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Frames of reference


Top view of a road with two people standing on opposite sides. A car drives past.

Consider two people standing, facing each other on either side of a road. A car drives past them, heading West. For the person facing South, the car was moving toward the right. However, for the person facing North, the car was moving toward the left. This discrepancy is due to the fact that the two people used two different frames of reference from which to investigate this system. If each person were asked in what direction the car were moving, they would give a different answer. The answer would be relative to their frame of reference.

What is a frame of reference ?

Frame of Reference

A frame of reference is the point of view from which a system is observed.

In practical terms, a frame of reference is a set of axes (specifying directions) with an origin. An observer can then measure the position and motion of all points in a system, as well as the orientation of objects in the system relative to the frame of reference.

There are two types of reference frames: inertial and non-inertial. An inertial frame of reference travels at a constant velocity, which means that Newton's first law (inertia) holds true. A non-inertial frame of reference, such as a moving car or a rotating carousel, accelerates. Therefore, Newton's first law does not hold true in a non-inertial reference frame, as objects appear to accelerate without the appropriate forces.

Why are frames of reference important?

Frames of reference are important because (as we have seen in the introductory example) the velocity of a car can differ depending on which frame of reference is used.

Frames of reference and special relativity

Frames of reference are especially important in special relativity, because when a frame of reference is moving at some significant fraction of the speed of light, then the flow of time in that frame does not necessarily apply in another reference frame. The speed of light is considered to be the only true constant between moving frames of reference.

The next worked example will explain this.

Relative velocity

The velocity of an object is frame dependent. More specifically, the perceived velocity of an object depends on the velocity of the observer. For example, a person standing on shore would observe the velocity of a boat to be different than a passenger on the boat.

The speedometer of a motor boat reads 5 m · s - 1 . The boat is moving East across a river which has a current traveling 3 m · s - 1 North. What would the velocity of the motor boat be according to an observer on the shore?

  1. R = ( 3 ) 2 + ( 5 ) 2 = 34 = 5 , 8 m · s - 1
    tan θ = 5 3 θ = 59 , 04

    The observer on the shore sees the boat moving with a velocity of 5,8 m · s - 1 at 59,04 east of north due to the current pushing the boat perpendicular to its velocity. This is contrary to the perspective of a passenger on the boat who perceives the velocity of the boat to be 5 m · s - 1 due East. Both perspectives are correct as long as the frame of the observer is considered.

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Questions & Answers

what is an isomer?
Kiba Reply
organic compounds with the same molecular formula but different structural formula
how is frequency directly proportional to kinetic energy max. and what is the relationship between the wavelength and the energy of a photon. eg if a photon with a greater wavelength and a photon with less frequency. what would be the energies. as in which one would be greater and why
Pearl Reply
relationship wave depend n enrgy greadt reflect of energy n the form of tex shape
what is Cell capacity?
liphelo Reply
what are hydrocarbons
Nyiko Reply
compounds that are made up of hydrogen and carbon only
They are compounds that consist only with hydrogen &carbon elements.
how do i calculate the work done on an object by a frictional force
Emmelda Reply
@ Emmelda it's W = FxCos180 Where 180° is the angle between Frictional force and where the object is going, it's always 180° because it opposes the motion. Then F is the magnitude of the force and x is the displacement
Ohm's law state that provide the temperature remains constant, that potential difference is directly proportional to the current flowing in the circuit
Ezile Reply
state ohms law
what does ohm's law state ?
Gugu Reply
ohm's law state that the potential difference across a conductor is directly proportional to the current in the conductor at constant temperature
ohm'law in metalic conductor is directly proportional to the potential difference provided the temparature
what is the principle on which generator is based
Nqobile Reply
electromagnetic induction
Explain it😌
mechanical​ energy is used to rotate turbines inside the generator where a coil placed between two fixed magnets.as the coil rotates it cuts the magnetic flux due to the magnets. the magnetic flux will then induce an electric current into the coil which will be transmitted to the brushes
may you Please instruct me on how to do Flemmings Right hand dynamo Rule
Fleming's right-hand rule shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field. It can be used to determine the direction of current in a generator's windings
 hold out the right hand with the first finger, second finger and thumb at the right angle to each other.forefinger represents the direction of the line of Force, the thuMb points in the direction of Motion or applied force, then seCond finger points in the direction of the induced Current
what is spectrum
Mpeh Reply
It is a different combinations of frequency of light or any types of waves.
Heii guys what's the topic? Am neww
Vannessa Reply
In nuclear reactions, is it necessary that there must only be one element on the product side and a radioactive particle or you can have a product that are both radioactive particles?
Jobena Reply
conservative force is a force when the work done in moving the object between the two points that does not depend on the path way
Bongiwe Reply
what is conservative force?
Mh Reply
It is a force , that does not have any external features affecting it
what does the alternating current do
Kwanga Reply
what is definition of hydrocarbon
jessha Reply
it is the atoms that Contain of C atoms and H atoms only
You mean molecules
yea 💯
what's the definition of le chatelier?
what is electromagnetic
differentiate between DC motor and Ac generator
dc motor converts electrical energy into mechanical energy while ac generator converts mechanical into electical
DC has a Split-ring but then Ac has a slip-rings.
what's the other name of galvanic cell?
galvanic cell is also called Voltanic cell
is the chain whereby there is only Hydrogen and Carbon atoms only.
what is the difference between AC and DC moto
please check the answer from Ophola and Joyous
AC motor consists of 2 slip rings and a DC motor has a single split ring commutator
ac have slip rings n DC has split rings

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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