object's diagram, and a missing contact force.
The test for any arrow: name the object that exerts that force, right now. No object, no force.
For example, here is the puck's correct diagram: weight down, normal force from the ice up, and a small friction force backward, against the sliding.
Now the same diagram with one more arrow: a forward force labeled 'the hit'.
Remove that arrow, and the diagram is correct again: weight, normal force, friction.
A binder slides across a desk after a shove. Its diagram shows weight down, normal force up, and friction backward. Which verdict fits this diagram?
A drink coaster is flicked across a table and slides on its own. Its diagram shows weight down, normal force up, friction backward, and a forward arrow labeled 'the force of the flick'. The flick has ended. Which verdict fits?
Now the same diagram with the friction arrow erased.
A moving box is dragged across a rough floor by a rope. Its diagram shows weight down, normal force up, and the rope's tension forward. Nothing else is drawn. Which verdict fits?
Last, friction goes back in place, and one new downward arrow is added: 'the force from the puck on the ice'.
So check every free-body diagram with three questions: does each arrow have an exerting object; does each force act on this object; is any touching object's force missing.
A cafeteria tray slides over a smooth counter. Its diagram shows weight down, normal force up, friction backward, and a forward arrow labeled 'the slide'. Evaluate the diagram.
Test the forward arrow: which object exerts 'the slide'? No object does.
A force needs an exerting object, so that arrow is not a force.
The other three arrows pass: Earth exerts the weight, and the counter exerts the normal force and the friction.
One error: an extra force that no object exerts. Remove the forward arrow.
A rolling wagon's diagram shows its weight, the normal force, and a downward arrow labeled 'the force from the wagon on the road'. Evaluate that third arrow.
Ask what the third arrow's force acts on: the road.
This is the wagon's diagram, and it may carry only forces acting on the wagon.
The force is real, but it is drawn on the wrong diagram.
One error: a force that belongs on another object's diagram (the road's).
A plastic storage tote slides across a garage floor. Its diagram shows weight down, normal force up, friction backward, and a second downward arrow labeled 'the force from the tote on the floor'. Which verdict fits?
A library book slides across a table after a shove, now on its own. Its diagram shows weight down, normal force from the table up, and friction backward. Which verdict fits?
A shuffleboard disk slides along the court on its own; the player let go a moment ago. Its diagram shows weight down, normal force up, friction backward, and a forward arrow labeled 'the force of the push'. Which verdict fits?