A flatbed truck travels along a straight, level road at a constant 12 m s⁻¹. A worker of mass 65 kg stands on the bed of the truck, remaining at rest relative to the truck throughout the motion.
(a) Calculate the translational kinetic energy of the worker as measured by an observer standing on the ground beside the road.
(b) Calculate the translational kinetic energy of the worker as measured by a second observer who rides on the truck and remains at rest with respect to the worker.
(c) Explain, using clear physical reasoning, why the two observers obtain different numerical values for the worker’s translational kinetic energy even though both describe the same physical situation.
(d) The truck comes to a uniform stop over a distance of 85 m. Assuming the worker continues to stand without slipping relative to the truck bed, determine the magnitude of the horizontal static friction force that must act on the worker’s feet during the braking interval. Include a discussion of how energy considerations relate to the force you calculate.