Physics Flashcards Preview

MCAT Prep > Physics > Flashcards

Flashcards in Physics Deck (109):
0

Force

Any influence capable of creating a mass to accelerate

A vector

Measured in Newtons (kg x m/s^2)

1

Newton's 1st Law (Law of inertia)

A object continues in a state of rest or motion at constant velocity, unless compelled to change that state by a net force.

Inertia: ability of an object to resist a change to its velocity

Mass: measure of object's inertia

2

Center of Mass Equation

Cmass= (r1m1 + r2m2 + r3m3 + ...)/mtotal

r: displacement vector between a reference point on each mass.
Step 1: choose reference point, "origin of coordinates" from which to measure each displacement vector.

3

Center of gravity

At the center of mass

4

Center of buoyancy

Center of mass of the fluid displaced by the submerged object

5

Newton's 2nd Law

Fnet= ma

Constant force will not an cause an object to accelerate faster and faster, it will cause a constant (non-changing) acceleration.

Cannot accelerate a ball horizontally across a room by throwing it

6

Newton's Third Law

Whenever an object exerts a force (via contact or field) on a second object, the second object always exerts an equal and opposite force on the first object.

7

Velocity vs speed

Velocity is a vector
Speed is a scalar

Velocity is change in displacement over change in time
Speed is change in distance over change in time

8

Constant velocity (or constant velocity)

Treat the same if and only if the distance traveled is on a straight line.

Constant velocity and constant speed THINK:
-no acceleration
-No note force
-all forces sum to zero
-no change in direction
-the object is in equilibrium

9

Acceleration

Change in velocity over change in time
Vector

If there is no net force, there can NEVER be an acceleration, however, there CAN be a force and no acceleration (forces cancel out)

10

Linear motion graphs:
How to interpret them

1. What does the slope represent?
2. Is this slope positive or negative? What does the sign of the slope tell you?
3. Is the slope constant (straight line) or non constant (curved line)? What does the observation tell you?
4. What value is on the y-axis?
5. Is the y-value (+) or (-) (are you above or below the x-axis)? What does that tell you?
6. Do you expect the value on the y-axis to be large or small at t=0?

11

Average velocity

v (ave)= (v1+v2)/2

12

Distance (or height) traveled

Distance= rate x time

13

Projectiles

Range: horizontal distance traveled
Range= vx x time

1. Horizontal velocity NEVER changes (ignoring air resistance)
2. Horizontal acceleration =0 always
3. Vertical acceleration always = 10 m/s^2 downward
4. Vertical behavior is exactly symmetrical (upward trip identical to downward trip)
5. Time in air depends on the vertical component of the velocity only
6. Range depends on both vertical and horizontal compounds
7. Time is always the same for both x and y components of the motion

14

X= 1/2at^2

Find distance

15

v=(2gh)^0.5 or v=(2ax)^0.5

Use when asked for final velocity given drop height

16

tair= 2v/g

Used ONLY to calculate "round trip" times, or, total time in the air

Variable V must be vertical component of vi

17

Air resistance characteristics

Factors that affect magnitude of air resistance

1. Cross Section Area: greater CSA impacting air, more air resistance

2. Shape: less aerodynamic, more air resistance

3. Velocity: greater velocity, more air resistance

Always assume AR is ignored! Unless specified.

18

At terminal velocity,
Fair= mg

Forces of gravity and air resistance are now balanced

19

Friction (kinetic and static)

Static: Ff= UsFn or Ff= Usmgcosx

Kinetic: Ff= UkFn or Ff= Ukmgcosx

Max static friction: friction created before an object begins to slide will always remain equal to the net applied force which the friction is opposing

20

Inclined planes

F=mgsinx (force down inclined plane, = to surface)

F=mgcosx (normal force on an inclined plane)

vf=(2gh)^0.5 (velocity of a particle at bottom of inclined plane)

