1.Atraintravellingat20ms(-1次方),startstoacceleratewithconstantacceleration.Itcoversthenextkilometrein25seconds.Usetheequations=ut+1/2at²tocalculatetheacceleration.Findalsohowfastthetrainismovingattheendofthi

Question

1.A train travelling at 20ms(-1次方),starts to accelerate with constant acceleration.It covers the next kilometre in 25 seconds.Use the equation s=ut+1/2at² to calculate the acceleration.Find also how fast the train is moving at the end of this time.Illustrate the motion of the train with a velocity-time graph.
How long does the train take to cover the first half kilometre?
2.A train travelling at 55ms(-1次方)starts to accelerate,and 200 metres later she is riding at 7ms(-1次方),Find her acceleration,assumed constant.

Answer 1

1解已知火车初速度v=20m/s,运动了t=25s,路程为s=1000m
根据运动学知识得
把v=20m/s,运动t=25s,s=1000m,代入公式 s=vt+1/2*at²
解得
加速度a=1.620ms(-2次方)
末速度V=v+at=60m/s
速度时间图（velocity-time graph）在这里无法画出来,楼主抱歉了,不过已知条件都有了你可以自己画一画.
2解火车做匀减速运动,初速度v=55m/s,末速度V=7m/s,运行路程为s=200m
由运动学知识得：
把v=55m/s,V=7m/s,s=200m代入公式s=1/2*(V+v)*t
解得
火车运动时间t=25/3秒
把v=55m/s,s=200m,t=25/3秒代入公式s=vt-1/2*at²
解得
a=7.44ms(-2次方)