physics cheatsheet unit 3 ver.3

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



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



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.

physics cheatsheet unit 3 ver.3

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