Unit Consistency: Exam Prep in 8 Hours

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Homework Help Overview

The discussion revolves around the concept of unit consistency in physics, particularly in preparation for an upcoming exam. Participants are exploring which units exhibit consistency and the definitions related to consistent units, referencing both the MKS and CGS systems.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants are attempting to identify consistent units and question the definitions provided in their textbook. They discuss examples of units, such as km/h and kg/m³, and explore the dimensional relationships between different units.

Discussion Status

The conversation is active, with participants sharing their interpretations and seeking clarification on unit consistency. Some have offered definitions and examples, while others express confusion and seek further understanding of the concepts discussed.

Contextual Notes

There is mention of a specific time constraint due to an impending exam, and participants are working within the framework of their textbook's definitions and examples of consistent units. Some participants also note potential typos or inaccuracies in the definitions provided.

jaimieee
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Ok, this is not a homework. But we have an exam later 8 hours from now.

On our book there is an activity called the Unit Consistency.

The instruction says: Which of the units below do no exhibit unit consistency? Give the equivalent consistent units for those that are not consistent.

1) cm/s2 - Answer: cm/s2
2) km/s - Answer: km/h

Our Physics teacher gave us some of the consistent units like:

km/h
mi/h
kg/m3

but I'm not sure if only these are the consistent ones, are there still consistent units? Can you share it? Thank you.
 
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jaimieee said:
Ok, this is not a homework. But we have an exam later 8 hours from now.

On our book there is an activity called the Unit Consistency.

The instruction says: Which of the units below do no exhibit unit consistency? Give the equivalent consistent units for those that are not consistent.

1) cm/s2 - Answer: cm/s2
2) km/s - Answer: km/h

Our Physics teacher gave us some of the consistent units like:

km/h
mi/h
kg/m3

but I'm not sure if only these are the consistent ones, are there still consistent units? Can you share it? Thank you.

Is there a definition of "consistent units" offered by the book or instructor?
 
berkeman said:
Is there a definition of "consistent units" offered by the book or instructor?

Yes,yes! Sorry I wasn't able to define.

This is what is EXACTLY written :)

Unit Consistency

whenever you make calculations involving measurements, always remember to check for unit consistency; that is, the units should belong in the same system. Generally, if you are using meter for length, you should use gram for mass. Within the SI, the system of units based on the use of meter and kilogram is referred to as the MKS (meter-kilogram-second) system. Another system is the CGS system, which uses the centimeter, gram and second.

I didn't get it. It's confusing me :(
 
I think they mean which are dimensionally the same.
The basic dimensions are length, mass, time (plus temperature, and current)

So for example speed is length/time so could be given in miles/hour, m/s, km/min furlong/fortnight etc but could never be km/kg
So two sets of units are consistent if they refer to the set of same base units.
 
jaimieee said:
Yes,yes! Sorry I wasn't able to define.

This is what is EXACTLY written :)

Unit Consistency

whenever you make calculations involving measurements, always remember to check for unit consistency; that is, the units should belong in the same system. Generally, if you are using meter for length, you should use gram for mass. Within the SI, the system of units based on the use of meter and kilogram is referred to as the MKS (meter-kilogram-second) system. Another system is the CGS system, which uses the centimeter, gram and second.

I didn't get it. It's confusing me :(

Oh, I understand now. Well, except for the typo in the sentence that says "if you are using meter for length, you should use gram for mass." That is incorrect. As the rest of the paragraph states, the typical unit systems are:

MKS (SI) -- meter, kilogram, second

cgs -- centimeter, gram, second

There are similar consistent unit systems for goofy units like we use here in the US (pounds, whatever). This is a pretty good intro to units, with pointers out to other articles:

http://en.wikipedia.org/wiki/Units_of_measurement

So it looks like the units you posted in your original post (OP) are consistent. Do you see why the following would be inconsistent?

g/m^3
 
mgb_phys said:
I think they mean which are dimensionally the same.
The basic dimensions are length, mass, time (plus temperature, and current)

So for example speed is length/time so could be given in miles/hour, m/s, km/min furlong/fortnight etc but could never be km/kg
So two sets of units are consistent if they refer to the set of same base units.

Hmm for example the given is:
Newtons per centimeter , what will be the consistent unit?

mgb_phys said:
So for example speed is length/time so could be given in miles/hour, m/s, km/min furlong/fortnight etc but could never be km/kg

I think km/min is not the consistent one? I think it should be km/h? What do you think?
 
berkeman said:
Oh, I understand now. Well, except for the typo in the sentence that says "if you are using meter for length, you should use gram for mass." That is incorrect. As the rest of the paragraph states, the typical unit systems are:

MKS (SI) -- meter, kilogram, second

cgs -- centimeter, gram, second

There are similar consistent unit systems for goofy units like we use here in the US (pounds, whatever). This is a pretty good intro to units, with pointers out to other articles:

http://en.wikipedia.org/wiki/Units_of_measurement

I kinda get it but I'm not sure if my understanding is correct.

berkeman said:
So it looks like the units you posted in your original post (OP) are consistent. Do you see why the following would be inconsistent?

g/m^3

I think the answer should be kg/m^3? I used the MKS (SI).
 
jaimieee said:
I think the answer should be kg/m^3? I used the MKS (SI).

Correct. kg/m^3 or g/cm^3 would be consistent sets of units, if I understand the definition correctly.
 
jaimieee said:
Hmm for example the given is:
Newtons per centimeter , what will be the consistent unit?
It should be Newtons per meter or dynes per centimeter(surface tension)



jaimieee said:
I think km/min is not the consistent one? I think it should be km/h? What do you think?
It is MKS(Meter Kilogram Second) so it is km/s
 
  • #10
Hi. We just had our exam awhile ago but this one was not included. T__T Anyway thank you guys ;)
 
  • #11
vin300 said:
It is MKS(Meter Kilogram Second) so it is km/s

Given the earlier definition, km/s is not consistent by m/s would be.

See Post #1, Item 2 ... km/h is considered to be "consistent units".
 

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