Angular Momentum: Rotating Object

In summary, the conversation is about a student trying to understand an example from their textbook on angular momentum. They are confused about where the cosine theta term in the example came from and how it relates to calculating the magnitude of torques. Another person explains that the cosine is used to find the moment arm in this case and gives three different ways to understand it. The student thanks the person for their explanation.
  • #1
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Homework Statement



I'm trying to understand an example from my textbook about angular momentum. This is the example given:
upload_2014-12-11_16-18-1.png


For the part in red: I don't understand where the cosine theta term came from. When you're calculating the magnitudes of torques, don't you just use FRsin(theta)? If someone could clear that up for me, it would be great! Thank you.
upload_2014-12-11_16-18-1.png

upload_2014-12-11_16-18-1.png
 
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  • #2
Speedking96 said:
I don't understand where the cosine theta term came from. When you're calculating the magnitudes of torques, don't you just use FRsin(theta)?
If the theta is the angle between the force vector and the distance vector, yes. But in the diagram, the angle between the vectors is the angle between the vertical and the seesaw. Theta is the angle between the horizontal and the seesaw.
 
  • #3
Ok. I think I get it; the cosine is used to get the moment arm in this case, correct?
 
  • #4
Speedking96 said:
Ok. I think I get it; the cosine is used to get the moment arm in this case, correct?
That's one way to look at it. There are at least 3 ways, leading to the same answer:
- distance cos (theta) = moment arm.
- force cos (theta) = component of force perpendicular to distance
- vector product of force and distance = force * distance * sin(90-theta)
 
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  • #5
Ok, I understand. Thank you very much.
 

1. What is angular momentum?

Angular momentum is the measure of an object's tendency to keep rotating. It is the product of an object's moment of inertia and its angular velocity. In simpler terms, it is the amount of rotational motion an object possesses.

2. Why is angular momentum important?

Angular momentum is important because it is a conserved quantity, meaning it cannot be created or destroyed. This property is crucial in understanding the behavior and movement of rotating objects, such as planets, stars, and other celestial bodies.

3. How is angular momentum calculated?

The formula for angular momentum is L = Iω, where L is angular momentum, I is moment of inertia, and ω is angular velocity. Moment of inertia is a measure of an object's resistance to changes in its rotational motion, and angular velocity is the rate at which an object rotates.

4. What is the relationship between angular momentum and torque?

Angular momentum and torque are directly related. Torque is the force that causes an object to rotate, and it is proportional to the change in angular momentum over time. This relationship is described by the equation τ = dL/dt, where τ is torque, dL/dt is the change in angular momentum over time.

5. How does angular momentum apply to everyday life?

Angular momentum can be observed in various everyday activities, such as spinning a top, throwing a frisbee, or riding a bicycle. It is also essential in many industrial processes, such as the operation of turbines and motors. Understanding angular momentum can help us design and improve technologies that rely on rotational motion.

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