Newton's Second Law-Pulley problem

  • Thread starter Precursor
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In summary, the conversation discusses a problem involving a 4.0-kg oak block attached to a hanging 2.0-kg object through a string and pulley system. The force of friction on the block is given as 11.8 N. The participants attempt to solve for the acceleration and tension in the string using the equations F_net=ma and F_g=mg. However, there is a discrepancy in the results, with one person getting 15.7N for tension and another getting 17N. They discuss possible errors in their calculations and try to find a solution.
  • #1
Precursor
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Homework Statement


A 4.0-kg oak block on a horizontal, rough oak surface is attached by a light string that passes over a light, frictionless pulley to a hanging 2.0-kg object. The magnitude of the force of friction on the 4.0-kg block is 11.8 N.
http://img353.imageshack.us/img353/3743/pulleyai7.jpg​

Homework Equations


[tex]F_{net}[/tex]=ma
[tex]F_{g}[/tex]=mg


The Attempt at a Solution


I used [tex]F_{net}[/tex]=[tex]F_{g}[/tex]+[tex]F_{F}[/tex] and substituted in the values, but I don't seem to get the correct answer.
 
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  • #2
Either I am blind...or I don't see a QUESTION...
 
  • #3
What are you looking for? Tension?
 
  • #4
Assuming you're looking for tension...

http://www.geocities.com/chmode12/pulley.JPG
 
  • #5
I was looking for acceleration.
 
  • #6
Ok. Well, do you see how you can use my demonstration to solve for acceleration as well? Put T on one side of both italicized equations, set them equal to each other, then solve for acceleration.
 
  • #7
I am in the same class as Precursor, and I am also stuck on this question. I was able to get the acceleration, but I am unable to get the tension in the string. I got 15.7N. Dvd, you also got 15.7N, but our answer key for this assignment says that the answer is 17N. So, are we wrong or is our answer key wrong?
 
  • #8
temaire said:
I am in the same class as Precursor, and I am also stuck on this question. I was able to get the acceleration, but I am unable to get the tension in the string. I got 15.7N. Dvd, you also got 15.7N, but our answer key for this assignment says that the answer is 17N. So, are we wrong or is our answer key wrong?


What did you calculate for an acceleration?

Casey
 
  • #9
I calculated [tex]1.3 m/s^{2}[/tex] for the acceleration.
 
  • #10
Perhaps your Force of Friction is incorrect? Double check and make sure all the numbers given is the same, and get back to me.
 
  • #11
Dvdqnoc - I apologize in advance in case I've missed something, but, on the right side of your calculations, you have

T - 11.8 = 2a

Should it be

T - 11.8 = 4a

since that block has a mass of 4g?
 
  • #12
Thank you - this was kind of helpful :)
 

1. What is Newton's Second Law?

Newton's Second Law is one of the three laws of motion developed by Sir Isaac Newton. It states that the acceleration of an object is directly proportional to the net force acting on the object, and inversely proportional to its mass. In other words, the greater the force applied to an object, the greater its acceleration will be, and the greater the mass of the object, the less its acceleration will be.

2. How does Newton's Second Law apply to pulley problems?

In pulley problems, Newton's Second Law can be used to determine the acceleration of the system. The total mass of the system is taken into account, as well as the net force acting on it. By applying the equation F = ma (force equals mass times acceleration), the acceleration of the system can be calculated.

3. What is a pulley and how does it work?

A pulley is a simple machine that consists of a wheel with a groove around its circumference and a rope or belt that runs through the groove. It is used to change the direction of a force or to lift objects. As the rope is pulled, the pulley rotates, allowing the force to be transferred to another object in a different direction.

4. How do you solve a pulley problem using Newton's Second Law?

To solve a pulley problem using Newton's Second Law, first draw a free body diagram of the system, identifying all the forces acting on each object. Then, apply Newton's Second Law to each object to determine the net force and acceleration. Finally, use the equations of motion to solve for any unknown variables, such as the tension in the rope or the acceleration of the system.

5. What are some common misconceptions about pulley problems and Newton's Second Law?

One common misconception is that the tension in the rope is always equal throughout the system. This is not necessarily true, as the tension can vary depending on the mass and acceleration of each object. Another misconception is that the acceleration of the system will always be equal to the acceleration of the hanging mass. In reality, the acceleration of the system is determined by the net force and the total mass of the system, not just one object. Lastly, many people think that the direction of the acceleration will always be in the direction of the hanging mass. This is not always the case, as the acceleration can be in any direction depending on the forces acting on the system.

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