CH 202 Problem 5.14

5.14 (a) What is the kinetic energy in joules of a 850-lb motorcycle moving at 66 mph?

remember that kinetic energy (KE) = 1/2 mv2; if we want to calculate this quantity in joules, then the mass must be in kg and the velocity in m/sec. Remember the conversion factors on the inside rear cover of your text?

(b) By what factor will the KE change if the speed of the motorcycle is decreased to 33 mph?

well, we could actually calculate this but that would be extremely inelegant, considering that the speed is decreasing by half (66 to 33 mph.) Since KE is proportional to v2, if we decrease the speed by a factor of 2, we decrease the KE by a factor of 4. Be sure you understand why!!

(c) Where does the KE of the motorcycle go when the rider brakes to a stop?

When the brakes are applied, the KE is transferred to friction between the brakes and the wheels. Most of this energy is then dissipated as heat.

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DAC 7/8/08