# Rotational Dynamics

### Problems

Problem : Calculate the net torque exerted by F 1 = 30 N and F 2 = 50 N in the figure below. You may assume that both forces act on a single rigid body.

Two forces acting on a single rigid body

Problem :

Two cylinders of the same mass and shape, one hollow and one solid, are set on a incline and allowed to roll down. Which cylinder will reach the bottom of the incline first? Why?

Problem :

A simple pendulum of mass m on a string of radius r is displaced from vertical by an angle θ , as shown below. What is the torque provided by gravity at that point?

A gravitational force acting on a simple pendulum

Problem :

See the last problem. What is the angular acceleration of the pendulum at that point?

Problem :

A revolving door is common in office buildings. What is the magnitude of the torque exerted on a revolving door of mass 100 kg if two people push on opposite sides of the door with a force of 50 N at a distance of 1 m from the axis of the door, as shown below? Also, the moment of inertia of a revolving door is given by I = . Find the resultant angular acceleration assuming no resistance.

Two people pushing on a revolving door

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