Question

1. a) What is the weight of a railway car of mass 70 T. ? b) Given that a force of 700 N. will just keep this car rolling, what is the coefficient of rolling friction?

Answer 1

a) Weight is the force acting on the surface on which the mass is located:

$$W=mg=70\space000\cdot9.8=686\space000\text{ N}.$$

b) The force of friction is defined as

$$f=\mu_rN=\mu_rW,\\ \mu_r=f/W=700/686000=1.02\cdot10^{-3}.$$