Passage III

In 5 trials, a toy cart rolled on a track after being released from rest at Point S (see Figure 1).

For each combination of cart mass, M, and loop radius, r, Point S was at the minimum height, h, required for the cart to roll completely through the loop. The cart was stopped at Point V.

The cart's gravitational potential energy, PE, at Points S and U and the cart's kinetic energy, KE, at Point U are shown in Table 1. When the cart was on the floor, PE was zero. The cart's total mechanical energy, TME, at a given point equaled PE + KE at that point. TME at Point S minus TME at a second point equaled the amount of TME converted to heat by friction between Point S and the second point.

Figure 1 and Table 1 adapted from T. M. Cooney, D. F. McGrail, and R. D. Unruh, eds, Physics Resources and Instructional Strategles for Motivating Students. @1993 by The College of Natural Sciences, The University of Northern lowa.

14. The total mechanical energy of an object is said to be conserved if the total mechanical energy of the object has the same value everywhere along the path of the object. In which of the trials, if any, was TME conserved while the cart rolled from Point S to Point U?

Answer and Explanation

Your Answer is

Correct Answer is J

Explanation

The car at point S only has potential energy PE=mgh;

At point U, the car has kinetic energy and potential energy, so when the PE+KE of the car at point U is equal to PE at point S, the energy is conserved .

Look at table 1, all trails do not meet the above conditions