# Proof of relativity formula

Samkiwi
Homework Statement:
I'm having trouble finding the proof of the relativistic acceleration formula starting from the velocity formula, I've been working on it for a long time but I can't solve this question. :)
Relevant Equations:
electromagnetism and relativity
It is an electron initially pushed by the action of the electric field. The vectors of force and velocity are parallel to each other.

Here's the question

A possible expression of speed as a function of time is the following:

$$v(t) = \frac{At}{\sqrt{1 + (\frac{At}{c})^2}}$$

where is it $$A =\frac{qE}{m}$$
Taking into account that  can be written in the equivalent form.
$$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$

verify by deriving and substituting that the function v (t) defined by  satisfies 

Homework Helper
Gold Member
2022 Award
What do you get when you differentiate ##v(t)##?

Samkiwi
i get this

$$\frac{dv}{dt}=(1+(\frac{At}{c})^{2})^{-\frac{3}{2}}(\frac{At}{c})^2$$

Homework Helper
Gold Member
2022 Award
i get this

$$\frac{dv}{dt}=(1+(\frac{At}{c})^{2})^{-\frac{3}{2}}(\frac{At}{c})^2$$
You need to use the product rule. I'm not sure how you got that. As a first step you should have something like:
$$\frac{dv}{dt} = \frac{A}{\sqrt{1 + (\frac{At}{c})^2}} + \dots$$

• Samkiwi
Samkiwi
and i know that

$$A=\frac{qE}{m}$$
and
$$At=\frac{qE}{m}t$$

$$F=maϒ^3$$

then

$$A=aϒ^3$$

Samkiwi
yes yes I was wrong I considered t a constant, when in reality it is a variable
You need to use the product rule. I'm not sure how you got that. As a first step you should have something like:
$$\frac{dv}{dt} = \frac{A}{\sqrt{1 + (\frac{At}{c})^2}} + \dots$$
yes yes I was wrong I considered t a constant, when in reality it is a variable

• PeroK
Samkiwi
You need to use the product rule. I'm not sure how you got that. As a first step you should have something like:
$$\frac{dv}{dt} = \frac{A}{\sqrt{1 + (\frac{At}{c})^2}} + \dots$$
I obtain this

$$\frac{dv}{dt}=\frac{Ac^{3}}{(c^{2}+A^{2}t^{2})\sqrt{c^{2}+A^{2}t^{2}}}$$

and now?  Homework Helper
Gold Member
2022 Award
I obtain this

$$\frac{dv}{dt}=\frac{Ac^{3}}{(c^{2}+A^{2}t^{2})\sqrt{c^{2}+A^{2}t^{2}}}$$

and now?  That's correct, although you've changed the format of the term ##1 + (\frac{At}{c})^2## to ##c^2 + (At)^2##.

You can simplify what you have to the correct answer. But, it's not good technique to change things that start in the format required for the final answer to something else that looks different.

• Samkiwi
Samkiwi
okay, now I try, anyway thank you very much yes it was very useful I don't know how to thank you
That's correct, although you've changed the format of the term ##1 + (\frac{At}{c})^2## to ##c^2 + (At)^2##.

You can simplify what you have to the correct answer. But, it's not good technique to change things that start in the format required for the final answer to something else that looks different. Homework Helper
Gold Member
2022 Award
okay, now I try, anyway thank you very much yes it was very useful I don't know how to thank you
I see now there is a further step needed. What you should get, when you simplify that expresson is:
$$\frac{dv}{dt} = A(1 + (\frac{At}{c})^2)^{-\frac 3 2 }$$
And, what you are aiming for is: $$\frac{dv}{dt} = A(1 - (\frac{v}{c})^2)^{\frac 3 2 }$$
That means that you to show: $$\frac{1}{1 + (\frac{At}{c})^2} = 1 - (\frac{v}{c})^2$$

• Samkiwi
Samkiwi
I see now there is a further step needed. What you should get, when you simplify that expresson is:
$$\frac{dv}{dt} = A(1 + (\frac{At}{c})^2)^{-\frac 3 2 }$$
And, what you are aiming for is: $$\frac{dv}{dt} = A(1 - (\frac{v}{c})^2)^{\frac 3 2 }$$
That means that you to show: $$\frac{1}{1 + (\frac{At}{c})^2} = 1 - (\frac{v}{c})^2$$
I didn't understand how I should demonstrate the last step

