- #1

jameslat

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## Homework Statement

If a tennis ball with a mass of 57 g is shot out of a spring cannon at a 45 degree angle and goes 10m find the answer to the following questions:

What is the velocity.

What spring constant must the cannon have.

What would be the constant converted into lbs/in.

## Homework Equations

FS=-kx

vf^2=vi^2 + 2ax

x=(vi^2(sin^2(theta)))/-a

F=ma

## The Attempt at a Solution

ok i started off with finding the vi

10=vi^2(.5)/9.8

vi^2=100

vi=10

and with that then i can now try and find the acceleration of the spring right?

from 0 to 10m/sec

vf^2=vi^2+2ax

10^2=0+2ax

50=ax

50/x=a

first question is this x referring to x as how far i must pull back the string or the distance that i will be shooting at? (10 m)

f=ma

F=-kx

so

ma=-kx

.057(50/x)=-kx

2.85/x=-kx

2.85=-kx^2

good so far?

then at this point I am supposed to guess a K and use that.

so a 20 lbs/in has 2.2597 N/m

( http://www.boltscience.com/pages/convert.htm )

2.85/2.2587=x^2

so x = 1.12 m

so i would have to pull in back a meter?

if i set k to 31

2.85/31=x^2

x=.30 which is abt a foot right?

but 31 N/m is like almost 300 lbs/in spring!

what am i doing wrong?

Thanks for any input.

-James

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