21

Acceleration on projectile

Always 10 m/s^2 (downard) during entire flight. Doesn't change at max height.
Acceleration NEVER becomes 0, even at the exact peak
VELOCITY does reach an instantaneous zero
Velocity vector is the only vector that changes direction during projectile motion

22

Vectors

Magnitude and direction
Examples:
Velocity
Displacement
Acceleration
Force
Weight
Electric field
Magnetic field
Momentum
Impulse
Torque

23

Scalars

Magnitude only
Examples:
Mass
Temperature
Speed
Work
Energy
Charge
Time
Density

24

Universal law of gravitation

F= Gm1m2/r^2
G= 6.67E-11 m^3/kgs^2
True everywhere
-Near earth, we assume gravity is 10 m/s^2, simplifying to F=mg

GIVES FORCE DUE TO GRAVITY

25

Gravity

Field that exists between any two objects switch mass

26

Field

Invisible influence capable of exerting a force on a mass or charge

27

Gravitational potential energy

PE= mgh (near earth)

PE= -Gm1m2/r (in space, near earth if not assuming g=10m/s^2)

PE per unit volume of fluid = ro(p)gh
Ro(p) = density (mass/volume)

28

g=Gm/r^2

-Gravity
-"Strength of gravitational field"
- Acceleration due to gravity

29

Hooke's Law
(Springs, resilient solids, rubber, bonds between atoms)

F= k delta(x) (delta x is displacement of spring from its equilibrium point, NOT length of spring)

30

How to calculate spring constant of a hanging weights

For delta x, enter displacement from equilibrium point for one trial or difference in 2 trials. For F, use force applied in one trial or difference in force between 2 trials.
- Convert mass to force (F=mg)

31

Elastic potential energy

PE stored in a compressed spring
PE = 1/2 k (deltax)^2

This formula more likely to be used in connection with CONSERVATION OF ENERGY

32

Simple Harmonic Motion (SHM)

Anything that oscillates back and forth that can be represented by a sine wave.
Examples: pendulum and a mass on a spring
Essentially any mount that oscillates about an equilibrium position and shows the characteristics of sinusoidal pattern

33

Mass on a Spring

T= 2pi(m/k)^.5 (T=period)

34

Pendulum

T=2pi(L/g)^.5

35

T=1/f

Period is the inverse of frequency

36

Equilibrium

Terminal velocity
Constant velocity
Objects at rest
Balanced fulcrums or boards hanging from strings
Objects floating in a liquid

37

How to solve equilibrium problems?

Set forces or torques EQUAL to each other
Fleft=Fright, Fup=Fdown, etc

Draw Free body diagram

38

Torque

T=Fl or T=mgl or T=Frsinx
l= lever arm
r= distance between the force and the point of rotation
rsinx(theta)= l (always)
r=l only when x(theta)= 90 deg

39

When do I use T = Frsinx?

Use when FORCE applied is not perpendicular to r
Most are at 90deg, so sin90=1, T=Fr

40

ZERO NET TORQUE

If an object with a point of rotation is "stationary", or "exactly balanced" then it must be in equilibrium and there must be ZERO NET TORQUE

41

Systems NOT in Equilibrium

Inclined Planes:
Fdown plane due to gravity= F=mgsinx
Ffriction is always parallel to the plane (opposite direction of sliding)

Two-Dimensional: up/down or left/right

42

Centripetal Force

Fc = mv^2 / r

Fc is ALWAYS caused by some other responsible force (friction, tension, gravitational force)

43

Centrifugal Force

Forms action-reaction pair with Fc
Ball and string example:
Fc= string is pulling on ball
Centrifugal force= ball's force on string
Newton's 3rd law

44

Centripetal Acceleration

ac= v^2/r
Direction of the vector: points radially toward center of circle

45

How to solve Centripetal Motion problems?

