To move maximum height the friction force can be assumed to be
negligible.
Reasons:
The component of the weight acting on the incline, w = m·g·sin(θ)
Length of the ramp = 15 m
Potential energy of the box = 15 × sin(θ) ×20 = 300·sin(θ)
F × 15 = 300·sin(θ)
F = 20·sin(θ)
Work done by net force = Potential energy gained by the box
Work done by net force = (40 - 20·sin(θ)) × 15 = 300·sin(θ)
40 = 20·sin(θ) + 20·sin(θ) = 40·sin(θ)
sin(θ) = 1
θ = arcsin(1) = 90°
The ramp is at an inclination of 90°, therefore, the ramp is oriented vertically
The height of the ramp = The length of the ramp = 15 m
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