# Mastering physics :pulleys and friction

The conversation discusses finding the maximum value of Q in a friction problem. The solution involves setting up equations for the forces and solving for Q, with the correct answer being 270. In summary, the conversation discusses finding the maximum value of Q in a friction problem and the solution involves setting up equations and solving for Q to get a final answer of 270.

## Homework Statement

http://img7.imageshack.us/img7/8717/friction.png

## The Attempt at a Solution

T-W_a=0
W=mg=-186.6
T=186.6
T-Q-fs=0
186.6-Q-fs

fs=uN
N=60+147=207
fs=82.8
186.6-Q-82.8=0
Q=103.8
what is wrong??

Last edited by a moderator:
Note: They want the maximum value of Q.

wait so should I take the direction of friction to the right? 82.8+186.2=269 .. but that also isn't in the choice list

Edit: wait I have things to two significant figures right? so is 270 the right answer?

wait so should I take the direction of friction to the right? 82.8+186.2=269 .. but that also isn't in the choice list

Edit: wait I have things to two significant figures right? so is 270 the right answer?
Yes.

Hello,

Thank you for sharing your attempt at the solution to this problem. It appears that you have correctly identified the forces acting on the system (tension, weight, normal force, and friction) and have set up the equations correctly. However, there may be a few errors in your calculations.

Firstly, it seems that you have used the incorrect value for the weight of the block. The image shows the weight to be 147 N, not -186.6 N. This may have resulted in some discrepancies in your final answer.

Additionally, it is important to note that the coefficient of friction, u, is not given in the problem. Without this information, it is difficult to accurately calculate the value of friction. It is possible that this is the reason for the discrepancy in your final answer.

It may also be helpful to double check your signs in the equations. For example, the tension T should be equal to the weight W minus the applied force W_a, so it should be T = W - W_a, not T - W_a = 0 as you have written.

Overall, it seems that your approach to the problem is correct, but there may be some errors in your calculations. I would suggest reviewing the given information and equations, and double checking your calculations to see if you can identify where the error may have occurred. Additionally, if you are still having trouble, it may be helpful to seek assistance from a teacher or classmate.

Best of luck in solving this problem!

## 1. What is a pulley and how does it work?

A pulley is a simple machine that consists of a grooved wheel and a rope or belt. It is used to change the direction of a force and make it easier to lift heavy objects. When a force is applied to one end of the rope, the pulley rotates and transfers the force to the object being lifted.

## 2. How does friction affect the motion of objects in a pulley system?

Friction is a force that opposes motion and it can have a significant impact on the efficiency of a pulley system. Friction between the rope and the pulley can cause the rope to slip or the pulley to rotate less smoothly, resulting in a loss of energy and decreased efficiency.

## 3. What are the different types of pulleys and how do they differ?

There are three main types of pulleys: fixed, movable, and compound. Fixed pulleys have a stationary axle and are used to change the direction of a force. Movable pulleys have a movable axle and are used to increase the mechanical advantage of a system. Compound pulleys combine both fixed and movable pulleys to provide a greater mechanical advantage.

## 4. How can I calculate the mechanical advantage of a pulley system?

The mechanical advantage of a pulley system is equal to the number of ropes supporting the object being lifted. For example, if a system has three ropes supporting the object, the mechanical advantage would be 3. The mechanical advantage can also be calculated by dividing the output force by the input force.

## 5. What are some real-world applications of pulleys and friction?

Pulleys and friction are used in a wide range of applications, including elevators, cranes, and zip lines. They are also used in everyday objects like window blinds and bike gears. In addition, understanding the principles of pulleys and friction is important in fields such as engineering, physics, and mechanics.

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