Define normal force (N), including the direction of the normal force.

Record data in the table, assuming typical values (e.g., mass per block = 15 g, tray + spring balance mass ≈ 50 g for illustration). Total mass determines normal force $N = mg$. Maximum static friction $f_\text{max}$ is the spring balance reading when tray just starts moving.

Example table (substitute your measurements):

Plot $f_\text{max}$ (y-axis) vs $N$ (x-axis): straight line through origin.

Graph slope $=$ coefficient of static friction $\mu$.

The graph is a straight line passing through the origin, showing $f_\text{max} \propto N$. Slope $\mu \approx 0.12$ (from example data).
Normal force acts perpendicular to surface (vertically upward on tray base).

(Q2)
As blocks added, total mass increases $\to N$ increases $\to f_\text{max} = \mu N$ increases (direct proportion).

(Q3)
Blocks increase $N$ without changing contact surface. Pulling tray slowly measures $f_\text{max}$ at limiting friction (tray just slips).

(Q4/Q5)
Total $N = (m_\text{tray} + m_\text{blocks} + m_\text{spring})g$. Omit $\to$ underestimate $N$, incorrect $\mu$.

Final answers:

Graph: linear, $f_\text{max} = \mu N$, $\mu =$ slope.