Worked examples — before it moves
A wooden block of weight 60 N stands on a horizontal table. The limiting friction between the block and the table is 18 N. Three examples, all of them before the block has moved at all.
Example 1 — find the friction
The problem
The block is pushed horizontally with a force of 8 N and it does not move. Find the frictional force acting on the block.
Solution
The block is not moving, so the forces on it balance.
friction = applied force
friction = 8 N (static friction)
Not 18 N. Static friction supplies only as much as it needs to balance the push; 18 N is merely the most it could supply if asked.
Example 2 — find the normal reaction
The problem
Find the normal reaction of the table on the same block.
Solution
The table is horizontal and nothing else pushes or pulls vertically.
R = weight
R = 60 N
The 8 N push is horizontal, so it does not enter this at all. Vertical forces balance vertical forces.
Example 3 — on the point of moving
The problem
The push is increased until the block is just about to move. What is the push now, and what is the frictional force?
Solution
"Just about to move" means friction has reached its maximum.
friction = limiting friction = 18 N
The block is still at rest, so the forces still balance:
push = 18 N
This is the one moment when the applied force and the limiting friction are the same number — which is exactly why activity 01 could measure the limiting friction with a spring balance.
Remember the method
Before anything moves, write friction = applied force and go no further. The limiting value is not the answer — it is only the test of whether the object stays still at all.