4.2 - Angular Motion

Question 1

what is angular motion?

Answer 1

movement around a fixed point
- occurs when a force is applied outside the centre of mass (eccentric force)

Question 2

what is an eccentric force?

Answer 2

an off-centre force (applied outside the centre of mass)

Question 3

what are the 3 axis of rotation?
include an example

Answer 3

transverse - somersault
sagittal - cartwheel
longitudinal - multiple spin in ice skating

Question 4

what is torque?

Answer 4

the rotational consequence of a force (turning force)
- angular motion occurs as a result of a torque
- causes an object to turn about its axis of rotation

Question 5

how can you increase torque?

Answer 5

• increase the size of the eccentric force applied
• apply the same force further away from the axis of rotation

Question 6

how do you calculate the moment of a force/torque?

Answer 6

MOF (Nm) = Force (N) x perpendicular distance from the fulcrum (m)

Question 7

how can you increase the moment of a force?

Answer 7

the perpendicular distance of the force from the pivotal point (moment arm)

Question 8

what is Newton’s first law of angular motion?

Answer 8

a rotating body will continue to turn about its axis of rotation with constant angular momentum unless a torque is exerted upon it

Question 9

give an example of Newton’s first law of angular motion.

Answer 9

ice skater spinning in the air
- they will continue to spin until they land on the ice
- torque is exerted from the ice onto their skates
- changes their state of angular momentum

Question 10

what is Newton’s second law of angular motion?

Answer 10

the rate of change of angular momentum of a body is proportional to the torque causing it and the change that takes place is the direction in which the torque acts

Question 11

give an example of Newton’s second law of angular motion.

Answer 11

cricket bowl
- greater the torque exerted, the faster the rotation will be

Question 12

what is Newton’s third law of angular motion?

Answer 12

when a torque is applied by one body to another, the second body will exert an equal and opposite torque on the other body

Question 12

what is Newton’s third law of angular motion?

Answer 12

when a torque is applied by one body to another, the second body will exert an equal and opposite torque on the other body

Question 13

give an example of Newton’s third law of angular motion.

Answer 13

when a goalkeeper tips the ball over the bar
- they throw their arms up (the eccentric action force on the body)
causes the lower part of their legs to go back (reaction force)

Question 14

what is angular displacement?

Answer 14

the smallest change in angle between the start and finish point of a rotation
- measured in radians

Question 15

what are radians?

Answer 15

unit of measurement for angles
- 1 radian = 57.3 degrees

Question 16

what is angular velocity?

Answer 16

the rate of change of angular displacement
- vector

Question 17

how do you calculate angular velocity?

Answer 17

angular velocity (rad/s) = angular displacement (rad) / time taken (s)

Question 18

what is angular acceleration?

Answer 18

the rate of change of angular velocity

Question 19

how do you calculate angular acceleration?

Answer 19

angular acceleration (rad/s^2) = change in angular velocity (rad/s) / time taken (s)

Question 20

what is the moment of inertia?

Answer 20

the resistance of a body to angular motion (rotation)

Question 21

what does the moment of inertia depend on?

Answer 21

the mass of the body
the distribution of mass around the axis

Question 22

how does the mass of the body/ object affect the moment of inertia?

Answer 22

greater the mass, greater the resistance to change and therefore greater the moment of inertia
- e.g ten-pin bowling ball (more difficult to roll along the ground than a football but once it starts rolling, more difficult to stop than the football)

Question 23

how does the distribution of mass from the axis of rotation affect the moment of inertia?

Answer 23

closer the mass to the axis of rotation, the easier it is to turn as the moment of inertia is low

Question 24

what happens if you increase the distance of the distribution of mass from the axis of rotation?

Answer 24

increases the moment of inertia
and is more difficult to turn

Question 25

give an example of how the distribution of mass from the axis of rotation affects the moment of inertia?

Answer 25

tucked somersault - distribution of mass is closer to the axis of rotation, moment of inertia decreases so the angular velocity increases and the spin is easier

layout (open) somersault - distribution of mass is further away from the axis of rotation, moment of inertia increases, angular velocity decreases and the spin is harder to perform

Question 26

what is angular momentum?

Answer 26

quantity of rotation a body possesses (spin.)

Question 27

how do you calculate angular momentum?

Answer 27

angular momentum = moment of inertia x angular velocity
- they are inversely proportional (if moment of inertia increases, angular velocity decreases)

Question 28

where can angular momentum be conserved in sport?

Answer 28

during flight,
on ice and snow
(where there is hardly ant friction)

Question 29

give an example of angular momentum in sport.

Answer 29

when a diver performs a double front somersault:
- amount of angular momentum stays the same during the flight
only changes when the diver hits the water on entry
- or if they change their body position

Question 30

use the example of a figure skater spinning to explain the conservation of angular momentum.

Answer 30

ice = friction free surface therefore no resistance to movement
- the figure skater can manipulate their moment of inertia to increase/decrease the speed of the spin
at the start of spin, arms and legs are stretched out
- increases distance from AoR, large moment of inertia and large angular momentum. enables spin to be started but the speed of rotation is slow
when figure skater bring arms and legs back in line with the rest of the body (closer to the AoR)
- reduces the moment of inertia, increases angular velocity and the skater spins very quickly