Newton's Third Law and Pressure Definition

Steps:

• Hammer applies force F_h to nail; by action-reaction, nail applies equal force back to hammer.
• Nail applies force F_n to wood; with negligible nail mass, net force on nail is zero, so F_h = F_n.
• Pressure p = force / contact area.
• Hammer-nail contact area (nail head) is larger than nail-wood contact area (nail tip), so p_h < p_n.

Why D is correct:

• Negligible mass implies F_h = F_n (Newton's second law, net force ≈ 0); p_h < p_n since p = F/A and A_h > A_n (pressure formula).

Why the others are wrong:

• A: F_h = F_n (not >); p_h < p_n (not >).
• B: p_h < p_n (not >).
• C: F_h = F_n (not >); p_h ≠ p_n.

Final answer: D