# Week 15 (Nov. 18 - 22)

**Reading:**

**Topics:**Rotational inertia, torque, and angular momentum

**No quiz this week**

**Week 15 HW problem:**

**Ladder problem (****solution****):**A ladder weighing 400 N and a length 10 meters is placed against a smooth (frictionless) vertical wall. A person weighing 800 N stands on the ladder 2 meters from the bottom as measured along the ladder. The foot of the ladder is 8 meters from the bottom of the wall. The floor exerts a friction force, f, that keeps the ladder from sliding out. Calculate the normal force exerted by the wall, the friction force exerted by the floor, and the normal force exerted by the floor on the ladder.**Two masses suspended from pulley:**Suppose that, like in the previous problem from last week, a 10 kg mass is suspended from a string which is wrapped a bunch of times round the disk/pulley. But now suppose the other end of the string is attached to another (3 kg) mass suspended from the pulley. So as the 10 kg mass falls, the 3 kg mass rises. Repeat the previous problem. Here is a video solution.

**Rotational inertia laboratory:**This week, we will be working with a rotational inertia apparatus. The goal is to predict how much time it will take for a falling mass to strike the ground. The falling mass, however, is suspended from a string which is wrapped around a rotatable shaft.

- First, carefully measure the masses on the crossbeam and their locations. From this, determine the rotational inertia of the apparatus. Be sure to include an estimate of the rotational inertia of the vertical shaft.
- Now measure the mass that is to be dropped. Also, measure the distance between the mass and the floor.
- Set up a free body diagram and attempt to predict the acceleration of the falling mass. Don't forget that the tension in the string is supporting it!
- From the acceleration and the distance, predict the time of fall. Put a box around your prediction. Now get your instructor to come watch your falling object. Determine the percent difference between your prediction and your experimental results.
- How might your experiment be done better?