Static and kinetic friction
When the block won't budge, how big is the friction?
Pull a resting block with a steadily increasing force. While it does not move, the static friction is equal in magnitude and opposite in direction to the pull. Once the pull exceeds the maximum static friction, the block starts to slide and the friction suddenly drops to kinetic friction, which then no longer changes with the pull. Kinetic friction is proportional to the normal force and depends on the roughness of the contact surfaces.
Friction vs. pull
When the pull reaches the maximum static friction, the friction suddenly drops to kinetic friction.
Think about it
Once the block is sliding, does the kinetic friction change if you keep increasing the pull?
No. Kinetic friction f = μN depends only on the normal force and the contact surfaces, not on the pull or the speed; the extra pull accelerates the block.
Model assumptionsHorizontal board · Pull increases uniformly with time · Kinetic coefficient capped at the static one · Restarts every 6 s
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.