Question

A speedtrain of mass 10,000kg is moving at 20m/s North. What is its momentum? What is its kinetic energy? If it encounters a frictional force of 40,000N, how far will it go before it stops? What will the acceleration on the train during this time be and is it reasonable?

Answer 1

here,

the mass of train , m = 10,000 kg

initial velocity , u = 20 m/s j

the initial momentum , Pi = m * u

Pi = 10000 * 20 j kg.m/s

Pi = 2 * 10^5 j kg.m/s

the momentum of train is 2 * 10^5 kg.m/s to the north

the initial kinetic energy , KEi = 0.5 * m * u^2

KEi = 0.5 * 10000 * 20^2 J = 2 * 10^6 J

frictional force , ff = 40000 N

let the stopping distance be s

using Work energy theorm

Work done by friction =change in kinetic energy

ff * s * cos(180) = Kef - Kei

40000 * s * (-1) = 0 - 2 * 10^6

s = 50 m

the stopping distance is 50 m

the acceleration on the train during this time , a = - ff/m

a = - - 40000 /10000 m/s^2 = - 4 m/s^2