Net torque question? Block 1 and block 2 hanging on pulley?

[nchin]

In Fig. 10.45, block 1 has mass m1=460g, block 2 has mass m2=500g, and the
pulley, which is mounted on a horizontal axle with negligible friction ahs radius
R=5.0cm. When released from rest, block 2 falls 75.0cm in 5.00 s without the
cord slipping on the pulley.

(e) What is its rotational inertia?

problem and picture on page 2 problem #55

http://faculty.kfupm.edu.sa/PHYS/marzoug/Phys101/chapter 10 problmes.pdf




Ʃτ = (T2 - T1) R

Why doesn't mg1 or mg2 get calculated into the net torque and only the tensions?



thanks!

 

[Simon Bridge]

Why doesn't mg1 or mg2 get calculated into the net torque and only the tensions?

That happens later - there are three free-body diagrams.
It's good discipline for more complicated problems: the pulley knows nothing about the masses - all it feels is these strings pulling on it.
Note - the 1 and the 2 belong on the "m"'s not the "g"'s ... g is a constant.

 

[nchin]

That happens later - there are three free-body diagrams.
It's good discipline for more complicated problems: the pulley knows nothing about the masses - all it feels is these strings pulling on it.
Note - the 1 and the 2 belong on the "m"'s not the "g"'s ... g is a constant.

thanks!