1. Do now: Why does a tight-rope walker use a long pole? Watch video (30s) Mane, 7 Hakihea 2015

2. Just as force causes linear acceleration, torque causes angular acceleration. I is the “rotational inertia” which measures how hard it is to get an object spinning Compare with F = ma Units: kg m 2 Rotational inertia depends on: – mass – how the mass is distributed about the axis of rotation 20cm 5kg I = ? Board notes

3. Examples of Inertia’s Solid Cylinder Hollow Cylinder Solid Sphere Board notes

4. Example: Tom wants to push his friends on a roundabout. Will he find it easier if they all stand in the centre or on the outer edge? Why? Total mass is the same in both cases But rotational inertia is smaller when all the mass is nearer the centre of rotation, so easier to push Watch ice skating world record (15s) + demo

5. “Classic” Example: A mass of 0.18kg is used to accelerate a wheel of radius 0.20m. The mass accelerates downwards at 1.2ms -2 a) Calculate the tension force in the string b)Calculate the torque on the wheel c)Calculate the angular acceleration of the wheel d)Calculate the rotational inertia of the wheel

6. Do Activity 9B, Q1-4