Finding rotational inertia of a yo-yo

[chaose]

A yo-yo shaped device mounted on a horizontal frictionless axis is used to lift a 30kg as shown. The outer radius R of the device is 0.50m, and the radius r of the hub is 0.20 m. Wen a constant horizontal force F of magnitude 140N is applied to a rope wrapped around the outside of the device, the box, which is suspended from arope wrapped around the hub, has an upward acceleration of magnitude 0.80 m/s^2. What is the rotational inertia of the device about its axis of rotation?

http://img438.imageshack.us/img438/7076/problemci9.jpg

 

[Doc Al]

Well... where's your work?

Hint: Apply Newton's 2nd law to both the yo-yo and the box. Take advantage of the information given and you'll be able to solve for the rotational inertia.

 

[kyle8921]

Based on the given information, we can determine the rotational inertia of the yo-yo using the equation I = MR^2, where I is the rotational inertia, M is the mass of the object, and R is the distance from the axis of rotation to the outer edge of the object.

In this case, the total mass of the yo-yo is 30kg, and the distance from the axis of rotation to the outer edge is 0.50m. Plugging these values into the equation, we get a rotational inertia of 7.5 kg*m^2.

However, we also need to consider the rotational inertia of the hub, which is given by I = mr^2, where m is the mass of the hub and r is the radius of the hub. In this case, the mass of the hub is not given, but we can assume it is negligible compared to the mass of the yo-yo. Therefore, we can simply add the rotational inertia of the hub (0.04 kg*m^2) to the rotational inertia of the yo-yo to get a final value of 7.54 kg*m^2 for the rotational inertia of the entire device.

It is important to note that the applied force and the acceleration of the box are not directly related to the rotational inertia of the yo-yo. This information is used to find the tension in the rope and the force exerted by the yo-yo on the box, but it does not directly affect the rotational inertia.