用400N的竖直向上的力提重物，若物体受到的合力大小为100N，则物体受到的重力是（ ）
A．100N
B．300N
C．500N
D．300N或500N
（1）物体受到的重力方向竖直向下，拉力方向竖直向上，两者方向相反．
（2）当合力的方向竖直向上时，拉力大于重力，物体受到的重力：G=F上-F合=400N-100N=300N；
当合力的方向竖直向下时，拉力小于重力，物体受到的重力：G=F上+F合=400N+100N=500N；
故选D．
| Questions 61-64 are based on the following passage. "It’s like being bitten to death by ducks." That’s how one mother described her constant squabbles with her eleven-year-old daughter. And she’s hardly alone in the experience. The arguments almost always involve mundane matters—taking out the garbage, coming home on time, cleaning up the bedroom. But despite its banality, this relentless bickering takes its adolescents—particularly mothers—report lower levels of life satisfaction, less marital happiness, and more general distress than parents of younger children. Is this continual arguing necessary For the past two years, my students and I have been examining the day-to-day relation-ships of parents and young teenagers to learn how and why family ties change during the transition from childhood into adolescence. Repeatedly, I am struck by the fact that, despite considerable love between most teens and their parents, they can’t help sparring. Even in the closest of families, parents and teenagers squabble and bicker surprisingly often—so often, in fact, that we hear impassioned recountings of these arguments in virtually every discussion we have with parents or teenagers. One of the most frequently heard phrases on our interview tapes is, "We usually get along but..." As psychologist Anne Petersen notes, the subject of parent-adolescent conflict has generated considerable controversy among researchers and clinicians. Until about twenty years ago, our views of such conflict were shaped by psychoanalytic clinicians and theorists, who argued that spite and revenge, passive aggressiveness and rebelliousness toward parents are all normal, even healthy, aspects of adolescence. But studies conducted during the 1970s on samples of average teenagers and their parents (rather than those who spent Wednesday afternoons on analysts’ couches) challenged the view that family storm and stress was inevitable or pervasive. These surveys consistently showed that three-fourths of all teenagers and parents, here and abroad, feel quite close to each other and report getting along very well. Family relations appeared far more pacific than professionals and the public had believed. |
The parents-children relationship changes from the relative positive to the relative negative when ______.
A．the children reach 7 or 8 years of age
B．the children reach 13 or 14 years of age
C．the parents begin to have too many household responsibilities
D．the parents begin to feel there is too much burden in the house