Question 2
A crate on a floor just begins to move when the horizontal push on it reaches 50 N. What does this tell you about the friction between the crate and the floor?
The right answer is the one marked above.
50 N — it is the limiting friction. The force needed to just start the motion is equal to the maximum friction those surfaces can produce.
The dynamic friction is a different, slightly smaller number, and it applies only once the crate is sliding. Activity 01 measures the limiting value in exactly this way.