A typical mechanical problem is analyzed in this paper. A heavy ball is hanged below the ceiling by a rope，and another rope is tied at the bottom of the ball. If the end of the rope moves at a certain velocity，which rope will snap first. The results show that when the moving speed v is less than a certain critical speed Vc，the rope above the ball snaps first. Which rope snaps first when the speed exceeding Vc is decided by the maximum tensile force that the rope can sustain. The rope under the ball snaps first when the speed v>>Vc .