We were given a study guide with the ansers only to review forour final next wee

Question

We were given a study guide with the ansers only to review forour final next week. Here is the Problem: A 12 kg projectile is launched with an initial vertical speedof 20 m/s. It rises to a maximum height of 18m above the launchpoint. How much work is done by the dissipative (air) resistiveforce on the projectile during it's ascent? The ans. is -2.8 Kj I can't figure out how to come up with that. Can anyone giveme a hand? We were given a study guide with the ansers only to review forour final next week. Here is the Problem: A 12 kg projectile is launched with an initial vertical speedof 20 m/s. It rises to a maximum height of 18m above the launchpoint. How much work is done by the dissipative (air) resistiveforce on the projectile during it's ascent? The ans. is -2.8 Kj I can't figure out how to come up with that. Can anyone giveme a hand?

Explanation / Answer

using conservation of energy, the energy at the bottom shouldequal the energy at the top. but air resisence prevents that. sofind the difference in energy at the top to the bottom.at bottom the energy is kinetic, a top it is gravatioanlpotential. bottom: E = (1/2)mv2 = (1/2)(12)(20)2 = 2400J top: E = mgh = (12)(9.81)(18) = 2119 J top - bottom = 2119 - 2400 = 281 J

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