1. Aim: How do we explain the force of friction?

2. Turn and Talk to Solve Conceptual Friction Problem 1
In three experiments, three different horizontal forces are applied on the same block lying on the same countertop. The force magnitudes are F1=12 N, F2=8N, and F3= 4N. In each experiments the block remains stationary in spite of the applied force. Rank the forces according to (a) the magnitude fs of the static frictional force on the block from the countertop and (b) the maximum value f s max of that force, greatest first.
The static friction is greatest in case 1 where it is 12 N, next greatest in case 2 where it is 8 N, and smallest in case 3 where it is 4 N. Since each object is at rest, the applied force is equal to the static friction force.
The maximum static friction is the same in each case.

3. Turn and Talk to Solve Conceptual Friction Problem 2
In the figure below the box is stationary and the angle ᶱ of force F is increased, do the following quantities increase, decrease, or remain the same:
Fx -DECREASES
Fs- DECREASES
N-INCREASES
fs max-INCREASES
If instead the box is sliding and ᶱ is increased, does the magnitude of the frictional force on the box increase, decrease, or remain the same?
The magnitude of the frictional force increases since the normal force increases