circular motion phya 4 further mechanics. many objects follow circular motion the hammer swung by a...
TRANSCRIPT
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Circular Motion
PHYA 4
Further Mechanics
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How can we make an object travel in a circle?
• Hint: think about Newton’s 1st law...
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Circular motion• Remember Newton’s 1st law?
– an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force
• So what is needed to make something go around in a circle?– A resultant force
• Remember Newton’s 2nd law?– F=ma
• So a body travelling in a circle constantly experiences a resultant force (and is accelerated) towards the centre of the circle– This is not an equilibrium situation! An unbalanced
force exists!
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A bucket of water on a rope
• If we spin the bucket fast enough in a vertical circle, the water stays in the bucket– Why?
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A mass on a string
• Speed of rotation remains constant• Velocity is constantly changing, so mass is
constantly accelerating towards centre of circle• So there is a constant force on the mass
towards the centre of the circle– Tension in string (until you let go!)
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Talking about circular motion
The radian
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Rotation and speed
• No gears, so as the pedals are turned, the wheel goes round with them with a period T
• The wheel rim is travelling faster than the pedals, although both are rotating at the same frequency, f
• Speed of rim: rfT
rv
22
So the speed an
object moves depends on the frequency of
rotation and the radius
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s
r
Talking about circular motion
• Angular displacement = no. of radians turned through
• Angular speed () = no. of radians turned through per second
(sometimes called angular velocity)
rvr
v
t
rs
dt
dT
f
or ,/
22 speed
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Worked example: Calculating
A stone on a string: the stone moves round at a constant speed of 3 ms-1 on a string of length 0.75 m
•What is the instantaneous linear speed of the stone at any point on the circle?
•What is the angular speed of stone at any point on the circle?
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Practice Questions
• Examples 1: Radians and angular speed
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Centripetal acceleration
• Acceleration directed towards centre– Centripetal means “centre seeking”
• Size depends on:– How sharply the object is turning (r)– How quickly the object is moving (v)
dt
da
v vector
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Centripetal acceleration
r
vra
rv
vat
t
v
t
va
vvv
v
r
s
22
,
.
so
Remember
so
,but
so ,
object
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Centripetal Force
• Force acts towards the centre of the circle, not outwards!
• Not a special type of force
rmr
mvF
maF
22
,
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Examples of sources of centripetal force
Planetary orbits gravitation
Electron orbits electrostatic force on electron
Centrifugecontact force (reaction) at the walls
Gramophone needlethe walls of the groove in the record
Car cornering friction between road and tyres
Car cornering on banked track
component of normal reaction
Aircraft bankinghorizontal component of lift on the wings
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Worked Example: Centripetal Force
A stone of mass 0.5 kg is swung round in a horizontal circle (on a frictionless surface) of radius 0.75 m with a steady speed of 4 ms-1.
Calculate:
(a) the centripetal acceleration of the stone
(b) the centripetal force acting on the stone.
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No such thing as centrifugal force...
• Centrifugal means “centre fleeing”
• It is an “effective force” you feel when in a rotating frame of reference
• e.g., cornering car
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No such thing as centrifugal force...
• Car applies a force towards the centre of the circle
• Driver feels a force pushing him outwards– Reaction force
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• Physics joke...
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Practice Questions
• Centripetal force sheet
• Whirling bung experiment
• Examples sheet 2
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Hump-backed bridges
• Centripetal force provided by gravity
• Above a certain speed, v0, this force is not enough to keep vehicle in contact with road
gr vso 0
20 r
mvmg Note: independent of mass...
