题干

如图所示,在匀速转动的水平圆盘上离转轴某一距离处放一小木块,该木块恰能跟随圆盘做匀速圆周运动而不发生相对滑动,则在改变下列哪种条件时,物体仍能与圆盘保持相对静止(   )

A:增大圆盘转动的角速度

B:增大木块到转轴的距离

C:增大木块的质量

D:改变上述的任一条件都不能使木块与圆盘保持相对静止

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C

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 Secrets of Web Travel Sites

Booking a trip on an online travel siteis convenient, but comes with its own set of problems.

1 They who's on a Mac and who's on a PCand who's going to spend more.

Last year, US travel research companyOrbitz tracked people's online activities to test out whether Mac users spendmore on travel than PC users. On average, Mac users lay out US$20-30 more pernight on hotels and go for more stars. according to the Wall Street Journal. Asa result, online• travel sites show these users more expensive travel optionsfirst. To avoid inadvertently paying more, sort results by price.

 Their software doesn't always hook up to thehotel's system.

A guaranteed reservation is almostimpossible to come by anywhere — but the risk of your flight or hotel beingoverbooked increases with third-party providers. The middle-man's softwareisn't immune to system errors, so always call the hotel or airline to make sureyour booking was processed.

 Don't be fooled by packages: Often, they'relow-end items grouped together.

Ever notice how travel sites recommend ahotel, a rental car, and tour package all in one click? These deals usuallyfeature travel that no one wants, like flights with multiple layovers.

Check the fine print.

 You could miss out on loyalty points.

Third-party providers can get betweenyou and frequent flyer miles or points. Many hotel loyalty programmes don'trecognise external sites, others award only minimum points and exclude specialoffers, like double points on hotel stays.

 Once your trip is purchased, you're on yourown.

An online travel agency can't provideassistance the same way an agent can if a flight is cancelled or a room issubstandard. Basically, when you arrive at the airport or hotel, you're justanother client who booked at the lowest rate.