“A body is moving unidirectionally under the influence of a source of constant power. Find the relationship between displacement $s$ and time $t$.”

I proved it using dimensional homogeneity. Power has dimensions $${[ML^2T^{-3}]}.$$

As it is constant and mass M is also constant, So, $${[L^2T^{-3}]}$$is constant.

Hence ${L^2}\propto{T^3}$

So, L $\propto{T^{3/2}}$

But this method feels like a trick proof or maybe not work in all situations (for example, here we had constant mass, but in other times we may not have that)

I feel that there is probably a better and concrete method through calculus. Can someone please help me out here.

Any help will be greatly appreciated.

  • 1
    $\begingroup$ I find your solution much better and much more elegant that suggested in the answer. $\endgroup$ – sleepy May 23 at 10:57
  • $\begingroup$ yes there is one method through calculus [commenting so I can get back to it and be forced to answer :)] $\endgroup$ – Adil Mohammed May 23 at 16:18

Is this the sort of thing you're looking for?

If the power is applied from time $t=0$, when the object is at rest, then at time $t$, $$\frac12 m v^2 =Pt.$$ Writing $v=\frac{ds}{dt}$, separating variables and preparing to integrate, $$\int_0^{s_1} ds =\int_0^{t_1} \sqrt{\frac{2Pt}{m}} dt$$ So $$s_1=\frac23 \sqrt{\frac{2P}{m}} t_1^{\ 3/2}$$ We can now drop the '1' subscript. It has served its purpose.

  • $\begingroup$ Thanks, this is exactly what I was looking for! $\endgroup$ – InfiniteCool23 May 24 at 13:42
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    $\begingroup$ I love your dimensions approach, though, as you know, it won't yield dimensionless numerical factors. $\endgroup$ – Philip Wood May 24 at 13:47
  • $\begingroup$ Yes, that's why I wanted to know the calculus way, because I needed to put in real values and get the answer (which would have only been possible by knowig the proportionality constant) $\endgroup$ – InfiniteCool23 May 24 at 13:51

Your method works. You can also do this by considering that power $$P=\vec F \cdot \vec v = Fv$$ since the problem says "unidirectional" meaning $\vec F$ and $\vec v$ are in the same direction. Note also, that $$F=m\frac{v}{t}$$ so that $$P=m\frac{v^2}{t}$$ or $$Pt=mv^2$$ and noting that power and mass are constant, we can write $$v^2=kt$$ for some constant $k$. Remembering that $$v=\frac{x}{t}$$ we can also write $$\frac{x^2}{t^2} = kt$$ or $$x^2=kt^3$$ or $$x \propto t^\frac{3}{2}$$

  • $\begingroup$ How did you write F=mv/t? $\endgroup$ – InfiniteCool23 May 23 at 6:21
  • $\begingroup$ Acceleration a = v/t only when a is constant (w.r.t time) and initial velocity =0 $\endgroup$ – InfiniteCool23 May 23 at 6:21
  • $\begingroup$ Because $F=ma$ and $a=v/t$. $\endgroup$ – joseph h May 23 at 6:22
  • $\begingroup$ If the force is constant, then so is $a$. It also doesn't matter if the the initial velocity was not $0$. You get the exact same result. Try it. $\endgroup$ – joseph h May 23 at 6:24
  • $\begingroup$ Force is not constant. Power is constant $\endgroup$ – InfiniteCool23 May 23 at 6:24

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