Cross products vanish Classical Mechanics

In summary, the conversation is about an object moving in the x-y plane and the question of why the cross terms vanish in equation 8.9. The answer is that they vanish due to the definition of the center of mass. The book provides an explanation with mathematical equations and suggests looking at equation 5.56 and 5.58. The concept of the center of mass in the center of mass frame of reference is also mentioned. There is also a discussion about the integral of the cross product of r' and v' not being equal to zero.
  • #1
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As seen in the picture, this question is about an object moving in the x-y plane. But I don't get why in equation 8.9 the cross terms vanish? If anyone can help me, that would be really nice.
 

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  • #2
... they vanish due to the definition of "center of mass".
The text explains with maths in the passage right below eq8.9.
Have you tried working out the cross terms for some example?
 
  • #3
The book answers your question already. You need to think about the center of mass in the center of mass frame of reference. Also the author suggests hou look at equation 5.56
 
  • #4
Equation 5.58 is just R_cm = sum m_i r_i / M.

I get why r' is zero in the frame of the center of mass. But if that's the case, why is the integral of the cross product of r' and v' not equal to zero also?
 

1. What is a cross product in Classical Mechanics?

The cross product is a mathematical operation that takes two vectors and produces a new vector that is perpendicular to both of the original vectors. In Classical Mechanics, this operation is used to calculate the torque or rotational force on an object.

2. How do you calculate the cross product of two vectors?

To calculate the cross product of two vectors, you first need to determine the magnitude of the vectors, then multiply them together and multiply by the sine of the angle between the two vectors. This can be represented by the formula: A x B = |A| x |B| x sin(θ).

3. Why do cross products vanish in Classical Mechanics?

In Classical Mechanics, the cross product of two vectors is said to "vanish" or be equal to zero when the vectors are parallel or antiparallel to each other. This means that the two vectors are either pointing in the same direction or in opposite directions, resulting in a cross product of zero.

4. What does it mean when the cross product vanishes?

When the cross product of two vectors vanishes, it means that there is no torque or rotational force being applied to an object. This can occur when the forces acting on an object are balanced, resulting in no net rotation.

5. Are there any real-world examples of cross products vanishing in Classical Mechanics?

Yes, there are many real-world examples of cross products vanishing in Classical Mechanics. One example is a seesaw or teeter-totter where two children of equal weight sit on opposite ends. The force of gravity acting on both children is equal and opposite, resulting in a net torque of zero and no rotation of the seesaw.

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