有一类竞赛题 :带电粒子在匀强磁场、匀强电场和重力场中运动 ,所受的洛伦兹力大小和方向随时间而变化 ,运动过程复杂 ,常规解法难于求解此类问题。因该类问题中带电粒子所受的重力或电场力是恒力 ,故可以采用构造速度的方法来巧妙地解答该类问题。构造速度法就是根据带电粒子 There is a class of contest questions: charged particles move in uniform magnetic fields, uniform electric fields and gravitational fields. The magnitude and direction of Lorentz force vary with time, and the motion process is complex. Conventional solutions are difficult to solve such problems. Because the gravity or electric field force of the charged particles in this type of problem is a constant force, the method of constructing the velocity can be used to solve this problem skillfully. Constructive velocity method is based on charged particles