A box of mass 15 kg is lifted smoothly from the floor to a tabletop 1.3 m above

Question

A box of mass 15 kg is lifted smoothly from the floor to a tabletop 1.3 m above the floor. a) Assuming no acceleration, what force is applied to lift the box? b) How much work is done on the box? What is the box's change in gravitational energy? A crane lifts a crate of mass 80 kg at constant speed through from the ground to a height of 20 m. potential energy? a) What is the crate's increase in gravitational b) How much work was done on the crate? c) If the cable on the crane snaps at the top of this motion, how much kinetic energy will the crate have just before it hits the ground? (Neglect air resistance.)

Explanation / Answer

m = 80 kg , h = 20m

(a) U = mgh = 80*9.8*20

U = 15680 J

(b) W = change in potential energy

W = 15680 J

(c) from work energy thorem

W = change in kinetic energy

K = 15680 J

(d) K = 0.5mv^2

15680 = 80*v^2

v = 14 m/s

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