Set Fc=mv^2/r equal to the equation of whatever force is actually causing the force in the situation

46

Angular Motion

w=v/r or w=2pif
w=angular velocity (in rad/sec)
v= tangential velocity (in m/s)
r= radius (in m, C=2pir)
f= frequency (in Hz)

47

Angular Frequency

-Scalar
-Magnitude of the angular velocity vector
-In MCAT, ang. freq. and ang.velocity uses interchangeably (rad/sec)

48

Conversion from radians - degrees

2pi radians = 360 degrees

pi radians = 180 degrees

49

Rotational Equilibrium

If it's not rotating, or rotating with a constant angular velocity (like normal equilibrium)

50

Momentum

p = mv

-In kg m/s

Momentum= inertia increased by velocity
Always conserved in a system
Collisions

51

Impulse

Change in an object's momentum
Impulse = delta p
Impulse = m delta v
Impulse = Faverage x time (remember air bag example)

If no change in velocity, there CAN BE NO IMPULSE.
Car collisions

52

Collisions

Elastic: momentum and KE are both conserved
-Use conservation of Energy (ignore signs)
1/2m1v1^2 + 1/2m2v2^2 = 1/2m1v1^2 + 1/2m2v2^2 (relative speed same before and after)

Inelastic: momentum conserved, KE not conserved
-Use conservation of momentum (use SIGNS)
m1v1 + m1v2 = m1v1 + m2v2

Perfectly inelastic (Stuck together)
m1v1 + m2v2 = (m1+m2)v3 where v3 is new velocity of stuck objects

53

Solids

Stress=Force/Area
Strain= change in dimension/ original dimension

54

Moduli of Elasticity

ME= stress/strain (general formula)

55

Young's Modulus

Tensile or compressive stress/strain modulus (simultaneously pushing or pulling forces on both sides of an object; the two forces must be exactly aligned in both vertical or horizontal planes)

56

Shear Modulus

Shear stress/strain modules (simultaneously pushing or pulling forces; two forces are NOT aligned)

57

Bulk Modulus

Bulk stress/strain modulus (simultaneous compression from ALL sides)

58

Thermal Expansion

When solids are heated, they expand. When cooled, they shrink.

Delta L = alpha x L0 x Delta T

59

Energy

KE = (1/2)mv^2
PE(gravitational) = mgh or -Gmm/r
PE (elastic) = (1/2)kx^2
PE (electrical) = Kqq/r or qEd or qV
PE (capacitor) = 1/2QV or 1/2CV^2 or 1/2Q^2/C

60

Mechanical Energy

ME = KE + PE
In the absence of non-conservative forces such as friction, drag, air resistance, etc., mechanical energy is ALWAYS CONSERVED (total energy)

61

Law of conservation of energy

Energy in an ISOLATED system is always conserved

62

Open System

Both mass and energy can be exchanged with the surrondings

63

Closed System

Energy, but NOT mass, can be exchanged

64

Isolated system

NEITHER energy nor mass can be exchanged

65

Work= change in energy

1) W = Change in Energy. Think like this FIRST, if energy changed, then it's WORK
- Change in velocity (change in KE=work, most common)
- Change in height (change in grav. PE = work)
- Change in position of masses/planets/etc. in space ( change in grav. PE)
- Change in position of charge (change in electrical PE = work)
- Compression of a spring (change in elastic PE = work)
- Friction (change in internal energy = work)

66

Work = Fdcos(o)

2) Think like this second. Any time force is applied across a displacement, work has been done.

UNITES = Joules
Work POSITIVE= Force and displacement in same direction
Work NEGATIVE= Force and displacement in opposite directions

67

1st Law of Thermodynamics

E = W + Q (W = energy transfer via a force, Q = energy transfer via energy flow from hot to cold)

68

Work-Energy Theorem

If a net force does work on a rigid object, the work done on that object is equal to the change in the KE of the object.
W = KEfinal - KEinitial

69

Machines

They reduce the amount of force necessary to perform a given amount of work. MACHINES NEVER REDUCE OF CHANGE THE AMOUNT OF WORK.

