Reactions with crate and pulley

In summary, the conversation discusses a problem involving a suspended crate with an unknown center of gravity and equal tension in cables around pulleys. The question is how to analyze this system, particularly when summing moments about a point. The recommended approach involves looking at the crate separately and finding the force in the green string, then using a graphical method to solve for the unknown forces at the pin joint and roller.
  • #1
stinlin
72
1
So I had an exam yesterday and ran into a problem like this. The crate was suspended in such a way that it's CG didn't seem to line up with the given measurements. Also, the tension in the cables around the pulleys are equal (nothing is said about the tension in the blue cable). There's a pin joint at the bottom area and a roller on that incline. How would you go about analyzing this system? Mainly my question is - if you sum moments about a, what do you do about the 150 lb crate and those pulleys? Thanks!

http://img507.imageshack.us/img507/9034/lamelh0.png

^^ Image
 
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  • #2
Try looking at the crate separately. Which forces are acting on it? You should be able to find the force in the green string. Now, look at the system without the crate with the force in the green string acting on it. Write out the equations of equilibrium.

P.S. There is a neat graphical way to solve this. The force in the roller must be perpendicular to the direction of rolling. Further on, the force in the green string has a known direction, and the point of coincidence of the lines of these two forces is thereby known. Now, by drawing a line through that point and the point of the pin joint, you'll find the direction fo the force in the pin joint. Since you now have three forces, all of which directions are well known, and since you know the magnitude of one of them (the force in the green string), you can calculate your reactions graphically by simple vector addition and proper length and force scale selection.
 
  • #3
for reference


Hi there! It sounds like you encountered a challenging problem on your exam. Let's break it down and see if we can figure out how to analyze this system.

First, it's important to remember that when dealing with pulleys, the tension in the cable is equal on both sides. This means that the tension in the blue cable is also equal to the tension in the red and green cables. This will be helpful in our analysis.

Next, we can use the fact that the crate is suspended by the cables to determine the forces acting on it. The weight of the crate, 150 lbs, is acting straight down. Since the crate is not moving, the tension in the cables must be equal to the weight of the crate. This means that each cable is supporting 50 lbs of the crate's weight.

Now, let's look at the pin joint at the bottom and the roller on the incline. These act as supports and can be treated as reaction forces. We can use the principles of static equilibrium to determine the forces acting at these points.

When you sum moments about point A, you will need to consider the forces acting on the crate and the forces acting at the pin joint and the roller. The weight of the crate will create a clockwise moment, while the reaction forces at the pin joint and the roller will create a counterclockwise moment. You will need to set these two moments equal to each other to solve for the unknown reaction forces.

I hope this helps! Remember to always draw a free body diagram and use the principles of static equilibrium to solve problems like this. Good luck with your exams!
 

Related to Reactions with crate and pulley

What is a crate and pulley?

A crate and pulley is a simple machine that consists of a wooden or metal crate attached to a pulley system. The crate can be lifted or moved by pulling on the rope or chain attached to the pulley.

How do reactions occur with a crate and pulley?

Reactions occur with a crate and pulley when an external force is applied to the system. The external force causes the pulley to rotate, and the rotation of the pulley creates a reaction force that lifts or moves the crate.

What are the types of reactions that can occur with a crate and pulley?

There are two types of reactions that can occur with a crate and pulley: tension and compression. Tension occurs when the rope or chain attached to the pulley is pulled, causing the pulley to rotate in the direction of the external force. Compression occurs when the weight of the crate causes the pulley to rotate in the opposite direction of the external force.

How can the mechanical advantage of a crate and pulley be calculated?

The mechanical advantage of a crate and pulley can be calculated by dividing the load force (weight of the crate) by the effort force (force applied to the rope or chain). For example, if the load force is 100 pounds and the effort force is 20 pounds, the mechanical advantage would be 5. This means that the pulley system can lift or move the crate with 5 times less effort than if the crate were lifted manually.

What are some real-world applications of reactions with crate and pulley?

Crate and pulley systems are commonly used in industries such as construction, manufacturing, and transportation. They are also used in everyday tasks, such as lifting objects with a pulley in a garage or using a pulley system for a zip line in a recreational setting.

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