Q1.
But how can you check if the force of friction is indeed different for different surfaces?
Answer
Measure it directly with a spring balance, as Activity 6.2 does.
- Lay the spring balance horizontally on the surface and check that it reads zero.
- Hook a wooden block to it and pull with a slowly increasing force.
- Note the reading at the instant the block just begins to move. That reading is an approximate measure of the force of friction between the block and that surface.
- Repeat on each of the four surfaces and compare the readings.
Why this measures friction: just as the block begins to move it is still travelling with essentially no change in velocity, so its acceleration is zero and the net force on it is zero. The only two horizontal forces are the pull of the spring and friction, so they must be equal in magnitude. Whatever the spring reads, friction equals.
Check it yourself: the surface on which the stack of coins travelled the largest distance in Activity 6.1 should give the smallest spring balance reading here. Two different activities, one consistent answer.