Is this correct?

Mathmasteriw

Mathmasteriw

Junior Member
Joined
Oct 22, 2020
Messages
85
Hi Everybody,
Just a quick post to ask if this is correct? Find attached the question and my answer.
Thank you!
3676ED66-1C1A-49B2-9B04-689FCEF67409.jpeg
 

Attachments

  • 70A79D17-F837-44F7-B20A-DE1FFD703F41.jpeg
    70A79D17-F837-44F7-B20A-DE1FFD703F41.jpeg
    1.1 MB · Views: 5
Mathmasteriw said:
Hi Everybody,
Just a quick post to ask if this is correct? Find attached the question and my answer.
Thank you!
View attachment 24083
Click to expand...
Your work is mostly correct. However:

  1. You have 'claimed' to have differentiated 'v' with respect to 'x' and derived \(\displaystyle \frac{dv}{dx}\) - but you were given 'v' as a function of 't'.

    So you should derive \(\displaystyle \frac{dv}{dt}\). Fix that!

  2. You need to finally calculate \(\displaystyle \frac{dv}{dt}\) at t = 5 for answer.
 
No, the units will NOT be volts.

dv/dt ~ (change in v)/(change in time) = (change in volts)/(change in time) = (change in volts)/ (change in seconds) = volts/second. That is the units of dv/dt is volts/sec
 
Mathmasteriw said:
A yes see them now, will correct those.
Also, what would I be correct in saying that the unit of measure for the final answer would be Volts?
Click to expand...
In your original equation you did not indicate the unit 'v' nor did you indicate the unit of 't'. What are those units?
 
Ok , so this 620 is not correct?
Not sure where to go from here to be honest guys. ?
 

Attachments

  • image.jpg
    image.jpg
    1.2 MB · Views: 4
Make up your mind. Does dv/dt = 4t^3 + 24t or does dv/dt = 620? Or are they both the same, that is 4t^3 + 24t always equal 620?

What happened to the units in dv/dt??
 
Well, what I have tried to do is put t=5 into the equation as you can see when reading my steps you can quite clearly see that.
I'n not sure if it is correct or not that's what I am asking.
 
Mathmasteriw said:
Well, what I have tried to do is put t=5 into the equation as you can see when reading my steps you can quite clearly see that.
I'n not sure if it is correct or not that's what I am asking.
Click to expand...
[math]\dfrac{dv}{dt}[/math] is a function of t just like f(x) is a function of x. We can write [math]\dfrac{dv}{dt}\ as \ \dfrac{dv}{dt}(t)[/math]. You want to evaluate dv/dt at t=5 so we write [math]\dfrac{dv}{dt}(5) = 620[/math] (what are the units??).

Think about this. What if you were also asked to find dv/dt = 2sec. How can you write dv/dt = 620, dv/dt = 80 and dv/dt = 4t^3 + 24?? dv/dt can't possible equal all of those expressions!
 
Ok thanks for that I am getting my head around this now.
So, dv/dt = 4t^3 + 24 would be correct?
but to evaluate dv/dt = 4t^3 + 24 at 5 seconds would be: dv/dt x (5) =620
would this then be the correct way to finish the question?
thanks for your time by the way I know I'm a novice but i'm getting no help or support form tutors, so i'm very greatful for any help.
 
Just like f(7) does NOT mean f*7, dv/dt (5) does not mean dv/dt * 5!! As I wrote before, it is just dv/dt(5).
Again, can you please state the units that go with the 620? Hint: I gave it to you earlier.
 
Technically that question cannot be answered!

The OP did not specify that the unit of time was "second" - in the stated voltage function.
 
Mathmasteriw said:
Assuming the 'OP' did specify the unit of time was ''seconds'', then would that be correct?
Click to expand...
Yes...

And the unit of 'v' should be assumed to be 'volt'. Then unit of \(\displaystyle \frac{dv}{dt}\) would be \(\displaystyle \frac{volt}{second}\)
 
Mathmasteriw said:
Assuming the 'OP' did specify the unit of time was ''seconds'', then would that be correct?
Click to expand...
No, the units will be the units of dv divided by the units of dt. I posted what these units are in post #6. Read it!
 
Jomo said:
No, the units will NOT be volts.

dv/dt ~ (change in v)/(change in time) = (change in volts)/(change in time) = (change in volts)/ (change in seconds) = volts/second. That is the units of dv/dt is volts/sec
Click to expand...
Just a little nit-pick:

While specifying "unit" we do not use plural. Thus

units of dv/dt is volts/sec
 
You must log in or register to reply here.
Top