例1:(2006年全国理综卷)一位质量为m的运动员从下蹲状态向上起跳,经△t时间,身体伸直并刚好离开地面,速度为v。在此过程中, A．地面对他的冲量为mv+mg△t,地面对他做的功为1/2mv2 B．地面对他的冲量为mv+mg△t,地面对他做的功为零C．地面对他的冲量为mv,地面对他做的功为1/2mv2 D．地面对他的冲量为mv-mg△t,地面对他做的功为 Example 1: (2006 National Synthetic Volume) A player of mass m jumps upward from the squatting state. After Δt, the body is straight and just off the ground at a speed of v. In this process, A. The impulse to the ground is mv+mgΔt, and the work done by the ground to him is 1/2mv2. The impulse to the ground is mv+mg△t, and the work done by the ground to him is zero. The impulse to the ground is mv, and the work done by the ground to him is 1/2mv2. The impulse to the ground is mv-mg △ t. The ground’s work for him is