# Need help finishing this problem off

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

help me solve for Vi please

Related Introductory Physics Homework Help News on Phys.org
robphy
Homework Helper
Gold Member
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.

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

robphy
Homework Helper
Gold Member
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 $$C$$?

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

4.33Vi

robphy
Homework Helper
Gold Member
almost... correct coefficient...
check the units of that term

i dont understand why that isnt correct

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

robphy
Homework Helper
Gold Member
that term should have a Vi^2

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

robphy
Homework Helper
Gold Member
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.

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

Is this the starting point:

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

yes that is the starting point

except it's (10/ViCos30)^2

the whole thing squared

n/m

Last edited: