Three blocks are pushed along a rough surface by a force with magnitude P as shown aboveFc is the magnitude of the contact force between blocks 2 and 3 and Ff F and Fg are themagnitudes of the frict

There are different ways to draw a free-body diagram for block 3, but one possible correct diagram is:

 Fg
  ↓

┌───▼───┐ │ │ Ff│ │Fc │ m3 │ │ │ └───▲───┘ │ P↓

In this diagram, the downward gravitational force Fg is shown as a vector pointing from the center of mass of block 3 towards the ground. The friction force Ff is shown as a vector pointing to the left, opposite to the direction of motion (assuming the blocks are moving to the right). The contact force Fc is shown as a vector pointing to the right, perpendicular to the contact surface between blocks 2 and 3. The normal force is not shown explicitly, but it is present as a perpendicular force that balances the weight of block 3 and allows it to remain in contact with the surface. The applied force P is shown as a vector pointing to the right, parallel to the surface. The arrows indicate the direction of each force, and their lengths can be proportional to their magnitudes (relative to a chosen scale).