Unit 2 PE Angular Motion, Torque and Angular Momentum

How do you define angular motion? What kinds of angular motion are there?

Angular motion: Rotation around an axis

• Rotation or spinning around a fixed point or axis by the body or an object

• Occurs when an eccentric force creates an unbalanced force that does not pass directly through the axis

• Can be:

  • Internal – inside the body (e.g. knee joint)

  • External – outside the body (e.g. equipment, door hinge)

How is general motion a combination of two other motion types? How common is this motion?

General Motion: The body or object moving with a mixture of straight line movement and rotations

• A combination of both angular and linear motion.

• Most sporting movements are examples of general motion.

Example: Running – legs rotate about the hips while the body moves forward

Define linear motion. What is required by the body/object in order to be moving in linear motion?

Linear motion: The body or object moving in a straight line (or curved path)

• All parts of the body or object move in the same direction at the same time

Example: Ice skater gliding

What is an eccentric force? How do eccentric forces affect movement?

Eccentric force: A force that doesn’t act in a line that passes through the centre of mass (or gravity) of an object

• Causes the object to rotate as well as move in a straight line

• Produces a rotational effect called torque

• Applying the force further from the centre of mass increases rotation because torque increases

EG Kicking a soccer ball off-centre to curve it

How can torque be defined? eand what determines its size?

Torque: The tendency of a force to cause rotation about an axis

• The rotational effect produced when an eccentric force acts on an object

Formula: Torque = Force × Moment Arm

Torque increases when:

• More force is applied

• The force is applied further from the axis (larger moment arm)

What is the moment arm? What is it used to measure?

Moment arm: Distance from the centre of gravity to the applied force

• Torque = Force x Moment arm

Increasing the moment arm:

• Increases torque

• Increases the moment of inertia

• Trades off speed for power by decreasing angular velocity while conserving angular momentum

What is angular momentum?

Angular momentum: The amount of rotation of a body around an axis

Angular momentum = Moment of inertia x Angular velocity

• Greater moment of inertia can increase angular momentum if angular velocity is maintained

• Greater angular velocity increases angular momentum

Examples: Somersaults, dives, spins in gymnastics and figure skating

What is angular velocity?

Angular velocity: How fast an object rotates about an axis (degrees/seconds)

• Describes how fast an object is spinning

• Greater angular velocity means faster rotation

What is moment of inertia? What does it describe?

Definition: The tendency of a body to remain its state of angular motion

• Describes how difficult it is to change angular motion of an object

• Depends on: Mass and distance of mass from the axis of rotation (radius)

• Moment of inertia = Mass x Radius²

What are features of objects/bodies with low and high moment of inertia?

Lower moment of inertia:

• Smaller mass

• Shorter radius

• Mass closer to axis

• Easier to rotate

Higher moment of inertia:

• Larger mass

• Longer radius

• Mass further from axis

• Harder to rotate

What is the benefit of increasing moment of inertia?

Angular Momentum = Moment of Inertia × Angular Velocity

• Greater angular momentum gives the body a greater tendency to maintain it's state of angular motion

• Therefore can improve stability and control during rotation

• However ↑ moment of inertia generally results in decreased angular velocity due to the conservation of angular momentum