A 7.50kg block slides down a frictionless ramp of height 2.25m at a velocity of 1.50 m/s. it crosses a rough patch that is 2.5m long and exerts a force of 3N. It then continues to the other side of the ramp which is angled at 35 degrees(I dont think this matters b/c it is frictionless). How can I find how many times the block will cross the rough patch? I tried a few equations which didn't make sense.... W=ΔE=Ef-Ei W=FdCosθ KE=1/2mv^2 PE=mgh I tried to find the work being done and used Fdcos180 because the patch acts against the direction of motion so it would be -Fd where F=3N and d=2.5m. Then I set Ef-Ei=W and then it started to fall apart. I am just not sure how to approach this problem I guess... Ef would be 0 if we are looking for how many times it crosses the rough patch right? so then -Ei=-Fd, but I just am having trouble relating things... help would be appreciated, thank you!