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A 25-gram paper cup falls from rest off the edge
of a tabletop 0.90 meter above the floor. If the
cup has 0.20 joule of kinetic energy when it hits
the floor, what is the total amount of energy
converted into internal (thermal) energy during
the cup’s fall?
(1) 0.02 J (3) 2.2 J
(2) 0.22 J (4) 220 J

Sagot :

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So a cup falls from the table top to the ground and acquires 0.20 J of kinetic energy in the moment it hits the ground and we want to know what is the total amount of energy converted into thermal energy. While it is on the table, the cup has potential energy and the kinetic energy is 0. The potential energy of the cup is Ep=m*g*h where m=mass of the cup, g= gravitational acceleration of 9.81m/s^2 and h=height of the table. So Ep=0.025*9.81*0.9=0.220725 J. That potential energy converts into kinetic energy and into thermal energy via friction that the air is causing. So Ep=Ek+Efriction. From here: Efriction=Ep-Ek=0.220725 J-0.20J=0.020725J So the correct answer is (1) 0.02J
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