问题 选择题

一个带正电的小球穿在一根绝缘的粗糙直杆上,杆与水平方向成角,所在空间存在竖直向上的匀强电场和垂直于杆且斜向上的匀强磁场,如图所示,小球沿杆向下运动,通过a点时速度是4m/s,到达c点时速度减为零,b是ac的中点,在小球运动过程中

A.小球通过b点的速度为2m/s

B.小球的电势能的增加量一定大于重力势能的减少量

C.绝缘直杆对小球的作用力垂直于小球的运动方向

D.到达c点后小球可能沿杆向上运动

答案

答案:D

题目分析:小球在运动过程中受重力、电场力、洛伦兹力、摩擦力、杆施加的两个支持力,其中一个平衡洛伦兹力。根据动能定理WG+W+Wf=ΔEk ①,摩擦力随洛伦兹力的减小在减小,因此,ab段和bc段合力不同,合力做功不同,则动能变化不同,则小球通过b点的速度一定不是2 m/s,A错误;由①式得: W+Wf=ΔEk-WG=ΔEk+ΔEp,由于不明确电场力与重力的大小,小球的电势能的增加量与重力势能的减少量电势能无法比较,B错误;绝缘直杆对小球的作用力有:两个支持力垂直于杆和摩擦力沿杆向下,这三个力的合力一定不垂直于杆,C错误;小球运动到C点后,由于不明确电场力与重力的大小有可能静止,也有可能沿杆向上运动,  D正确。

单项选择题
单项选择题

Questions 11~15
It’s estimated that every year 100,000 children aged 16 and under run away from home. The London Refuge, an unremarkable house on an unremarkable street, is the only place in Britain that will give them a bed. Last year it gave sanctuary to 238 children of whom the youngest was 11. What happened to the other 99,762 Nobody knows, although it’s a fair bet that some of them ended up on the streets, that some fell into inappropriate and dangerous company, that some didn’t survive. "The mere fact that they’re running away puts them at risk," says Lorna Simpson, the refuge’s deputy manager. "On the streets they’ 11 mix with other young people. They’ re so naive; they don’t understand that people who are nice to them will want payback. Our job is to make them safe. "
Simpson, a former social worker, is a calm woman of great warmth. The refuge has six beds and has been open since 1993, often with the threat of closure hanging over it. The problem has nothing to do with the quality of its service and everything to do with funding. A week’s placement costs £2. 278 and three successive governments have argued that the annual running costs of £720. 000 should be locally funded. But because it is used by children from many parts of London, and beyond, local authorities are reluctant to contribute.
The Government has now agreed to work on a strategy to support runaway children in England and Wales, which is rich after its withdrawal of funding from the refuge in December. Since then the NSPCC, National Society for the Prevention of Cruelty to Children, which runs the refuge in conjunction with St Christopher’s Fellowship, has financed it through a donation from an individual, but that money will last only until late next year. "Without this facility there’s nothing; children who run away are on the street," says Nasima Patel, the assistant director of the NSPCC. "One of the strengths of the refuge is that children who have left home can ring up directly and will get a bed and supportive staff without having to go through a process of assessment".
The refuge accommodates six children plus staff. Many of the admissions are at night and children can stay up to 21 days in three months, although most stay for three to five days. They find it through social services, through ChildLine and through word of mouth.
"Children run away from everything you can think of," Simpson says. "Arguments with step-parents, sexual abuse, alcoholic parents, being left to bring up their younger siblings, neglected children who have been failed by social services, girls who have been trafficked. We get doctors’ and lawyers’ children who run away because they want more pocket money, or want to stay out later than their parents allow. They’ve been given everything, they get to 15 and no one thinks to pull the reins in. By that time it’s too late; they rebel. "
Most of the children are from families known to social services, and for them the refuge’s ordered regimen is a welcome contrast to the chaos they know. Staff listen without judging and without encouraging dependency, trying to establish why the children have run away. The aim is to get them home or into the care of social services and, after discharge from the refuge, a family support worker is available.

According to the passage, the London Refuge used to be funded by ______.

A. the Government
B. the NSPCC
C. individual donors
D. local authorities