Question

a. three blocks are pushed along a rough surface by a force with magnitude f, as shown below.

diagram of three blocks (block 1, block 2, block 3) with a force p pushing block 1 to the right

$f_1$ is the magnitude of the contact force between blocks 2 and 1, and $f_f$, $f_n$, and $f_g$ are the magnitudes of the friction, normal, and gravitational forces on block 1, respectively. which of the following is a correct free - body diagram for block 1?

a. diagram with $f_n$ up, $f_g$ down, $p$ right, $f_c$ right, $f_f$ left

b. diagram with $f_n$ up, $f_g$ down, $p$ right, $f_c$ left

c. diagram with $f_n$ up, $f_g$ down, $f_c$ right, $f_f$ left

d. diagram with $f_n$ up, $f_g$ down, $f_c$ right

Explanation:

1. Vertical forces on Block 1: Gravitational force $F_G$ acts downward, and the normal surface force $F_N$ acts upward to balance it.
2. Horizontal forces on Block 1: The applied external force $P$ acts to the right. A contact force $F_C$ from Block 2 pushes back to the left (Newton's third law). Since the surface is rough, kinetic friction $F_f$ acts opposite to the direction of motion, so it points to the left.
3. Eliminate incorrect options:

• Option B: Missing friction force $F_f$.
• Option C: Missing applied force $P$.
• Option D: Missing both $P$ and $F_f$.

Answer:

A. The free-body diagram with upward $F_N$, downward $F_G$, rightward $P$ and $F_C$, leftward $F_f$