Really with this Free Response question for AP Physics

In summary: The forces acting on the man are normal force and the weight of the ladder. The force of friction between the ladder and the ground is T.
  • #1
AlphabetTown
4
0

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



[itex]T[/itex] = Torque

∑[itex]T[/itex] = [itex]T[/itex][itex]_{man}[/itex] + [itex]T[/itex][itex]_{ladder}[/itex] + [itex]T[/itex][itex]_{wall}[/itex]

The Attempt at a Solution



Well I tried to find the Force of Friction F[itex]_{f}[/itex] = μn; F[itex]_{f}[/itex] = μmgcosθ:

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

and

[itex]T[/itex] = F*r*sinθ
[itex]T[/itex] = (800 N)(10 m)(sin 60°)
[itex]T[/itex] = -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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  • #2
(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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
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.
 

Related to Really with this Free Response question for AP Physics

1. What is the purpose of a Free Response question in AP Physics?

A Free Response question in AP Physics is designed to test a student's understanding and application of key concepts and skills in physics. They often require students to solve problems and explain their reasoning and calculations.

2. How should I approach a Free Response question in AP Physics?

It is important to carefully read and understand the question, identify the relevant concepts and equations, and show all of your work and reasoning. It is also helpful to label all variables and units and clearly state your final answer.

3. How much time should I spend on a Free Response question in AP Physics?

In the AP Physics exam, you will have 90 minutes to complete three Free Response questions. This means you should aim to spend about 30 minutes on each question. However, it is important to manage your time effectively and not spend too much time on one question.

4. Can I use a calculator on Free Response questions in AP Physics?

Yes, you are allowed to use a calculator on Free Response questions in AP Physics. However, it is important to show all of your work and not rely solely on your calculator for the answers.

5. Are there any tips for scoring well on Free Response questions in AP Physics?

Some tips for scoring well on Free Response questions in AP Physics include practicing with past exam questions, reviewing key concepts and equations, and showing all of your work and reasoning. It is also helpful to manage your time effectively and double-check your answers before submitting.

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