According to my teacher, neglect friction means to neglect the work of friction. I believe that it means that the system is frictionless. Am I wrong?

The problem is specifically, "A 5 kg cylinder 0.5 m in diameter rolls down a plane 5 m long inclined 30° with the horizontal at the bottom of the incline. What is its (a) total kinetic energy, (b) linear speed, (c) angular speed, (d) rotational kinetic energy, and (e) translational kinetic energy? Neglect friction."


1 Answer 1


"The system is frictionless" and "the work done by friction is 0" are not equivalent statements!

If the system is frictionless, the cylinder will not roll down the plane, and instead will slide down the plane. In this case, its motion will be purely translational.

If, on the other hand, the work done by friction is 0, but the system is not frictionless, then the cylinder rolls without slipping. Since the contact point always has velocity 0, the (static) friction doesn't do work on the cylinder.

I'm assuming that you want to use the second case. In this case, you should take friction to be large enough that the cylinder can roll without slipping.

  • $\begingroup$ So is "neglect friction" neglecting the work done by friction or frictionless? $\endgroup$ Commented Nov 17, 2021 at 8:24
  • 1
    $\begingroup$ @student1928374 It means neglecting the work done by friction. $\endgroup$
    – ohwilleke
    Commented Nov 17, 2021 at 8:58

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