11.2: Angular Motion

Question 1

Angular motion definition:

Answer 1

• the movement around a fixed point
• occurs when a force is applied outside the centre of mass. An off-centre force is referred to as an eccentric force
• in the centre of mass is a concentric force

Question 2

What are the different axes?

Answer 2

• transverse: side-to-side across the body, e.g. somersault
• sagittal: runs from front to back, e.g. cartwheel
• longitudinal: runs from top to bottom, e.g. ice-skating spin

Question 3

Torque definition:

Answer 3

• ‘rotational consequence of a force’
• a torque is a turning force. It causes an object to turn about its axis of rotation
• increasing the size of forces increases the torque

Question 4

Torque equation:

Answer 4

• the moment of force or torque (measured in Newton metres (n/m)) can be calculated as follows:
  moment of force or torque = force (n/m) x perpendicular distance from the fulcrum (metres)

Question 5

Angular analogues of Newton’s laws of motion?

Answer 5

• First law
• Second law
• Third law

Question 6

What is the first law?

Answer 6

• a rotating body will continue to turn about its axis of rotation with constant angular momentum unless an external rotational force (torque) is exerted upon it.
• e.g. an ice-skater turning/spinning in the air until they hit the ice again during landing

Question 7

What is the second law?

Answer 7

• the rate of change of angular momentum of a body is proportional to the force (torque) causing it and the change that takes place in the direction in which the force (torque) acts.
• in practice, the greater the torque exerted, the faster the rotation will be.

Question 8

What is the third law?

Answer 8

• when a force (torque) of angular motion is applied by one body to another, the second body will exert an equal and opposite force on the body.
• e.g. in practice, as a goalkeeper tips the ball over the bar they’ll throw their arms up which causes the lower part of their legs to go back.

Question 9

Radian definition:

Answer 9

• the unit of measurement for angles

Question 10

Angular displacement definition:

Answer 10

• the smallest change in angle between the start and finish point of rotation.
• Measured in degrees and radians. (1 radian = 57.3 degrees)
• angular velocity: the rate of change of angular displacement
• angular acceleration: the rate of change of angular velocity.

Question 11

What is angular velocity?

Answer 11

• refers to the rotational speed of an object and the axis about which the object is rotating.
• the term is a vector quality as it makes reference to direction. This is because it refers to the angular displacement that is a certain time.

Question 12

Angular velocity equation:

Answer 12

• calculated by dividing the angular displacement in radians (rad) by the time taken (s)
  Angular velocity (rad/s) = angular displacement / time taken (s)

Question 13

Angular acceleration equation:

Answer 13

• it is calculated by dividing the change in angular velocity in radians per second by the time taken (s)

Question 14

Moment of inertia:

Answer 14

• ‘the resistance of a body to angular motion’
• moment of inertia depends on the body’s mass and the distribution of mass around the axis
• the greater the mass, the greater resistance to change and therefore the greater the inertia.
• distribution of mass from axis of rotation: the flower the mass is to the axis of rotation, the easier it is to turn as the moment of inertia is low. Increasing the distance of the distribution of mass from the axis of rotation will increase the moment of inertia.

Question 15

What is angular acceleration?

Answer 15

• the rate of change of angular velocity