Calculating Force P: Solving a Statics Problem with Friction and Pulley Neglect

[smashbrohamme]

Knowing that friction equals 0.2 at all surfaces of contact, determine the magnitude of the force P required to move the 40kg plate B to the left. (Neglect Pulley Friction)

This is a statics problem asking how much force will it take to move block B which is 40kg...the problem is it has another block of 80kg sitting on top of it...and a pulley is connecting the 80kg block and the 40kg block.

the answer is 549N but I can't seem to find how to get it.


the 40kg block is on top of a friction plane that is 0.2...and a block is sitting on top of it that weighs 80kg. so I am assuming you have to count both frictions into the equation...

for the 40kg block I got Force to move it by itself is 152.96...and I am getting it takes 156.96 to move the 80kg block as well. so if you neglect the pulley's friction and just create a Tension of 156.96 I am only getting a force of 313.92N.

the picture is a lil funky but there is no angles to worry about.

 

[p21bass]

Draw a FBD for each block. You'll see that there are two friction planes for the bottom block, plus the tension in the string.

 

[smashbrohamme]

I believe I did account for both frictions of the 40 kg block, I am still not coming up with the correct answer that was given. would the friction from both top and bottom of the 40kg block be going opposite of the p(force)

 

[PhanthomJay]

I believe I did account for both frictions of the 40 kg block, I am still not coming up with the correct answer that was given. would the friction from both top and bottom of the 40kg block be going opposite of the p(force)

Yes, the friction always opposes the direction of relative motion or impending relative motion between the 2 objects or surfaces in contact with the object you are examining for forces. Follow p21bass' advice, and be sure you are calculating the normal force between the lower block and floor correctly.