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问题 选择题

质量为m的小球以速度v与竖直墙壁垂直相碰后以原速率反向弹回，以小球碰前的速度为正方向，关于小球的动能变化和动量变化，下面的答案正确的是（   ）

A．0 , 0

B．mv2 , 0

C．0 , -2mv

D．0 , 2mv

答案

答案：C

动能是标量，只与速度大小有关，与速度方向无关，动能变化量为零，动量变化量为-mv-mv=-2mv，C对；

选择题

Last week Lily’s father bought her__________MP4 as___________birthday present.

A．/；a

B．an；an

C．a；a

D．an；a
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单项选择题

Estimating schedule activity costs involves developing an（）of the costs of the resources needed to complete each schedule activity. Cost estimating includes identifying and considering various costing alternatives. For example, in most application areas, additional work during a design phase is widely held to have the potential for reducing the cost of the execution phase and product operations. The cost estimating process considers whether the expected savings can offset the cost of the additional design work. Cost estimates are generally expressed in units of（）to facilitate comparisons both within and across projects.The（）describes the business need, justification, requirements, and current boundaries for the project. It provides important information about project requirements that is considered during cost estimating.

Estimating schedule activity costs involves developing an（）of the costs of the resources needed to complete each schedule activity.

A．accuracy

B．approximation

C．specification

D．summary
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