Question 8
A 20 N block on a table needs a force of 6 N to just start it moving. A second 20 N block is placed on top of it. The force now needed to just start the bottom block moving is:
The right answer is the one marked above.
More than 6 N, because the normal reaction has doubled. R has gone from 20 N to 40 N, and limiting friction rises with R.
Why not "exactly 12 N"? Because this chapter has only established that friction increases with the normal reaction, not the exact rule connecting them. Claiming 12 N is claiming more than the experiment shows.