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# Physics 221 Chapter 11 - PowerPoint PPT Presentation

Physics 221 Chapter 11. Problem 2 . . . Angular Momentum. Guesstimate the formula for the angular momentum? A. mv B. m  C. I  D. 1/2 I . Solution 2 . . . Angular Momentum. Guesstimate the formula for the angular momentum? Linear Momentum is mv Angular Momentum is I .

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Physics 221Chapter 11

• Guesstimate the formula for the angular momentum?

• A. mv

• B. m

• C. I 

• D. 1/2 I 

• Guesstimate the formula for the angular momentum?

• Linear Momentum is mv

• Angular Momentum is I 

• In the absence of any external torques, the angular momentum is conserved.

• If = 0 then I11 = I2 2

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• A. When her arms stretch out her moment of inertia decreases and her angular velocity increases

• B. When her arms stretch out her moment of inertia increases and her angular velocity decreases

• C. When her arms stretch out her moment of inertia decreases and her angular velocity decreases

• D. When her arms stretch out her moment of inertia increases and her angular velocity increases

• B. When her arms stretch out her moment of inertia increases and her angular velocity decreases

• I11 = I2 2

• So when I increases,  decreases!

• A X B is a vector whose:

• magnitude = |A| |B| sin 

• direction = perpendicular to both A and B given by the right-hand rule.

• Right-hand rule: Curl the fingers of the right hand going from A to B. The thumb will point in the direction of A X B

 = r x F

 = r F sin 

L = r x P

L =m v r sin 

= dL/dt

Proof

L = r x p

dL/dt = d/dt(r x p)

dL/dt = dr/dt x p + r x dp/dt

dL/dt = v x p + r x F

But v x p = 0 because p = mv and so v and p are parallel and sin 00 = 0

dL/dt = r x F

 =dL/dt

Given: r = 2 i + 3 j and F = - i + 2 j

Calculate the torque

 = r x F

 = (2 i + 3 j) x (- i + 2 j)

 = - 2 i x i + 4 i x j - 3 j x i + 6 j x j

 = 0 + 4 k + 3 k +0

 = 7 k