Really with this Free Response question for AP Physics

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SUMMARY

The discussion focuses on solving a physics problem involving an 80-kg man on a 10-meter ladder, which is positioned at a 60° angle against a frictionless wall. The key equations used include torque calculations (T = Torque) and the force of friction (F_{f} = μn). The participant struggled with identifying forces, drawing a free body diagram, and applying equilibrium conditions, ultimately receiving only 2 out of 10 points on the assignment. The correct approach involves accurately identifying all forces acting on the ladder and applying the conditions for equilibrium.

PREREQUISITES
  • Understanding of basic physics concepts such as torque and equilibrium.
  • Familiarity with free body diagrams and their components.
  • Knowledge of frictional forces and their calculations (F_{f} = μn).
  • Ability to apply trigonometric functions in physics problems (e.g., sin, cos).
NEXT STEPS
  • Learn how to draw and interpret free body diagrams in physics.
  • Study the principles of torque and equilibrium in static systems.
  • Explore the concept of friction, including static and kinetic friction coefficients.
  • Practice solving physics problems involving inclined planes and ladders.
USEFUL FOR

Students preparing for AP Physics exams, particularly those struggling with mechanics and equilibrium problems, as well as educators seeking to enhance their teaching methods for complex physics concepts.

AlphabetTown
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Homework Statement



1. An 80-kg man is one fourth of the way up a 10-m ladder that is resting against a smooth, frictionless wall. If the ladder has a mass of 20 kg and it makes an angle of 60° with the ground, find the force of friction of the ground on the ladder.



Homework Equations



T = Torque

∑T = T_{man} + T_{ladder} + T_{wall}

The Attempt at a Solution



Well I tried to find the Force of Friction F_{f} = μn; F_{f} = μmgcosθ:

mg = μmgcosθ
(20 kg)(-10 m/s^2) = μ(20 kg)(-10 m/s^2)cosθ
μ = 1.05

and

T = F*r*sinθ
T = (800 N)(10 m)(sin 60°)
T = -2438.5 N


I have no idea which of that is right. I only got 2 out of like 10 points for this Free Response question and I need to fix it, but I'm horrible at Physics and really need some help figuring it out. Thank you!
 
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(1) Start by identifying all the forces acting and drawing yourself a free body diagram.
(2) Careful! μN gives the maximum possible friction between surfaces, which is not what you want; You want the actual friction, which will be lower. Just label it Ff; you'll solve for it.
(3) Apply the conditions for equilibrium for both translation and rotation.
(4) Pick your 'axis' for calculating net torque wisely.
 
I'm sorry but that didn't help at all. I don't even know how to draw a free body diagram. I have absolutely no idea what to even do.
 
AlphabetTown said:
I'm sorry but that didn't help at all. I don't even know how to draw a free body diagram. I have absolutely no idea what to even do.
Do you have a textbook?

Draw a diagram of the ladder showing all the forces acting on it.
 
Yes and I've tried to use it but just makes no sense to me.

What are the forces? My guess would be normal force and the force of weight. What about tension? I never know whether to include that or not.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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