- #1
Okazaki
- 56
- 0
Homework Statement
A uniform cylindrical spool of mass M and radius R unwinds an essentially
massless rope under the weight of a mass m. If R = 12 cm, M = 400 gm and m = 50 gm, find the speed of m after it has descended 50 cm starting from rest.
Solve the problem twice: once using Newton's laws for torques, and once by application of energy conservation principles.
Homework Equations
I(spool) = (1/2)MR^2
τ = R * Fsin(90) (*it will be 90 degrees here due to the way the Gravitational Force pulls down)
τ = Iά
at = ά * R (tangential acceleration)
vf = sqrt(vi2 + 2at*Δx)
The Attempt at a Solution
So I used τ = R * Fsin(90) to come up with the torque (since I converted everything into meters and kilograms, my answer ended up being -5.88 x 10^-2 N-m)
Then, I basically plugged in values for I:
I = 0.5 * 0.4 kg * (0.12 m)^2 = 0.00288 kg m^2
After that, I thought about solving for ά. But ά is the angular acceleration, which is kind of useless in this case, since we're trying to find the velocity of mass m after is has fallen 50 cm. So I set
ά = τ/I = at / R
at = 0.12 m * -5.88 x 10^-2 N-m/0.00288 kg m^2 = -20.4 m/s^2,
and from here I used the equation: vf = sqrt(vi2 + 2at*Δx) and got -4.52 m/s
So, I honestly don't even know if I solved this problem right (more or less, where to even start if I was going to use conservation of energy principles to solve it again.) Any help would be greatly appreciated.