Do now:Copy and complete these statements:

Liner momentum is conserved if no external forces are acting.

Angular momentum is conserved if no external torques are acting

Units of L are: kg m2 rad s-1 = kg m2 s-1

Linear kinetic energy is:

Rotational kinetic energy is:

Unit of EK(Rot) are: Joules (J)

Wednesday, April 19, 2023

IL

p mv

212KE mv

21( ) 2K RotE I

Example:A turntable has a rotational inertia of 0.090 kgm2 spins freely with an angular velocity of 4.0 rads-1. A disc with rotational inertia 0.030 kgm2 is dropped on the turntable. Calculate the new angular velocity.

What does conservation of angular momentum actually mean?

• If no external torques are acting, then object will stay at rest or...

• Continue rotating at a constant angular velocity• Lbefore = Lafter

Demo

Ice skating

Bike wheel demos: – rotating plate

– from ceiling

Video clips: – riderless motorcycle

An object with a large angular momentum

will require a large torque to change the angular momentum

Blade rotation

Helicopter fuselage will start to spin against the rotors to conserve angular momentum

Tail rotor thrust to counter the torque on the helicopter

What is the angular momentum of a mass in orbit?

Example:A spacecraft is stationary in space. In order to rotate the spacecraft, two small rockets are fired for a fraction of a second.

The spacecraft is a cylinder with radius 2.0m and a rotational inertia of 1600 kgm2. The small rockets each fire 0.40kg of gas at a speed of 100ms-1.

Calculate the angular speed of the spacecraft.

Gas emitted:m = 0.40kgv = 100ms-1

Gas emitted:m = 0.40kgv = 100ms-1

Do Activity 9C and 9DHomework Set 6