# Sample Problem Kinetic Energy And Work

Sample Problem Kinetic Energy And Work

Shows constant forces acting on a box as the box slides rightward across a friction less floor. Force Fr is horizontal, with magnitude  is angled upward by, Two forces   act on abox that slides rightward across a friction less floor. The velocityof the box is v.

(a) What is the power due to each force acting on the box at that instant, and what is the net power? Is the net power changing at  that instant?

SOLUTION: A Key Idea here is that we want an instantaneous power, not an average power over a time period. Also, we know the particle’s velocity (rather than the work done on it). (b) If the in magnitude  is, instead, 6.0 N, what now is the net power, and is it changing?

SOLUTION: The same Key Ideas as above give us, for the power. Thus, the kinetic energy of the box is increasing, and so also is the speed of the box. With the speed increasing, we see from that the values , and thus also of will be changing. Hence, this net power of 3.0 W is the net only at the instant the speed is the given 3.0 m.

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