Impulse-momentum theorem Steps:

• Define initial velocity as +12 m/s (towards wall) and final as -8 m/s (bouncing back).
• Calculate change in velocity: Δv = -8 - 12 = -20 m/s.
• Compute change in momentum: Δp = m Δv = 0.5 × (-20) = -10 kg m/s (magnitude 10 kg m/s).
• Average force: F_avg = |Δp| / Δt = 10 / 0.1 = 100 N. Why D is correct:
• Impulse equals change in momentum, so F_avg = Δp / Δt yields exactly 100 N per the theorem. Why the others are wrong:
• A: 0.2 N from incorrectly using final speed difference (8-12=-4) and dividing by 20.
• B: 1 N from Δp / (m Δt) or similar unit error.
• C: 20 N from m Δv without dividing by time interval.

Final answer: D