您的位置: 首页  >  作业答案  >  英语  >  a car of mass 1500kg is moving up a straigt road,which is inclined at an angle *to the hrizontal,where sin*=1/14,the resistance to the motion of the car from non-gravitional forces is constant and is modelled as a single constant force magnitude 650N.The car's engine is working at a rate of 30kw.find the acceleration of the car at the instant when its speed is 15m/s.

a car of mass 1500kg is moving up a straigt road,which is inclined at an angle *to the hrizontal,where sin*=1/14,the resistance to the motion of the car from non-gravitional forces is constant and is modelled as a single constant force magnitude 650N.The car's engine is working at a rate of 30kw.find the acceleration of the car at the instant when its speed is 15m/s.

分类: 作业答案 • 2021-12-20 12:13:14
问题描述：

a car of mass 1500kg is moving up a straigt road,which is inclined at an angle *to the hrizontal,where sin*=1/14,the resistance to the motion of the car from non-gravitional forces is constant and is modelled as a single constant force magnitude 650N.The car's engine is working at a rate of 30kw.
find the acceleration of the car at the instant when its speed is 15m/s.

答

the resistance to the motion of the car from non-gravitional forces is constant and is modelled as a single constant force magnitude 650N.

这句话很奇怪，既然车向上开，那收到的阻力肯动和重力有关，就算是摩擦力也是重力导致的，怎么可能会From non-gravitional force？

这问题有瑕疵，就算是摩擦力好了，

那么，作用于小车上而且平行于斜面的力就是 cos（90-*）=F斜/ 重力
因为F斜/重力=1/14,所以F斜= 重力/14
如果g取9.8，重力=1500乘以9.8=14700
F斜=1050 N

车在斜面上受到的总阻力=F斜+650N摩擦力=1050+650=1700N

由于发动机的功率=30kw=30000瓦，根据功率=做功/时间，而做功又等于力/距离，
P=W/t，W=FS，所以 P= FS/t = F S/t =F V, 距离除以时间等于速度，所以功率=FV

如果算瞬时加速度要用F=a m 即 a = F / m

因为P=FV，所以当速度为15m/s时，引擎提供的力F=P/V=30000/15，F=2000N

引擎在斜面向上的力 减去 斜面的总阻力1700N 就是 车子的合力，2000-1700=300N。

a=F/m=300N/1500kg=0.2 m/s^2

答

重力在平行路面方向的分量=mgsinα=1500×9.8×(1/14)=1050(N)
∵发动机输出功率恒定在30KW
∴当小车速度为15m/s是,发动机输出的拉力=30KW/(15m/s)=2000N
∴此时小车的加速度a=(2000N-1050N-650N)/1500KG=0.2m/s²

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