PHYC10005 Study Guide - Final Guide: Total Internal Reflection, Elastic Collision, Centripetal Force
23 Jun 2018
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PHYC10005 NOTES
General
• METRIC PREFIXES
o Tera, T = 1012
o Giga, G=109
o Mega, M=106
o Kilo, k=103
o Deci, d=10-1
o Centi, c=10-2
o Milli, m=10-3
o Micro, μ=10-6
o Nano, n=10-9
o Pico, p=10-12
o Most commonly used are mega, kilo, centi, milli, micro and nano
o The numbers given are multiplying factors, you multiply to convert to the ‘default’
metric. Default metrics are units without a prefix, e.g metres, seconds, grams as
opposed to kilometres, microseconds or milligrams.
o E.g To convert 1 kilograms to its default metric of grams, you multiply the 1 by the
kilo factor of 103 to get 1000 grams.
• PHYC10005 USES SI UNITS:
o Length in metres
o Mass in kg
o Time in seconds
o Speed in m/s
o Frequency in Hertz Hz
o Force in N
o Pressure in Pa
o Energy in J
o Power in W
o Charge in C
• Converting between revolutions of a circle and radians for angular displacement
measurement. There are 2π radians in a revolution; Rev = Rad/2π.
• Rev Rad x 2π. Rad Rev is /2π.
Common Exam Style Conceptual Question
• Why does the feather and hammer fall at same rate on moon?
Because, all objects are subject to a force proportional to their own mass due to gravity. In
absence of other forces this causes all objects to accelerate at the same rate. These other
forces include air resistance, which has a presence on earth but not on the moon.
Lift Questions
• If question says ‘the scales read 60kg’, then normal force, N is 60g where g is 9.8. (Scales
only measure the mass portion of normal force; given by m=N/g as N=mg).
• If object is on lift is accelerated UPWARDS at ‘a’ m/s2. Then use N = ma + mg as going
upwards is adding more normal force, as you feel heavier going up on a lift.
• If object is on a lift accelerating DOWNWARDS then it is N + ma = mg, ma is on other side of
equation, decreasing normal force felt, as you feel lighter going down on a list.
Hooke’s Law
• F=-kx where k is spring constant and x is displacement from rest position of spring.
• The negative in front of the K is to show that the displacement ‘x’ is always in opposite
direction of spring force. So, on a graph the gradient is still positive, as shown below.
• F represents Fsp and it is directly proportional to displacement from natural length.
Uniform Circular Motion Conceptual Ideas
• Acceleration and velocity are perpendicular.
• Acceleration always towards centre of circle, pushing the object inwards, this is what keeps
the object in circular motion or else it’d go flying out.
• Speed is constant, as it is scalar and does not consider direction.
• Velocity is not constant as direction constantly changes, its magnitude (speed) is constant.
• If speed is constant, then it’s magnitude of acceleration is also constant.
Equilibrium of mass blocks hanging by two strings from ceiling.
• Use trigonometry to resolve the tension forces in the strings in the x and y directions.
• Static equilibrium: net force on object = 0 and the object is in equilibrium
• Dynamic equilibrium: net force on object = 0 and object is moving at constant non zero
velocity.
Note the positive gradient of the line.
Friction
• Static Friction: Friction on object when object is stationary.
• Kinetic Friction: Friction on object when object is moving.
• Static Friction is higher than kinetic friction
Celestial (solar planet) Mass Calculations
• To find gravitational force F between objects (objects are celestials) with masses m1 m2 (kg).
Use
where G is constant of proportionality. 6.67 x 10-11 and r is the distance of
their separation in m measured from the cores of the two objects
• For object near earth surface, object’s acceleration due to gravity is.
where the
mass of the object cancels out.
• Equating gravitational force and centripetal force u get
T is time in seconds to
go around once. r is distance from earth CENTRE to object, radius of orbit in metres.
• Gravitational force g is what keeps an object in orbit at a constant orbit altitude.
Torque
• Torque is the force that causes rotation to occur. Torque units are Newton metres, N m,
whilst force is just N.
• Can be represented by 2 formulas, these 2 formulas can also be equated. Also need at=r α
1. T= rFsin(ϕ). r is distance from rotational axis to force applied and F is applied force.
When ϕ=90 degrees, T is max for that specific radius and Force applied.
2. T=I α (F=ma), use when moment of inertia or angular accel is known.
3. at=r α is also used at times. Where r is radius.
• Moment of inertia I or rotational inertia is how difficult it is to rotate an object in kg m2
• I=mR2k where k is a constant that varies between round objects. m is mass of object being
rotated and R is distance (scalar) to central rotational axis from outside edge.
• Torque is also equal to rate of change of angular momentum.
Torque and Static Equilibrium
• Applying torque concept for a static equilibrium state with planks hanging instead of blocks
to find the net torque.
• At static equilibrium, net force = 0 and net torque about any rotational axis is 0.
• Rotational axis can be defined anywhere, as long as the directions defined are consistent.
• Define direction signs by clockwise or anti-clockwise.
• Can be used to find tension of string on either end of planks. This can be done by simsolving
net torque and net force.
• Do not assume COM is at rotation axis.
