1700990501
1700990502
t时刻，据质点动量定理，有
1700990503
1700990504
1700990505
1700990506
1700990507
（1）引入常量
1700990508
1700990509
1700990510
1700990511
1700990512
据①式，可解得
1700990513
1700990514
1700990515
1700990516
1700990517
即有
1700990518
1700990519
1700990520
1700990521
1700990522
将④式两边对t求导，得
1700990523
1700990524
1700990525
1700990526
1700990527
因du／dt＝a，即得
1700990528
1700990529
1700990530
1700990531
1700990532
（2）由②式，可解得
1700990533
1700990534
1700990535
1700990536
1700990537
即有
1700990538
1700990539
1700990540
1700990541
1700990542
由⑧式，可见，
1700990543
1700990544
1700990545
1700990546
1700990547
即质点在力作用下加速，其极限速度为真空光速c．
1700990548
1700990549
将⑧式两边对t求导，可得
1700990550