Homework Helper
Gold Member
2022 Award
I didn't understand how I should demonstrate the last step
It's a relatrively simple piece of algebra. Just calculate, using the expression you have for ##v##, the quantity: $$1 - \frac{v^2}{c^2}$$

• Samkiwi
Samkiwi
It's a relatrively simple piece of algebra. Just calculate, using the expression you have for ##v##, the quantity: $$1 - \frac{v^2}{c^2}$$
I may sound silly, but would you explain each step please, sorry I'm nagging

Homework Helper
Gold Member
2022 Award
I may sound silly, but would you explain each step please, sorry I'm nagging
You have to make an effort yourself. The first step is easy, so I'll do that: $$v^2 = \frac{(At)^2}{1 + (\frac{At}{c})^2}$$

• Samkiwi
Samkiwi
You have to make an effort yourself. The first step is easy, so I'll do that: $$v^2 = \frac{(At)^2}{1 + (\frac{At}{c})^2}$$
and after?

Homework Helper
Gold Member
2022 Award
and after?
That's up to you.

Samkiwi
That's up to you.
I'm not understanding anything anymore

differentiating I found this

$$\frac{dv}{dt}=A\frac{c^{6}}{(\sqrt{c^{2}+v^{2}})^{3}}$$

or this

$$\frac{dv}{dt}=Ac^{3}(1+\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$

Homework Helper
Gold Member
2022 Award
If $$v^2 = \frac{(At)^2}{1 + (\frac{At}{c})^2}$$ then what is $$1 - \frac{v^2}{c^2}$$?

Samkiwi
If $$v^2 = \frac{(At)^2}{1 + (\frac{At}{c})^2}$$ then what is $$1 - \frac{v^2}{c^2}$$?
$$\frac{c^{2}}{c^{2}+A^{2}t^{2}}$$

and now?

Homework Helper
Gold Member
2022 Award
That's it, isn't it? Apart from changing the format of the expresson.

• Samkiwi
Samkiwi
That's it, isn't it? Apart from changing the format of the expresson.
no, the form of the expression asking the question is this
$$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$

Homework Helper
Gold Member
2022 Award
no, the form of the expression asking the question is this
$$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$
See post #10. Note that ##A = \frac{qE}{m}##.

• Samkiwi
Homework Helper
Gold Member
2022 Award
no, the form of the expression asking the question is this
$$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$
You should check this, as it should be $$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{\frac{3}{2}}$$
PS you can see that it must be this, as we need ##\frac{dv}{dt} \rightarrow 0## as ##v \rightarrow c##.

• Samkiwi
Samkiwi
$$You should check this, as it should be$$\frac{dv}{dt}=\frac{qE}{m}(1-\frac{v^{2}}{c^{2}})^{\frac{3}{2}}$$PS you can see that it must be this, as we need ##\frac{dv}{dt} \rightarrow 0## as ##v \rightarrow c##. I was able to find this formula$$\frac{dv}{dt}=A(1+\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$the only problem is this part$$(1+\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$I tried what you said. but I don't solve anything like that. i should make sure that.$$(1+\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}=(1-\frac{v^{2}}{c^{2}})^{\frac{3}{2}}$$but how do I prove it? if I substitute v for the initial formula, I get$$\frac{\sqrt{(c^{2}+v^{2})^{3}}}{\sqrt{(c^{2}+2v^{2})^{3}}}$$Last edited: Science Advisor Homework Helper Gold Member 2022 Award I was able to find this formula$$\frac{dv}{dt}=A(1+\frac{v^{2}}{c^{2}})^{-\frac{3}{2}}$$That can't be right. Just take the limit as ##v \rightarrow c##. Instead, we got as far as this: If$$v^2 = \frac{(At)^2}{1 + (\frac{At}{c})^2}$$then what is$$1 - \frac{v^2}{c^2}?
You need to pick up from there.

• Samkiwi
Samkiwi
That can't be right. Just take the limit as ##v \rightarrow c##.

Instead, we got as far as this:

You need to pick up from there.
ok he succeeded, thanks a lot