Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information.... 
Home Conservation Laws Simplifying the Energy Zoo Examples Dropping a rock  
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Dropping a rockIf you drop a 1kg rock from a height of 1 m, how many joules of KE does it have on impact with the ground? (Assume that any energy transformed into heat by air friction is negligible.)
If we choose the y axis to point up, then F_{y }is negative, and equals
(1 kg)(g) = 9.8 N. A decrease in y is represented by a negative
value of Δy, Δy = 1 m, so the change in potential energy is (9.8 N)(1 m) ~ 10 J. (The proof that newtons multiplied by meters
give units of joules is left as a homework problem.) Conservation
of energy says that the loss of this amount of PE must be accompanied
by a corresponding increase in KE of 10 J.


Home Conservation Laws Simplifying the Energy Zoo Examples Dropping a rock 