- #1
JulienB
- 408
- 12
Homework Statement
Hi everybody! Still checking my comprehension of physics, and hopefully those posts will help other users!
A homogeneous wooden sphere with mass M and radius R rotates about a perpendicular axis going through its center of mass. At the top of this axis is a homogeneous metal bar of length 2R and of same mass M. A massless string is rolled around the ball (see attached picture) and on its other end goes through a wheel and has a suspended mass attached to it. There is no friction.
a) calculate the moment of inertia of the system wooden ball-metal bar regarding its rotation axis. The bar should be considered as infinitely thin. The moment of inertia of a sphere is JK = (2/5)MR2.
b) what is the acceleration of the mass m in the field of gravity of Earth?
c) what is the velocity of the mass m when it has gone down a distance h?
Homework Equations
Moment of inertia, torque, Newton 2nd and 3rd laws, conservation of energy
The Attempt at a Solution
Though it is not the hardest problem, I find the calculations messy and the risk of missing something high. Here is what I've done:
a) First I calculate the moment of inertia of the bar:
JS = (1/12)M(2R)2 = ⅓⋅MR2
⇒ Jtotal = (2/5 + ⅓)MR2 = (11/15)MR2
Im pretty sure I can add the moments of inertia that way since they have the same axis of rotation, right?
b) By Newton's 3rd law I know that the force of tension of the string on the mass is equal to the force of tension of the string on the sphere. I try to find an expression for T using the torque and the angular acceleration:
Στ = T⋅R
⇒ α = Στ/J = 15T/11M
The angular acceleration is also equal to tangential acceleration/radius aT/R, and in that case aT = a (the acceleration of the suspended mass):
⇒15T/11M = a/R ⇒ T = 11aR/15M
I plug that into the Newton's 2nd law:
a = ΣF/m = g - 11a⋅m⋅R/15M
⇒ a = g/(1 + 11m⋅R/15M)
c) For that part I set up an equation of conservation of energy. I assume that the system starts at rest:
m⋅g⋅h = ½⋅m⋅v2 + ½⋅J⋅ω2
Is that correct? I have a little doubt about the rotational energy.
Then I substitute ω by v/R. Here again I have a little doubt: I allow myself this substitution because I think the velocity of the string at the sphere should be the same as the velocity at the mass. Anyway that gives me for the velocity:
v = √(2⋅m⋅g⋅h/(m + 11M/15))
What do you guys think? It's always a bit tricky not to misjudge such a situation..Thank you in advance for your answers!Julien.