# Problem with vectors

1. Jan 7, 2013

### MMCS

Question

Forces of magnitude 3 Newtons, 5 Newtons and 7 Newtons act along the vectors (-3,-2,-9) (8,5,-9) (4,-1,-9)

Find the x component.

I have the answer at 4.968 however i can not arrive at it, attached is my working out. d has been used instead of c.

i have the x components as 2.531, -2.774 and -6.93 however these are not correct as they do not equal the answer of 4.968

**Im pretty sure the figures are correct, there is most probably error in the method

Thanks

#### Attached Files:

• ###### Working.jpg
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Last edited: Jan 7, 2013
2. Jan 7, 2013

### Joffan

Normalize the given vectors to unit length.

3. Jan 7, 2013

### MMCS

Is that not what i have done.For example, on F1 i have divided it by the magnitude of AB (13.038) then multiplied it by the magnitude of newtons given in the question (3N) ?

4. Jan 7, 2013

### Joffan

I don't know why you chose to multiply the vectors together first - there was no need.

5. Jan 7, 2013

### MMCS

I didnt multiply them, it isnt clear from the working out sorry, where it says ab it represents the vector a to b.

6. Jan 7, 2013

### Joffan

Ah. The three triplets are the vectors - the forces act from the origin towards those coordinates.

7. Jan 7, 2013

### NemoReally

Does the following help?

Notes:
• str2mat is user-defined function that converts a string representation of a vector into a Mathcad array
• the over-arrow is the vectorize operator (you probably know this, but ...)

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8. Jan 7, 2013

### MMCS

Im only new to mathcad so i wasnt aware of them functions but ill give them a go they look to solve the problem pretty easily thanks!

9. Jan 7, 2013

### Joffan

I recommend solving this without Mathcad, to get a feel for the process. Your initial problem with solution was not associated with Mathcad use, but with understanding what was specified in the problem.

10. Jan 7, 2013

### Staff: Mentor

Please do not do student's homework for them here on the PF. That is against our rules here.

Instead, give hints, ask questions, find errors in their work, etc. We need to have the students doing the bulk of their schoolwork themselves.