Help, I dont understand anything about the friction problems! I am stuck on all

Question

Help, I dont understand anything about the friction problems! I am stuck on all of them! If you have any suggestion let me know. Chapter 4 number 48 is really messing me up. The question is A student moves a box (25.0kg) by a rope with a force of 80.0 N at an angle up an incline of 25 degrees above the horizontal. The coefficient of kinetic force is .300. A) find acceleration of box B)student now starts up an incline of 10 degree keeping a 80 N force directed at 25 degrees above the line of incline. If the coefficient of friction has not changed whats the acceleration of the box?

Explanation / Answer

Ff = N = M g cos           frictional force opposing motion Fg = M g sin      gravitational force opposing motion F = M a = Fs - ( M g cos + M gsin )      net force moving box upincline F = 80 - (.3 * 25 * 9.8 * .906 + 25 * 9.8 * .423) =-90.2       the net force is down theincline a = F / m = -90.2 / 25 = -3.6 m /s2      down the incline Now Fs becomes 80 * cos 15 = 77.3N      the angle the rope makes with theincline is 15 deg F = 77.3 - ( M g cos + M g sin) = -37.6 N      the net forceis still down the incline a = -37.6 / 25 = -1.51      still down the incline

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