搜题请搜索小程序“爱下问搜题”
问题 选择题

下列关于超重和失重的说法中正确的是（　　）

A．在同一地点，超重就是物体的重力增加了

B．在同一地点，失重就是物体的重力减少了

C．在同一地点，完全失重就是物体的重力消失了

D．在同一地点，无论超重、失重还是完全失重物体的重力都是不变的

答案

A、超重是物体对接触面的压力大于物体的真实重力，在同一地点，物体的重力并没有增加，故A错误．

B、失重是物体对接触面的压力小于物体的真实重力，在同一地点，物体的重力并没有减小，所以B错误．

C、完全失重是说物体对接触面的压力为零的时候，在同一地点，物体的重力也不变，所以C错误．

D、不论是超重、失重，还是完全失重，物体所受的重力是不变的，只是对接触面的压力不和重力相等了，所以D正确．

故选D．

解答题

通过对一份中学生营养快餐的检测，得到以下信息：

①快餐总质量为300g；

②快餐的成分：蛋白质、碳水化合物、脂肪、矿物质；

③蛋白质和脂肪含量占50%；矿物质的含量是脂肪含量的2倍；蛋白质和碳水化合物含量占85%．

根据上述数据回答下面的问题：分别求出营养快餐中蛋白质、碳水化合物、脂肪、矿物质的质量．
查看答案
单项选择题

In fall 2006, the National Basketball Association (NBA) started using basketballs made with synthetic, or man-made, material instead of leather. They made the change because they wanted every basketball they use to feel and bounce the same. Not all leather balls are exactly alike in weight or how they bounce, but the synthetic balls are.
However, some players complained right away that the new balls bounced differently and were actually harder to control than the leather ones.
Physics helped answer questions about why the new balls felt different. For example, the scientists studied friction, which in this case affects the ability of a player to hold onto a ball. "The greater the friction, the better it will stick to his hand," explains a scientist.
Tests on both wet and dry balls showed that while the synthetic ball was easier to hold onto when dry, it had less friction and became much harder to hold onto when wet. That’s because sweat stays on the surface of the synthetic balls but gets taken into the leather balls.
The scientists also tested bounce and found that the logo printed on the new balls made their surface uneven and caused them to bounce a little strangely compared with the leather balls.
In January, the NBA went back to using the leather balls. They aren’t perfect, but for now, that’s just the way the ball bounces.

The synthetic ball bounces differently from the leather one mainly because ______.

A．it is wet

B．it is dry

C．there’s a logo on it

D．its weight remains the same
查看答案