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Roundabouts and corners
• What provides the centriptal force?– Friction
• What factors affect the maximum speed a vehicle can corner?– Radius of corner– Limiting frictional force
r
mvF
20
0 mg
: coefficient of friction (not examinable)
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Banked tracks• On a flat road, only friction
provides the centripetal force– Above a certain speed you
lose grip
• On a banked track there is a horizontal component of the reaction force towards the centre of the curve– No need to steer! (at least at
one particular speed)
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Optimum speed on a banked track
• Can you derive an expression for the speed at which no steering is required for a circular track of radius r, banked at an angle
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Banked tracks – speed for no sideways friction
• Resolving reaction force horizontally and vertically:
• so
mgnr
mvn
cos
sin2
tantan 22
grvgr
v or ,
Speed at which a vehicle can travel around a banked curve without steering
Wall of death Ball of death
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Fairgrounds• Many rides derive their excitement
from centripetal force– A popular context for exam questions!
– Read pages 26-29– Answer questions on p.29
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Simple Harmonic Motion
PHYA 4
Further Mechanics
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Oscillations in nature• Oscillation is nature’s way of finding equilibrium• This interplay can be found throughout nature:
– A swinging pendulum– Waves on water– A plucked string (and the eardrum of a listener)– Vibrating atoms in a lattice– Voltages and currents in electric circuits– Excited electrons emitting light– A bouncing ball– Ocean tides– Populations of predators and prey in an ecosystem...
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Simple Harmonic Motion
• Harmonic motion: motion that repeats itself after a cycle
• Simple: simple!
• Let’s look at some examples...
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time
disp
lace
men
t
time
time
velo
city
acce
lera
tion
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• Displacement/velocity/acceleration animation
• x/v/a Java applet
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Simple Harmonic Motion Summary
• What is SHM?
• What sort of systems display SHM?
• How can we describe SHM?
• What is happening to the energy of an ideal system undergoing SHM?
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Displacement of mass on a spring
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Mass on spring terminology
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When do you get SHM?
• A system is said to oscillate with SHM if the restoring force:– is proportional to the displacement from
equilibrium position– is always directed towards the equilbrium
position
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Force, acceleration, velocity and displacement
If this is how the displacement varieswith time...
... the velocity is the rate of changeof displacement...
... the acceleration is the rate ofchange of velocity...
...and the acceleration tracks the forceexactly...
... the force is exactly opposite tothe displacement...
Phase differences Time traces varies with time like:
/2 = 90
/2 = 90
= 180
zero
displacement s
force F = –ks
displacement s
cos 2ft
same thing
–sin 2ft
–cos 2ft
–cos 2ft
cos 2ft
acceleration = F/m
velocity v
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Mass on spring Energy transfer
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Mass on spring Energy
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SHM is like a 1D projection of uniform circular motion
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Phasors
• A rotating vector which represents a wave
• Length corresponds to amplitude, angle corresponds to phase
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Damping• In a real system there is always some
energy loss to the surroundings
• This leads to a gradual decrease in the amplitude of the oscillation– For light damping, the period is
(approximately) unaffected, though.
• The damping force generally is linearly proportional to velocity– Resulting in exponential decrease of
amplitude
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Damping
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Damping example
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Under-damping
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Critical Damping
• Critical damping provides the quickest approach to zero amplitude
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Over-damping
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Damping summary
• An underdamped oscillator approaches zero quickly, but overshoots and oscillates around it
• A critically damped oscillator has the quickest approach to zero.
• An overdamped oscillator approaches zero more slowly.
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What’s going on here?
• Example 2
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Free and Forced vibration
• When a system is displaced from its equilibrium position it oscillates freely at its natural frequency– No external force acts– No energy is transferred
• When an external force is repeatedly applied the system undergoes forced oscillation– energy is transferred to the system.
• Eg Barton’s pendulums
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Resonant driving
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Resonance
• If the system happens to be driven at its natural frequency the transfer of energy is most efficient: this is RESONANCE– Oscillation is positively reinforced every cycle– Amplitude quickly builds up
• Resonance can lead to uncontrolled, destructive vibrations– Bridges, glasses and opera singers, etc.
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Amplitude vs driving frequency
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Effect of damping on resonance
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Further investigation
• Pendulum lab
• Masses on springs