physics cheatsheet unit 3 ver.3
TRANSCRIPT
8/6/2019 Physics Cheatsheet Unit 3 Ver.3
http://slidepdf.com/reader/full/physics-cheatsheet-unit-3-ver3 1/3
Graphs
x-t m = velocity.
v-t m = accel. A=displac.
a-t A = Δvel.f-t m = accel. A=impulse
f-x A = work/energy
Newton’s laws1st: Every object continues to be at rest or
continues w. constant velocity unless it
experiences an unbalanced force (law of
inertia)
2nd: Accel of a body exp unbalanced force
directly proportional to net force, inversely
proportional to mass
3rd: Forces always occur in pairs. When one
body exerts a force on another body there is
an equal force in opposite direction
At constant velocity. F DF F =
Frame of referencevel.B relative to A = vel.B – vel.A
Inclined planes
F F mg Fdownslope −=Σ θ sin
θ cos
mg F N =
θ sin g a =
In Lifts
ma Rmg a =−↓ )(:
mamg Ra =−↑ )(:
Projectiles
Horizontal distance for flights that start &end at same height use:
g v R θ 2sin
2
=
where v is initial speed
Collisionsmv p = (kgms-1)
vector (remember direction)
pvmt F I ∆=∆=∆Σ= (N s)
22112211 vmvmumum+=+
elasticmomentum & energy conserved
Work, Energy & Power
Must use component of F parallel to x
F perpendicular to x does no work
xT Fx
mumv E Work
×==
−=∆=
θ cos
21
21 22
Power rate work is done
Fvt
Fx
time
work
P ===
(where v is constant)
Mechanical energy: P K E E +
Elastic PE = work = energy absorbed
(area under F-x graph)
Circular Motion
Centripetal forces
Since accelerating, must be an unbalanced
forcecentripetal force acting towards
centre circle. Must be supplied by real forces
eg. moon-gravity, car-friction, train-reaction
force from rails
r mvmg R F C
2
)( =−=
hence,r
m vmg R2
+=
since R>mg he feels ‘heavier’
Weightlessness only really exists when
reaction forces vanish (g=0), when (a=g)
(freefall) reaction forces also vanish giving
impression of apparent weightlessness
T Rv π 2=
22
2
22
44
Rf T
R
R
va π
π
===
Fg is grav. force, g is grav.field.strength
mg R
GMm
T
Rm
R
mv F g ==== 22
224π
g is measured in (N kg-1)
3
2
2
4π
GMT R =
GM
RT
3
2π =
R
GM V =
22
3
4π
GM
T
R=
geostationary T = 86400 s
Energy in circular orbits
- EP & EK constant
- No work done = no ΔE
Energy changes moving thru changing
gravitational field
Area = work done = ΔE
(Nm or J)
Electronics
Series Parallel
I = I1=I2 I = I1+I2
V=V1+V2 V=V1=V2
RV R I V I
t E P
22
====
Using a multimeter
Voltage – leads connect in parallel across 2
points whose voltage we wish to measure
Current – leads connect in series withcomponentResistance – one end of component
disconnected from rest of circuit. leadsconnect in parallel
switch-on voltage – forward-bias voltage
that gives normal operating current through
diode
limiting resistor – when LED starts
conducting, its resistance drops sharply.
The current is likely to get so high that it
damages the LED. The resistor limits the
max current that can flow.
saturation – collector current has reached max value, cannot change further
cutoff – transistor stopped conducting
current due to base voltage falling too lowbiasing (voltage amplifiers) – add a DC
current to ensure the AC voltage is greater
than 0 at all times
When average of signal is not zero, there is
mixture of AC & DC:
Decoupling capacitors – block DC
components of the input & output signals.
VB needs to be kept at particular values of
DC. Allowing other Dc values to add to this
might cause incorrect biasing. High
frequency AC passes much more easily than
low. The larger the capacitor, the easier for
AC current to pass through
8/6/2019 Physics Cheatsheet Unit 3 Ver.3
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Diodes: Allows current to flow one way but
not the other. In reverse bias resistance is
very high
Forward bias:
Capacitor:
A device used to store charge.
f R
π 21=
Thermistor:
resistance normally falls with increasing T
Transistor amplifiers- VIN required is AC signal biased with DCto ensure correct voltage
- Small changes in IB causes large changes inIC
V V V BE B 7.0==
E C I I ≈ BC I I >>
gain I I BC ×=
C S OUT V V V −=
OUT CC C V V V −=
B
CC B I
V R
7.0−=
C
OUT CC C I
V V R
−
=
C
OUT CC C RV V I −
=
E BE B V V V +=
E C E R I V ×=
E
BC R
V I
7.0−=
C
OUT CC C R
V V I
−
=
21
2
R R
R
V V CC B +×=
E OUT CE V V V −=
E
C GAIN R
RV
−
=
Photonics
LEDs
- convert electrical energylight energy
-emit light when current passes
- limiting resistor prevents too much current
through LED, too much power to LED will
cause it to blow
- can emit in many colours & infra-redregion
- much more efficient than incandescentlamps because they switch on much faster,
operate on much lower voltages(and power)& operate for many more hours before
failure
LDRs
- convert light energyelectrical energy
- relatively slow to respond to changes
- semiconductor
Photodiodes (requires power source)
Photodiodes are affected by light.
Can be used two ways -- in a photovoltaic(here it becomes a current source when
illuminated (solar cell), or photoconductiverole. To use a photodiode in its
photoconductive mode, the photodiode is
reverse-biased; the photodiode will then
allow a current to flow when it is
illuminated.
- respond rapidly to light changes
- convert light energyelectrical energy
(reverse bias)
- as light intensity↑, current↑- sensitive detectors of changes in light
intensity
(forward bias)
- solar cells
- zero current means max voltage
- zero voltage means max current
- max power point is b/w these two
- 20% efficient in conversion of light energy
to electrical energy
Phototransistors (don’t require power
source)
- Similar to ordinary transistors, except they
are controlled by light falling on collector-
base junction region rather than by an
external current into the base- IB generated when light detected at base
terminal (conventional – generated frominput voltage in circuit)
Intensity modulation (amplitude)
Modulation is altering the intensity of a
beam of light in a way that transfers
information. The information is encoded in
the time variation of the intensity.
Bandwidth refers to the range of
frequencies passed with limited attenuation
and is measured in Hz, MHz, bps, etc
Skin effect
Copper (electrical signals) have a skin effectat high frequencies (high bandwidth) so it
limits the amount of information that cantransmit before being lost or distorted
Structures
Strength: max stress
Toughness: area under graph (strain energy)
Brittle: no plastic region (fracture just past
the elastic limit)
Stiffness: Larger Y’s mod.
Flexible: Low Y’s mod
Ductile: large plastic region
21
2211
mm xm xm X CM
+
+=
Yield point end of elastic region
Fracture point end of plastic region
I-beams
I-beams provide larger surface areas tosections of beam which will be under most
stress. Decreases stress, reduced weight.
Struts (columns): compression
8/6/2019 Physics Cheatsheet Unit 3 Ver.3
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Ties (cables): tension
Pre-stressed concrete
If steel rods put under tension whilst beam ismade, rods exert permanent compressive
force on concrete. Hence when beam isloaded, although compression on lower
surface reduced, it still remains incompression – never enters the weak tensile
condition
Arches
The weight of the materials and curved
shape keep the individual blocks in
compression. It has the property that it
transfers the arch forces into nearly vertical
components at the support pillars. Arches
need buttressing at the ends in order to
balance any horizontal force components.