Work against gravity depends on vertical height, not path length

Steps:

• The barrel's weight is 2000 N, the gravitational force.
• The vertical height raised is 0.80 m, the lorry's height above the road.
• Work against gravity is force times vertical displacement: 2000 N × 0.80 m.
• This equals 1600 J, the gain in potential energy.

Why B is correct:

• By definition, work against gravity is mgh, where h is vertical height, yielding 2000 × 0.80 = 1600 J.

Why the others are wrong:

• A: Underestimates by using half the height (0.40 m), ignoring full 0.80 m rise.
• C: Incorrectly uses plank length (2.0 m) as displacement, not vertical height.
• D: Doubles the correct value, perhaps confusing with total weight or path.

Final answer: B