Thank for your help
But now I hv another problem..
A case as follows,
Now I place a box (1kg) on the ground.
Assume there's no friction between the ground and this box.
And I place another box (also 1 kg) above this box
The limiting static friction between this two box is 5N
And now...
As follows,
a box is placed on the ground
And let the limiting static friction be 5N
for instance an external force 2N is applied to the box
Then, will the friction on the box be 2N?
if yes, then .. there will be a friction of 2N on the earth? causes they are action and rxn pair
And will...