1. The problem statement, all variables and given/known data Block mass "m" is sliding on frictionless horizontal surface while traveling inside of hoop radius "R". Coefficient of f between block and wall is "u", therefore, speed of block is decreasing. In terms of "R", "v, block's veloc.", "m", and "u", find expressions for: 1. frictional force on the block: 2. block's tangential acceleration (dv/dt). 3. From #2 find time required to reduce speed of block from its original velocity to one-third of its original velocity. 2. Relevant equations fk = un F = ma ar = v^2/r <-- ar is radial acceleration 3. The attempt at a solution My process for 1: fk = un ar = v^2/R F = n = ma n = m(v^2/R) fk = (umv^2)/R 2: F = ma I have no clue on this one. Would it be from #1, use v^2 = fR/um v = sqrt[(fR)/(um)] And take the derivative of that? And it will be my tangential acceleration? but the thing is, you have to give the answer in terms of m, R, u, and v only. Can't put in terms of f - friction 3: I assume to be easy with #2? It's just simple kinematics right? I found a previous inquiry on this problem in the forums but it wasn't very clear, so I wanted to ask it myself.