# Torsion problem for strenght of materials help

#### duffman1278

1. Homework Statement 2. Homework Equations
tao=Tc/J
theta=TL/JG

3. The Attempt at a Solution

I got the torque of the motor as 210,085lb-in
Then I converted the 4* to .698 rad

From there I plugged it into the equations. I solved for T from tao=Tc/J then plugged that T into the theta=TLJG and put everything else in there.

When I did that I got a diameter of 3.44" or 1.72" radius. I don't get what I'm doing wrong?

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#### nvn

Science Advisor
Homework Helper
duffman1278: Conversion mistake; 4 deg is not 0.698 rad. Also, generally always maintain four (or
five) significant digits throughout all your intermediate calculations, then round only the final answer
to three significant digits, unless the final answer begins with 1, in which case round the final answer
to four significant digits.

Initially compute T from the given data, at which point T is known, not an unknown. Do not solve for
T in the stress nor deflection equation; solve for d. The unknown is d. Try it again.

#### duffman1278

The 4 degree's I put on here was just a type-o this is what I got when I redid it.

$$\phi$$= .069813
T(shaft)= 52,521.1312 lb-in
G= 12x106
$$\tau$$= 12,000psi
L= 120"

I solved for T as you said in which I got

T=$$\stackrel{\tau*J}{c}$$

I then plugged that T into the $$\phi$$=$$\stackrel{TL}{JG}$$

That then gave me $$\phi$$=$$\stackrel{L\tau}{Gc}$$

I solved for "c" which would be the radius and it gave me 1.72 or 3.43in still.

#### nvn

Science Advisor
Homework Helper
The answer for T is already given in the third line of post 3. Don't solve
for T again after that; just use it in your other equations thereafter to
solve for d (or c).

#### duffman1278

I love you!! omfg this stupid problem was so easy the entire time.

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