2.
Learning Outcomes
• Link 5 angular motion terms to linear
equivalents
• Describe centre of gravity/mass
• Explain Newton’s 3 laws of motion applied
to angular motion
• Explain how a figure skater can speed up
or slow down a spin using the law of the
conservation of angular momentum
3.
Angular Motion
• When a body or part of the body moves in
a circle or part circle about a particular
point called the axis of rotation
• E.g. the giant circle on the high bar in the
men’s Olympic Gymnastics
5.
Centre of Gravity / Centre of Mass
“The point at which the body is
balanced in all directions”
6.
Centre of Gravity & stability
• The lower the centre of gravity is –
the more stable the position
7.
Base of support
• The larger the base of support – the
more stable the position
8.
Line of Gravity
• An imaginary line straight down from the
centre of gravity / mass
•If the line of gravity is at the centre
of the base of support – the position is
more stable (e.g. Sumo stance)
•If the line of gravity is near the edge
of the base of support – the position is
less stable (e.g. Sprint start)
•If the line of gravity is outside the
base of support – the position is
unstable
9.
Which is the most
Which is the most
stable?
stable?
10.
To work out the centre of gravity of a 2D
shape• Hang the shape from one point & drop a
weighted string from any point on the
object
• Mark the line where the string drops
• Repeat this by hanging the object from
another point
• Mark the line again where the string drops
• The centre of gravity is where the two
lines cross
12.
Movement of force or torque
• The effectiveness of a force to produce
rotation about an axis
• It is calculate – Force x perpendicular
distance from the fulcrum
• Newton metres
• (Fulcrum – think of levers)
• To increase Torque – generate a larger
force or increase distance from fulcrum
13.
Angular Distance
• The angle through which a body has
rotated about an axis in moving from the
first position to the second (Scalar)
• Measured in degrees or radians
14.
Angular Displacement
• The shortest change in angular position. It
is the smallest angle through which a body
has rotated about an axis in moving from
the first to second position
• Vector
• Measure in degrees or radians
• 1 radian = 57.3 degress
15.
• Consider movement
form 1 to 2 clockwise
• Angular Distance –
o
270
• Angular
Displacement – 90o
16.
Terminology
Angular speed
• The angular distance
travelled in a certain time.
• Scalar
• Radians per second
Angular Velocity
• The angular displacement
travelled in a certain time.
• Vector quantity
• Radians per second
17.
Angular Acceleration
• The rate of change of angular velocity
• Vector quantity
• Radians per second per second (Rad/s 2)
18.
Newton’s First Law - Angular
• “ A rotating body continues to turn about
its axis of rotation with constant angular
momentum unless acted upon by an
external torque.”
• (Law of inertia)
19.
Newton’s Second Law - Angular
• “When a torque acts on a body, the rate of
change of angular momentum experience
by the body is proportional to the size of
the torque and takes place in the direction
in which the torque acts.”
• E.g.Trampolinist – the larger
the torque produced – faster
the rotation for the front
somersault – greater
the change in angular
momentum
20.
Newton’s Third Law - Angular
• “For every torque that is exerted by one
body on another there is an equal and
opposite torque exerted by the second
body on the first.”
• E.g. Diver – wants to do a left-hand twist at take
off – he will apply a downward and right-hand
torque to the diving board – which will produce
an upward and left-hand torque – allowing the
desired movement
21.
Angular Momentum
• The quantity of angular motion possessed
by a rotating body
• Kgm2/s
• Law of conservation of angular momentum
– for a rotating athlete in flight or a skater
spinning on ice – there is no change in AM
until he or she lands or collides with
another object or exerts a torque on to the
ice with the edge of the blade.
22.
Moment of inertia
• The resistance of a rotating
body to change its state of
angular motion
Angular momentum
= moment of inertia x angular
velocity
Moment does not mean a bit of
time (in this case)
– it is a value
23.
ANGULAR MOMENTUM – MOMENT OF
INERTIA (rotational inertia)
• If the body’s mass is close to the axis of rotation,
rotation is easier to manipulate. This makes the
moment of inertia smaller and results in an increase in
angular velocity.
• Moving the mass away from the axis of rotation slows
down angular velocity.
Try this on a swivel
chair – see which
method will allow you
to spin at a faster
rate? Note what
happens when you
move from a tucked
position (left) to a
more open position
(right).
25.
Questions
Task 1
Task 2
• Explain how a sprinter’s
• Explain how a figure
stability changes through the
skater can change their
three phases of a sprint start:
speed of rotation on a
“on your marks”, “set”, bang”
jump – to change the
(6)
move from a single
rotation to a double or
• A Diver performs a 2 tucked
triple rotation (4)
front somersaults in their dive
– draw a diagram/graph and
explain the Law of
Conservation of Angular
Momentum (4)
26.
Learning Outcomes
• Link 5 angular motion terms to linear
equivalents
• Describe centre of gravity/mass
• Explain Newton’s 3 laws of motion applied
to angular motion
• Explain how a figure skater can speed up
or slow down a spin using the law of the
conservation of angular momentum
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