# Need help finishing this problem off

1. Oct 3, 2005

### bmore007

1.05 = (ViSin30)10 * ViCos30+ 1/2(-9.8)(10/ViCos30)^2

help me solve for Vi please

2. Oct 3, 2005

### robphy

Can you do some algebra to separate out the factors of Vi in each term on the right-hand side?
You might want to take care in using parenthesis around (ViCos30), especially in the second term on the right-hand side.

3. Oct 3, 2005

### bmore007

all the #'s i'm getting are turning out wrong

4. Oct 3, 2005

### robphy

Let's go slowly... do algebra first (arithmetic last). It's easier to spot an algebra mistake.

Can you factor "(ViSin30)10 * ViCos30" ?
It may help to forget the numbers and think of this symbolically as $$(V_i A)B * V_iC$$. So, can you factor this expression and write it in terms of $$V_i$$, $$A$$, $$B$$, and $$C$$?

5. Oct 3, 2005

### bmore007

"Can you factor "(ViSin30)10 * ViCos30" ?"

4.33Vi

6. Oct 3, 2005

### robphy

almost... correct coefficient...
check the units of that term

7. Oct 3, 2005

### bmore007

i dont understand why that isnt correct

ViSin(30)*10 * ViCos(30)
Vi.5 * 10 * Vi .87
Vi 4.4

8. Oct 3, 2005

### robphy

that term should have a Vi^2

9. Oct 3, 2005

### bmore007

anychance you can give me a walk through numerically to the answer? i'm afraid that'll help me more then me struggling here

10. Oct 3, 2005

### robphy

In summary (up to this point):
ViSin(30)*10 * ViCos(30) = Vi^2 4.4

You've done fine with the coefficient, although you missed the fact that Vi is squared.
But you've gotten passed that. In other words, algebra (rather than arithmetic) is your problem.

Now, work on factoring "1/2(-9.8)(10/ViCos30)^2", which you should really write as (1/2)(-9.8)(10/ (ViCos30) )^2... especially if you try to plug something like this into your calculator.

11. Oct 3, 2005

### bmore007

unfortunately i'm not too good at more involved algebra like this

(1/2)(-9.8)(10/ (ViCos30) )^2

-4.9(100/Vi^2* .75)

so

4.4Vi^2 -4.9(100/.75Vi^2)

Last edited: Oct 3, 2005
12. Oct 3, 2005

### Integral

Staff Emeritus
I do not see anyway to give "a walk through numerically" with out doing the algebra first.

Is this the starting point:

$$1.05 = V_i Sin(30) * 10 * V_i Cos(30) + \frac {-9.8} {2} \frac {10} {{(V_i Cos(30))}^2}$$

13. Oct 3, 2005

### bmore007

yes that is the starting point

except it's (10/ViCos30)^2

the whole thing squared

14. Oct 3, 2005

### bmore007

n/m

Last edited: Oct 3, 2005