70

Ramps

Fmachine = mg(h/d)
- h is the height of the ramp and d is the distance along the hypotenuse. Fm is the force necessary to do the work with the machine, which will be less that doing it w/o the machine

71

Levers

Fm = mg(L1/L2)
L1 and L2 refer to the lever arms for the mass and the applied force, respectively.

72

Pulleys

Fm=mg/(# of vertical ropes directly lifting the mass)
CAUTION: not every rope that is vertically oriented should be counted and entered into the above equation. To be counted, a vertical section of rope must lift the mass DIRECTLY, either by being attached to the mass, or by lifting a pulley that is attached to the mass. To test, imagine grabbing only that rope and tugging it upward: does the mass lift??

73

Hydraulic Lifts

Fm=mg(h1/h2) or F= mg(A1/A2)
- h1 and h2 refer to the distance traveled by the large plunger and the small plunger respectively.
-A1 and A2 refer to the cross-sectional areas of the small plunger and large plunger, respectively.

74

Power

Think of POWER IN THIS ORDER
1) P = Change in Energy/time
2) P = W/t
3) P = Fdcos(o)/t
4) Pi = Fvcos(o) --- instantaneous power. Used when they ask for that
POWER IN WATTS (J/s)

75

Intensity (Waves)

- Power per unit area. They transfer energy from one location to another within a specified time.
-THE INTENSITY OF ANY SOUND OR MECHANICAL WAVE IS DIRECTLY PROPORTIONAL TO THE AMPLITUDE SQUARED AND THE FREQUENCY SQUARED
- I is proportional to A^2f^2
-Units of W/m^2
-Area of a sphere: 4pir^2 (accounts for the denominators of the units)

76

Decibel System

Intensity in Decibels = 10 x log(I/I0); where I is the intensity of the sound wave in W/m^2, I0 is the threshold of human hearing (1e-12)

77

Types of Waves

Transverse vs Longitudinal: transverse waves displace the medium perpendicular to their direction of travel. Longitudinal waves displace the medium parallel to their direction of travel.

Transverse: electromagnetic waves, wave on string)
Longitudinal: sound waves

78

Electromagnetic waves

No medium required, capable of propagating in a vacuum; transfer energy and momentum.
Transverse Only

79

Mechanical Waves

Require a medium to propagate; transfer energy only.
Transverse: strings on a musical instrument. Cannot propagate in liquids or gases, need stiff medium.
Longitudinal: Sound waves

80

Wave Speed

v= lf (l is landa)
1) Wave speed is determined by the medium and sometimes (for a "dispersive medium") wavelength and frequency.
2) Frequency NEVER changes when a wave moves from medium to medium
3) Wavelength DOES change when a wave moves from medium to medium

81

Wave Velocity in Mediums

v=sqrt(elastic/inertial)
On a string: v=sqrt(T/u); T:tension u:mass/length (thicker string < vel)
In a gas: v=sqrt(B/p). B: bulk modulus, p:denisty
In a solid: MUCH FASTER THAN ANY OTHER MEDIUM, since elastic moduli are much larger.

82

Superposition of waves

Constructive Interference: regions where the amplitudes or superimposed waves ADD to each other, increasing amplitude.

Destructive interference: regions where the amplitudes of superimposed waves subtract from each other, decreasing amplitude

Waves 360 degrees out of phase is the same as "in phase", so there will be constr. inter. 180 deg. out of phase, waves will cancel out. 270 deg. out of phase, there will be multiple areas of const. and dest. interference, creating a new wave form.

83

Standing Waves

Special case of simultaneous constr. and destr. interference between two waves with identical f's moving through the same medium in OPPOSITE DIRECTIONS. At points maximum destructive int. the waves cancel entirely (NODE) and at points of max const. interference (antinode)
Standing wave shows NO NET TRANSPORT OF ENERGY and does not PROPAGATE.

