Two blocks are positioned according to the figure. Block A (80 lb) is attached to a rope as such that it doesn't slide or fall from block B (120lb). Compute the smallest value of Force to be applied on Block B for it to not move parallel to the inclined plane.
F = fN
N = Wcos[tex]\theta[/tex]
The Attempt at a Solution
Here's how I did, I assumed that Block B is sliding downwards parallel to the inclined plane.
Then I compute the Frictional force between block A and B, so
W = 80 lb
N = 80cos20
Friction forceA&B = 0.2(80cos20) direction is upwards parallel to inclined plane, because it will resist the downward sliding motion of Block B.
Then compute for Friction force between Block B and plane, so
W = 80 +120 (is this correct?)
N = 200cos20
Friction forceB&plane = 0.2(200cos20) direction same with previous Ff.
Then I compute for the sliding magnitude of Block B, assuming it will slide because of none external force acting on it other than friction
F = 120sin20 (force not applying frictions)
F2 = 120sin20 - (0.2(80cos20) + 0.2(200cos20)) = Answer
I equate F2 as my answer because that's the only force left in the system needed to be countered..... so my force needs to be acting upward...
When I substituted all values.... it gives me a negative answer force... so does that mean... I'm wrong?