Dynamics: Finding force exerted on a box traveling down a slope
1. The problem statement, all variables and given/known data
This is problem 13.70 from Beer and Johnston Vector Mechanics 9th ed. A 300g pellet is released from rest at A and slides with friction along the surface shown. Determine the force exerted on the pellet by the surface (a) just before the pellet reaches B, (b) immediately after it has passed through B. The drawing shows a pellet A on a declining slope, distance 1.2 m to B which is almost at the bottom of the slope. C is a flat surface. The radius of curvature between B and C is 0.8 m and the angle between them is 30° 2. Relevant equations PE = mgz KE = 1/2mv^{2} F_{r} = mu_{k}*N Energy from F_{r} = F_{r}Δx 3. The attempt at a solution Since the pellet is at rest KE_{A} = 0J, however PE_{A} = (.3kg)(9.81m/s^{2})(1.2sin(30°)) (The angle between the horizontal and the 1.2m hypotenuse isn't given, but I just found it using opposite interior angles.) so PE_{A} = 1.7658 J At B we will have KE and PE. Assuming C is at z = 0m we can use the radius of curvature and angle given to find z_{b} z_{B} = 0.8 m  0.8 m * cos(30°) z_{B} = 0.10718 m so PE_{B} = (.3kg)(9.81m/s^{2})(0.10718 m) PE_{B} = 0.31543 J KE_{B} = 1/2(.3 kg)v_{B}^{2} Plugging all of this into the law of conservation of energy where E_{A} = E_{B} 1.7658 J = 0.31543 J + (1/2)(.3 kg)v_{B}^{2} v_{B} = 3.109 m/s I got hung up here because I think that the work due to the frictional force should be subtracted from KE_{B} + PE_{B} in order to get the correct velocity the pellet would be traveling at. However, there is no coefficient of friction given in the problem. I feel like I'm just missing something small here and my brain is just dead.. 
Re: Dynamics: Finding force exerted on a box traveling down a slope
Would you sketch this?

Re: Dynamics: Finding force exerted on a box traveling down a slope
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It is problem 13.70

Re: Dynamics: Finding force exerted on a box traveling down a slope
From my reading of the problem, you're supposed to solve for the friction force on the block at the given points.

Re: Dynamics: Finding force exerted on a box traveling down a slope
Our teacher just posted an announcement that the problem should be a pellet sliding without friction.
The answer is 2.55N, I'm just unsure how to get here.. 
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