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Vectors in 3D prism question

Hello. What am I doing wrong here? I can seem to get the rest but i can’t understand to see where the 3 came from? I am asking about ci)
Thank you.

(edited 2 years ago)

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Reply 1

mqb2766

It would be better to explicitly write in vector form CB and BG then do the vector addition.

You've effectively assumed your plane is perpendicular to the x-axis, which it isn't. There will be a non-zero x component in CG because of CB.

(edited 2 years ago)

Reply 2

Eris13696

Original post by mqb2766

I’m sorry if I don’t understand this, do you mean CB here is the x? I thought with its direction it’s the j or the y :0, also would the CDBG and OEFA be same to each other?

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Reply 3

mqb2766

Original post by Eris13696

I’m sorry if I don’t understand this, do you mean CB here is the x? I thought with its direction it’s the j or the y :0, also would the CDBG and OEFA be same to each other?

Tthe x-y-z axes are on the diagram and you use vectors in parts a) and b), so you need to use them in c as well.
CG = CB + BG
as you say but you need to work CB out correctly as OB-OC.

You seem to have the wrong plane picture in your head - forget about it and use vectors explicitly.