Finding Tension in Multibody Systems in AP Physics II

In summary, you should analyze the problem separately as two different equations if you want to find the tension or normal force between two objects.
  • #1
z-component
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We're now studying classical mechanics in AP Physics II (forces specifically) and I have a very broad question. When I'm given a multibody system and am asked to find specific forces, such as tension between two objects, I sometimes use the wrong value for mass. How do I know if I am supposed to add two masses or just use the one object's mass depending on what's asked?

For example, if I am to find the tension in a rope between two objects, do I use the mass of the two objects or of the whole system?
 
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  • #2
z-component said:
How do I know if I am supposed to add two masses or just use the one object's mass depending on what's asked?
It depends on what you are trying to find. If you are trying to find a property of the entire system (say the acceleration of the center of mass), then you'd use the mass of the entire system.
For example, if I am to find the tension in a rope between two objects, do I use the mass of the two objects or of the whole system?
In this case, the only way to find the tension is by analyzing the masses separately. (If you treat them as a single system, the tension will be an unknown internal force.)
 
  • #3
So if I'm finding tension or normal forces between two objects I analyze separately as in two different equations? Is there any general rule of thumb for these situations?
 
  • #4
Don't think of this as an additional "rule of thumb" that you have to memorize. It's just the way Newton's laws work. If you want to study the forces acting on a given object (like the tension or normal forces you mentioned), then apply Newton's 2nd law to that object.

Realize that if you take both objects together as a single system, you won't be able to learn anything about the forces between them, since they will be internal forces (and, per Newton's 3rd law, they will cancel out). This will all become obvious after you've solved a few hundred problems. :smile:

But, as I said earlier, often it will prove useful to analyze the problem both ways: As a composite system and as separate objects.
 
  • #5
Thank you, that helps. I am looking forward to the 100th time I solve a mechanics problem... ;)
 

1. What is meant by "tension" in multibody systems in AP Physics II?

Tension refers to the force that is transmitted through a string, rope, or cable when it is pulled at both ends. In multibody systems, tension is an important concept because it helps us understand how forces act on different objects in a system.

2. How do you calculate tension in a multibody system?

To calculate tension, you need to apply Newton's Second Law, which states that the net force on an object is equal to its mass multiplied by its acceleration. In a multibody system, the net force on an object is the sum of all the forces acting on it, including tension. By setting up and solving equations of motion for each object in the system, you can determine the tension in each string or cable.

3. Can tension be negative in a multibody system?

Yes, tension can be negative in a multibody system. This occurs when an object is pulling on a string or cable in the opposite direction of the force applied by the string or cable. In other words, the tension is acting in the direction opposite of the force applied.

4. What happens to tension when there are multiple strings or cables in a system?

When there are multiple strings or cables in a system, the tension is shared among them. This means that the total tension in the system is equal to the sum of the individual tensions in each string or cable. It is important to keep in mind that the tension in each string or cable may be different depending on factors such as the angle at which they are pulled.

5. How does finding tension in a multibody system relate to real-world applications?

Finding tension in multibody systems is important in understanding how forces act on objects in various situations. This can be applied to real-world scenarios such as determining the weight that a bridge can hold or calculating the tension in cables supporting a suspended object. It is also relevant in fields such as engineering, where understanding tension in structures is crucial for ensuring their stability and safety.

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