Work against gravity equals weight times vertical height

Steps:

• The weight of the barrel is 2000 N, the force due to gravity.
• The vertical height gained is 0.80 m, the lorry's elevation above the road.
• Work done against gravity is force times vertical displacement: 2000 N × 0.80 m.
• This equals 1600 J, independent of the plank's length or incline.

Why B is correct:

• By the definition of gravitational potential energy, work against gravity is mgh (or weight × height), using the vertical rise of 0.80 m.

Why the others are wrong:

• A: Uses half the correct height (0.50 m), underestimating the elevation.
• C: Likely misapplies plank length or average height, ignoring vertical-only displacement.
• D: Incorrectly uses full plank length (2.0 m) as displacement, treating path as vertical.

Final answer: B