First test on equilibrium tomorrow, and I'm very sure there is going to be a ladder-type problem on it where I have to determine normal forces, frictional forces when someone is on the ladder up a certain distance, and how far can the person go up before it starts to slip. I don't know if you guys are familiar with masteringphysics.com or not, but it is an online homework system where it automates questions similar to that in the book. I've gotten pretty far in the problem, but for some reason I can't complete the 2nd two problems. Here is the exact problem on mastering physics:(adsbygoogle = window.adsbygoogle || []).push({});

I drew a diagram and a free-body diagram of the problem, and was able to get the answer to A through the requirements for equalibrium. Here is my work:A uniform ladder 5.0 m long rests against a frictionless, vertical wall with its lower end 3.0 m from the wall. The ladder weighs 160 N. The coefficient of static friction between the foot of the ladder and the ground is 0.40. A man weighing 740 N climbs slowly up the ladder.

A)What is the maximum frictional force that the ground can exert on the ladder at its lower end?

B)What is the actual frictional force when the man has climbed 1.0 m along the ladder?

C)How far along the ladder can the man climb before the ladder starts to slip?

(Fnet)x: Fs-Fw=0

(Fnet)y: Fn-Fp-Fg=0

We have Fp and Fg, so Fn = Fp+Fg orFn = 900

Since Fs <= Us*Fn we have (Fnet)x: Us*Fn-Fw=0, or Fw=(.4)*(900), soFw = 360. And since Fs<=Us*Fn, Fs = Fw, soFsmax=360. This is the answer to A.

I am confused as hell when it comes to B. It is difficult to explain because you cannot see my free-body diagram, but I will do my best to describe it.

Don't know how to use tao, so I'll just use lowercase t.

(tnet)=tn+ts+tp+tg+tw=0. I am choosing the origin to be at the lower end of the ladder where Fn and Fs are, so those would equal zero because they have no moment arm. So here is the new equation:

(tnet): -(Fp)(R)Sin()-(Fg)(R)Sin()+(Fw)(R)Sin()=0. With the values we have

(tnet): -(740n)(1m)sin(53.13)-(160n)(2.5m)sin(53.13)+(Fw)(5m)sin(36.87)=0.

I found 53.13 and 36.87 through basic trig. Here are my questions: I am solving for a new Fw (normal force coming from the wall) in this equation right? i.e. I would NOT be using the value I obtained earlier. After I get that I plug it back in to (Fnet)X to get Fs, which would be my answer - right?

My final answer was 301.3, which is NOT right - The correct answer is 171n. Why

When it comes to part C I'm not sure how to go about it because I don't fully understand the answer to part B. I'm guessing you solve for the unknown moment arm in the tnet equilibrium equation.

Thanks guys

pink floyd says hi

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# Homework Help: A simple ladder problem (equilibrium)

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