lecturenotes-Feb8

# lecturenotes-Feb8 - Exam#1 This Section#9 Average 74 Median...

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Exam #1 This Section #9 Average 74 Median 75 8 As—18% 14 Bs--31% 19 Cs—42% 0 Ds—0% 4 Fs—9% A B C D
Exam #1: All Sections

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Exam #1: Question #3 The blocks have non-zero but not necessarily equal masses. They are connected by a string that passes over a frictionless pulley. Both the string and the pulley have no mass. There is no friction and both blocks start at rest. When they are released then move with increasing velocity. Which of the condition(s) is/are consistent with this observation. a) The weight of Block A is smaller than the weight of block B. b) The weight of Block A is equal to the weight of block B. c) The weight of Block B is smaller than the weight of block A. d) None of the above. Top block T=Ma Side block mg T = ma Solution T = mM m + M g Consider the tension in the string connection the blocks. What conditions(s) must be met? a) The tension is equal to the combined weight of both block. b) The tension is equal to the weight of block A. c) The tension is equal to the weight of block B. d) The tension is smaller than the weight of block A. e) The tension is smaller than the weight of block B
Quick Review: Work-Kinetic Energy Theorem If F net is not a function of x then W net F net d = Δ KE No work is done on an object by a force unless there is a component of the force along the objects line of motion Work-Kinetic Energy Theorem change in the kinetic energy of an object net work done on the particle ( ) = ( ) Note: Work is the dot product of F and d

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Quick Review: Work done by special forces If an object is displaced upward ( Δ y positive ), then the work done by the gravitational force on the object is negative . If an object is displaced downward ( Δ y negative ), then the work done by the gravitational force on the object is positive . Spring Force W spring = F spring x ( ) d x = kx ( ) dx x 1 x 2 = k xdx x 1 x 2 = 1 2 k ( ) x 2 [ ] x 1 x 2 = 1 2 k x 2 2 x 1 2 ( ) WORK due to friction is ALWAYS NEGATIVE - Energy is transferred OUT - Kinetic energy decreases or Δ KE < 0 (slow down)
HW #12 Problem A block of mass m is dropped onto a spring. The block becomes attached to the spring and compresses it by distance d before momentarily stopping. While the spring is compressed, what work is done on the block by: a) the gravitational force on it b) the spring force? c) What is the speed of the block just before it hits the spring? d) From what height h was the box dropped? e) How high will it go back. a ): Work by Gravational Force F g  = mg ˆ j W g = FD = mgd b ): Work by Spring Force F s  = + ky ˆ j W s = kydy 0 d = kd 2 2 c ): Speed of block at x=0. Work = Δ E KE W s = kd 2 2 + mgd = 0 mv 2 2 d ): height of block h. Work = Δ E KE from y=h to y=0 W s = mgh = mv 2 2 0 e ): How hight will it go up h' Just do the problem backwards.

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