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HWK17s_11G

# HWK17s_11G - expense of distance but it cannot multiply the...

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HWK 17 ( 11 th Grade) SOLUTIONS A M Kolchin/ SAR Physics Read pages 108 - 118. Do: 8, 9, 15, 31, 32 (For problem 31: Think about force and think about energy). 8. A boulder above ground with a potential energy relative to the ground of 200 J is dropped. Just before it hits the ground, its potential energy has all turned into kinetic energy. So its kinetic energy at this point is 200 J. 9. Suppose an automobile has 2000 J of K.E. When it moves at twice the speed, what will its K.E. be? Since K.E. = 1/2 m v 2 , doubling v , will quadruple K.E. (8000 J). If you triple the speed, the K.E. will increase nine-fold (18000 J). 15. Work that is put in a machine will equal the work put out by a machine. A machine can transform energy from K.E. to P.E. or vice-versa, or it can trnasfer energy from one object (or position) to another. A machine can multiply a force at the
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Unformatted text preview: expense of distance, but it cannot multiply the work put into it. 31. A rock projected by a slingshot will go faster if the rubber is stretched an extra distance because: 1. Stretching the rubber further increases its potential en-ergy gets transformed into kinetic energy associated with the rock. Since K.E.=1/2 m v 2 , this results in a greater velocity for the rock. 2. Stretching the rubber increases the force associated with the impulse imparted to the rock, thereby increasing its momentum. Since momentum = m × v, this results in a greater velocity for the rock. 32. An object with momentum has a nonzero speed and will there-fore always have kinetic energy. An object can be at rest in a force ﬁeld in which case it will have potential energy but no momentum. 1...
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