84

Beat Frequency

When two waves with close to the same freq. interfere
fbeat = l f1-f2 l

85

The Doppler Effect

Δf/fs= v/c
Δλ/λ= v/c
The Doppler shift perceived by the observer is DEPENDENT upon the relative velocity between the source and the observer. THE GREATER THE RELATIVE V THE GREATER THE SHIFT IN F OR λ

COLOR SHIFT DUE TO DOPPLER
White light can shift blue if the Doppler effect causes an INCREASE in f (decrease in λ) and red if Doppler effect causes decrease in f (increase in λ)


86

IMPORTANT NOTE ON THE DOPPLER EFFECT

-Add to the frequency (or SUBTRACT from the wavelength) if relative motion is TOWARD EACH OTHER.

-Subtract from f (add to wavelength) if relative motion is AWAY FROM EACH OTHER.

87

Sound

Sound is ALWAYS CREATED BY A VIBRATING MEDIUM. Vibrations propagate through liquids or solids, and generate pressure waves that propagate through gases such as air. SOUND CAN'T PROPAGATE IN A VACUUM.

88

Pitch

Higher pitch sounds have HIGHER f. Lower pitch sounds have LOWER f.
Infrasound: sound of a frequency too LOW to be perceived by ear
Ultrasound: sound of f too high to be perceived by human ear.

89

Harmonics

L = nλ/2 (string or pipe with matching ends- both nodes or both antinodes). λ= 2L/n
- Gives all harmonics n=1,2,3...

L = nλ/4 (one node and one antinode; pipe open at one end only) λ=4L/n
-Gives only ODD harmonics n=1,3,5..
IT IS IMPOSSIBLE TO HAVE A NODE AT THE OPEN END OF A PIPE AND IMPOSSIBLE TO HAVE AN ANTINODE AT THE CLOSED END.

90

Fundamental f

Frequency of the first harmonic.
Frequency of any harmonic = fundamental f x n
-Each harmonic always has ONE MORE NODE, AND ONE MORE ANTINODE, than the previous harmonic.

91

Overtones

The 2nd harmonic is called the 1st overtone, and so on.
FOR OSCILLATORS WITH MATCHING ENDS, THE λ OF THE 2ND HARMONIC = LENGTH OF STRING OR PIPE (λ=L)

92

Light
Photoelectric Effect

Observation that electrons are ejected from a material when light of sufficiently high frequency is used, but not until a threshold frequency is reached.

93

Energy of a Photon

E=hf, remember c=fλ, so E=h(c/λ)

94

Young's Double Slit Experiment

Set up: Young shone a MONOCHROMATIC light through a screen with a SINGLE SLIT in it. The purpose of this slit was to create coherent wavefronts. Behind the 1st screen he placed a SECOND SCREEN with TWO NARROW, PARALLEL SLITS. These created the DIFFRACTION pattern Finally, behind the 2nd screen he placed a 3RD SCREEN. Light traveled through the firs two screens and formed alternating pattern of LIGHT AND DARK BANDS on the 3rd screen.
-Light traveling through each of the two slits in the middle must be COHERENT AND HAVE THE SAME f and polarization.

95

Young's Experiment

x=λL/d

x- distance between fringes
λ- wavelength of light used
d- distance between the two slits
L- distance between the double slit (2nd screen) and final screen
- Strictly true only when x is much smaller than L

96

Diffraction

Tendency of light to spread out as it goes around a corner or through a slit. Without diffraction the characteristic interference patterns would not be formed.

