# Common derived units of the physical quantities

#### nasadall

Hi all!

I came across 2 questions on my course work, and i don't know what is required here, can some one help please?

Q.1 Define the common derived units of the following physical quantities; and express them in terms of the base units: Mass, Length, Time and Electric charge (or Electric current);

Force, Velocity, Acceleration, Energy, Density, Pressure, Power, Momentum, Speed

Q.2 Define the common derived units of the following physical quantities:

P = mv - P = Kg.L/T

EK = ½ mv2 - EK = ½.Kg.(m/s)2
Ep = mgh - Ep = Kg.m/s2.m/s.s

v2 = u2 + 2as. - (m/s)2 = (m/s)2+2.m/s2.m/s.s

S = ut + ½ at2 - m/s.s = m/s.s.s+½.m/s2.s2

P = w/t - P = Kg.m/s2.s

F = ma - F = Kg.m/s2

t = t’/√(1 – (v2/c2)) no idea

Thank you for your help

Last edited:
Related Introductory Physics Homework News on Phys.org

#### schip666!

It's called "dimensional analysis"...I just saw a thread here that made it very clear why E should equal MC^2 because of the units used to measure energy...

Have a look at my little cheat sheet in this thread:
https://www.physicsforums.com/showthread.php?t=424058&highlight=power

I think that they want you to reduce the various usual physical measurements to their basic MKS components -- strangely (and conveniently) enough there are only a few actual "irreducible" quantities, like mass. For instance:
Force -- Newton -- Mass times Acceleration ( F = MA ) -- Killograms times Meters per Sec^2: (Kg x M) / S^2

#### nasadall

I still need some help with this, if someone could help me please.
I'm desperate now.

Thank you

#### tiny-tim

Science Advisor
Homework Helper
hi nasadall! (try using the X2 icon just above the Reply box )

Start with p = mv …

what are the units for m? what are the units for v?

now multiply them toegther Mass and speed

#### tiny-tim

Science Advisor
Homework Helper
yes, and what are the units of mass and speed? M-Kg
V-L/T

#### nasadall

am i on the right path?
i have done some on top. i think that is what is required.

#### tiny-tim

Science Advisor
Homework Helper
M-Kg
V-L/T
yes, the units for m are Kg

and the units for L/T are … ? #### nasadall

yes, the units for m are Kg

and the units for L/T are … ? meters and seconds

ok i get it, so i have to put every thing in units?

#### tiny-tim

Science Advisor
Homework Helper
yes!! #### nasadall

yes!! On Q1 do i only have to put like for example

Force - F=M.L/T2

???

#### tiny-tim

Science Advisor
Homework Helper
On Q1 do i only have to put like for example

Force - F=M.L/T2

???
hmm …

looking again at Q1, i find it rather confusing …

it does seem to indicate that M L T etc are the base units, so that ML/T for example would be a derived unit

but that's not the way i understand derived units …

i'd say you have to use kg m s etc …

and wikipedia (http://en.wikipedia.org/wiki/SI_derived_unit" [Broken]) points out that the SI system has officially designated derived units such as the joule for energy: but there is no such named derived unit for eg momentum (mv) as in Q2 (though there is a "Compound unit derived from SI units") sooo … i really don't know what the question wants Last edited by a moderator:

#### nasadall

Not a major issue, i will ask some one at college tomorrow. lol

#### nasadall

Q2.1
a) Using the information provided in the appendix, calculate the amount of time required for an electromagnetic signal sent by the astronauts landed on the surface of the Moon to reach the control base on Earth.

Appendix:
Distance of Moon from the control base: 4.0x107 km;
Speed of electromagnetic radiation in free space (& atmosphere) = 3.0x 108 m/s

I used this formula t=s/v

where t=4.0x1010(to achieve meters)/3.0x 108

and i get an answer of 133.333

am i ok??

#### tiny-tim

Science Advisor
Homework Helper
I used this formula t=s/v

where t=4.0x1010(to achieve meters)/3.0x 108

and i get an answer of 133.333

am i ok??
Yup! (except ridiculously accurate )

#### nasadall

Thanks.

This next one is a bit confusing, not so much the first part, its the second, i think i got it wright, i just dont know how to put it on peper to present it to the tutor.

b) A car is being driven along a straight and level road at a steady speed of
25 ms-1 when the driver suddenly notices that there is a fallen tree blocking the
road 65 metres ahead. The driver immediately applies the brakes giving the car
a constant retardation of 5 ms-2.

i) How far in front of the tree does the car come to a halt?

u=25 m/s v=0 m/s a=-5 m/s2 s=?

so using v2=u2+2as

my result was

02=252+2(-5)s
0=625+(-10)s
10s=-0-625
s=-625/-10
s=62.5

then i take away the 62.5 meters treveled while deccelerating, from the 65 meters from where the tree was spoted.

65-62.5 = 2.5 meters that the car came to a halt in front of the tree.

#### nasadall

ii) If the driver had not reacted immediately and the brakes were applied one second later, with what speed would the car have hit the tree?

u=25 m/s a=-5 m/s2 s=65 m - (1 sec reaction time = 25 m/s) = 40 m???? v=?

same formula

v2=252+(-5)40
v2=625+(-10)40
v2=625+(-400)
v2=225
v=√225
v=15

The car hit the tree with a velocity of 15 m/s

#### tiny-tim

Science Advisor
Homework Helper
uhh? that's completely different start a new thread! (and i'm going to bed :zzz:)

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving