How do I find the correct magnitude using vectors and magnitudes?

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The discussion focuses on calculating the correct magnitude of a velocity vector in relation to a position vector, specifically in the context of an astronaut's speed towards an airlock. The user initially misunderstands the requirement but clarifies that the task involves determining the component of the velocity vector parallel to the vector pointing from the astronaut to the airlock. The correct approach involves using the dot product formula, |\overline{v}||\overline{u}|cos \varphi = |\overline{v}\cdot\overline{u}|, to find the angle and subsequently the magnitude of the velocity vector component.

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Hey guys, I am posting this question because I don't know what it is asking of me. Here it is:

opwh14.jpg


Any tips or help would be nice. Thanks.
 
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You probably have to detemine the i vector component of the astronaut's speed to hit the airlock.
 
How do I go about doing that? The pictures confuses me.
 
Sorry I got it wrong, you have to determine the part of the velocity verctor that is parraler to the verctor pointing from astronaut to the airlock. Which is |\overline{v}| cos \varphi. Using |\overline{v}||\overline{u}|cos \varphi = |\overline{v}\cdot\overline{u}| you should get it.
 
OK thanks that makes sense. So would I Subtract Ra from Rg to get a vector and use the dot product with the velocity vector?
 
Right.
 
What am I doing wrong here. Here is the steps I was doing.
1. U = Rg - Ra = 72.2i + 100j + 154k

2. U . V (dot product) = -880

3. magnitude of V = 4.798. magnitude of U= 197.3

4. -880/ (197.4 x 4.798) = -0.9296 = 158.374 degrees (using cos)

5. magnitude of of V x Cos(158.374) = -4.46 <------ not correct

Please help!
 

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