97

Electromagnetic Spectrum

In order of decreasing f
Gamma rays (e24-e20) > X rays (e20-e17) > UV (e16) > Visible (e15) > IR (e14-e12) > microwave (e10) > radio waves (e8-e6)

λ: Gamma (e-16-e-12) < X rays (e-10) < UV (e-8) < VISIBLE (390-700nm) < IR (e-6-e-4) < microwaves (e-2) < radio waves (e0-e2)

VISIBLE: ROY G VIB
Red light LOWEST ENERGY (lowest f and longest λ)
Violet light HIGHEST ENERGY (highest f and lowest λ)

98

Snell's Law

Index of refraction: n=c/v (if n1, speed of light in medium is smaller than speed of light in vacuum)

Snell's Law: n1sinθ1=n2sinθ2

99

Total Internal Reflection

For a light crossing a boundary from a slower to a faster medium (like from glass or water into air), if the angle of refraction would be 90 or more, the incident light does not enter the second medium at all, 100% of the light is REFLECTED off the boundary and back into the first medium.
FOR TOTAL INTERNAL REFLECTION, LIGHT MUST BE PASSING FROM A HIGHER INDEX MEDIUM TO LOWER INDEX MEDIUM (n1>n2)
CRITICAL ANGLE: angle of incidence for which the angle of refraction will be 90. sinθcritical= n2/n1, since n2sin90=n1sinθc would make sin90=1

100

Dispersion

Change in index of refraction based on the frequency (or wavelength) of a wave. In a material with dispersion, different f's (or wavelengths) will be refracted to different angles, for the same incident angle. PRISM.

101

Lenses & Mirrors Image types

Virtual- there is NO ACTUAL LIGHT EMANATING FROM OR REACHING THE IMAGE (image formed behind a plane mirror)
Real- There is ACTUAL LIGHT AT THE IMAGE ( image formed on retina)

102

Lenses (single-lens systems)

Converging (aka, CONVEX, positive) = USUALLY produces PRI image.
-When object inside the focal point = NVU image

Diverging (concave)= ALWAYS produces NVU image

FOR OBJECTS FAR AWAY, ASSUME LIGHT RAYS HIT LENS PARALLEL. Considering convex lenses, the rays will be focused to the focal point at a distance f AWAY from the lens. As the object approaches the lens, however, the image will no longer be exactly at the focal point f.

103

Mirrors

Concave= like converging (convex) lens (PRI outside f, NVU inside)
Convex= like diverging (concave) lenses (ALWAYS NVU)
Also there are PLANE MIRRORS (the image and object will always be equal distances on either side of the mirror)

104

LENS/MIRROR CALCULATIONS

f=(1/2)r (for mirrors only)
1/f = 1/di + 1/do (thin lens equation, good for mirrors also)
M= -di/do = hi/ho

105

KEEP TRACK OF SIGNS
IF GET - # for M, the image is INVERTED

Four Rules (Lens and Mirrors)- single lens systems only!
1) Object distances (do) are ALWAYS +
2) Image distances (di) or focal point distances (f) are + IF THEY ARE ON THE SAME SIDE AS THE OBSERVER and - if on OPPOSITE SIDE.
3) The observer and object are on the same side for a MIRROR and on OPPOSITE sides for a lens (you have to be behind your glasses to see through them to view the object on the other side)
4) PRI/NVU: Positive, real, inverted and Negative, virtual, upright ALWAYS STAY TOGETHER. Positive means on the same side.

106

Near vs Far-Sighted

Near Sighted- ABLE TO FOCUS CLEARLY ON CLOSE OBJECTS, but not DISTANT OBJECTS. Image formed IN FRONT of retina.
Far Sighted- ABLE TO FOCUS ON FAR OBJECTS, but not CLOSE OBJECTS. Image formed BEHIND the retina.

107

Optical Power

P=1/f
When ciliary muscles contract, lens curvature increases, decreasing focal point, as focal point decreases, optical power INCREASES.

108

Two Lens System

Binoculars, telescopes, etc. THE IMAGE FORMED BY THE FIRST LENS BECOMES THE OBJECT FOR THE SECOND LENS.
Magnification: M= m1+m2
Power: P=p1+p2