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Helpp A Level Math (C4) Differential equations?! edexcel

Plz help with this maths question!!

'An object is moving so that its velocity is inversely proportional to its displacement at a fixed point. Write this as a differential equation, using k as the constant'
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Avatar for Zacken
Zacken
22
Original post by family'sfailure
Plz help with this maths question!!

'An object is moving so that its velocity is inversely proportional to its displacement at a fixed point. Write this as a differential equation, using k as the constant'


Velocity is dx/dt where x is displamenent.

So dx/dt = k/x.
Velocity is dx/dt where x is displamenent.

Thank you for the answer!!

But how do you know that velocity is displacement with respect to time (dx/dt)?
Original post by family'sfailure
Plz help with this maths question!!

'An object is moving so that its velocity is inversely proportional to its displacement at a fixed point. Write this as a differential equation, using k as the constant'


Moved this to the Maths forum for you :h:
Original post by Zacken
Velocity is dx/dt where x is displamenent.

So dx/dt = k/x.


Also as the general solution, I am getting

s = 2(kt+c)^0.5 which is wrong but I can't work out why?
Original post by family'sfailure
Velocity is dx/dt where x is displamenent.

Thank you for the answer!!

But how do you know that velocity is displacement with respect to time (dx/dt)?


Not so formally: Speed=Distance/Time
Reply 6
Avatar for Zacken
Zacken
22
Original post by family'sfailure
Also as the general solution, I am getting

s = 2(kt+c)^0.5 which is wrong but I can't work out why?


1. You should know that velocity is the rate of change of displacement with respect to time.

2. Why do you think what you've written is wrong?
Original post by Zacken
1. You should know that velocity is the rate of change of displacement with respect to time.

2. Why do you think what you've written is wrong?


This is on MyMaths so it marks the answer straight away and it says it's wrong
Original post by family'sfailure
This is on MyMaths so it marks the answer straight away and it says it's wrong


Send a picture of the question.
IMG_1375.jpg
Original post by Math12345
Send a picture of the question.
Original post by family'sfailure
IMG_1375.jpg


You entered (2kt+2c)0.5(2kt+2c)^{0.5} which is not the same as 2(kt+c)0.52(kt+c)^{0.5}.

2(kt+c)0.5=40.5∗(kt+c)0.5=(4kt+4c)0.52(kt+c)^{0.5}= 4^{0.5} * (kt+c)^{0.5} = (4kt+4c)^{0.5}.
(edited 7 years ago)
Reply 11
Avatar for Zacken
Zacken
22
Original post by family'sfailure
IMG_1375.jpg


It's asking you to find the value of k...

You know that v = k/x so when v=2, x=3 means that 2 = k/3 means that k = 6.

Hence your answer to the next part should not contain k.

Further to this, s=2(kt+c)1/2s = 2(kt +c)^{1/2} is not equal to (2kt+2c)1/2(2kt + 2c)^{1/2}.

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