## Teacher Notes

A productive question for this model is: What must be true for the block to stay at rest?

Encourage students to compare two quantities:

1. The parallel driving force: the combined effect of the spring force and the component of weight along the slope.
2. The maximum static friction: the largest frictional force available before slipping begins.

If the parallel driving force is smaller than or equal to the maximum static friction, the block can stay at rest. If it is larger, the block must slip.

Suggested sequence: prediction, run, observe, and explain. Ask students to justify each case with both the animation and the force comparison message.