题目

用F=20 N的拉力竖直向上拉重为15 N的物体，2 s内物体受到的合力的冲量大小为_________. 答案：解析：利用I=Ft求解.方法一：先求合力再求合冲量.物体上升过程中所受合力为F合=F-G=20 N-15 N=5 N，方向向上.故合力的冲量为I=F合t=5 N×2 s=10 N·s.方法二：先求各力的冲量再求合冲量.上升过程中拉力的冲量为I1=Ft=20 N×2 s=40 N·s，方向竖直向上.重力的冲量为I2=Gt=15 N×2 s=30 N·s，方向竖直向下.合冲量为I=I1-I2=10 NIf I 　　   your advice, I 　　　　so much trouble. A. had taken; would not have met　　　　　　 B. took; would not have met C. have taken; would not meet　　　　   D. take; will not have met