A block of mass 2 kg, moving with the initial speed of 3 m/s comes to rest on a rough horizontal surface after travelling a distance of 3 m. The magnitude of the frictional force is:

Using the work-energy theorem, the work done by friction equals the change in kinetic energy. Initial KE = (1/2)(2)(3)² = 9 J. Final KE = 0. So work done by friction = -9 J. Work done = -f × d, where f is the frictional force and d = 3 m. Hence f × 3 = 9, giving f = 3 N. Therefore, the frictional force has a magnitude of 3 N.