This might look like a homework question, but really isn't. I am teaching myself some basic physics and am shamefully stuck in one place. I have designed the problem below to help myself understand the situation better.

Suppose a motorbike weighing 100 kg is at rest and begins producing a constant force of $F=$ 800 N. The resistance to motion is $R=$ 500 N, so the the net force of 300 N causes acceleration in the positive direction, $a = (F-R)/m = 300/100 = 3$ m/s. The acceleration is constant, so the speed is given by v = at. The power produced is given by $P = Fv = Fat$, so it is increasing linearly over time and in direct proportion to speed. Suppose the bike accelerates for the first 5 seconds and then reaches a certain maximum power output that remains constant. Using the numbers above, the speed reached after 5 s is 15 m/s, and so the maximum power is 12 kW.

If my understanding is correct, at an instant when $P$ becomes constant, $v$ will still be increasing, so $F$ will have to be decreasing in inverse proportion, $F = P/v$. Since $F$ is decreasing, the acceleration will also be decreasing, $a = (F - R)/m$. So we have a three-way circular relationship where to find the function of $F$ we need the function of $v$, to find the function of $v$ we need the function of $a$, and to find the function of $a$, we need the function of $F$. How we find the curves for $F$, $a$ and $v$?

Also, is my understanding correct in that $v$ will drop down until $a = 0$, at which point $F = R$, so the maximum velocity allowed by the maximum power output $= P/F = P/R = $ 12000/500 = 24 m/s ?


The equation you're missing is the direct relationship between power $P$ and kinetic energy $K$. Specifically, for a constant power input, we have that $K=K_0+Pt$ where $K_0$ is the initial kinetic energy. Since we also know the definition of kinetic energy, $K=\frac{1}{2}mv^2$, we can say that


so we can solve for $v$ directly:


Once you have $v$ as a function of time, you can of course solve for the net force and $a$ as well.

One thing you can immediately see, though, is that the vehicle is always accelerating; there is no maximum allowed velocity. This makes sense, because you have required, in your definition of the problem, that there is a constant input of power regardless of speed; in other words, you have required that the kinetic energy of the vehicle always increases at a constant rate. Increasing kinetic energy implies increasing velocity for a vehicle of constant mass, so therefore $a>0$ for all time (though it does approach zero as kinetic energy gets higher and higher).

  • $\begingroup$ thanks for answer, but I am still confused. The function of $v(t)$ you derived implies $v$ is increasing over time. As $v$ is increasing, gross $F$ will be decreasing, since $F=P/v$. When $v$ increases up to a point when $F$ drops down to $R$, $a$ must be 0 as per Newton's 2nd law. So $v$ cannot increase after this point? $\endgroup$
    – Mihael
    Aug 23 '19 at 13:41
  • $\begingroup$ Is it too much to ask to produce graphs like this? I have made the graphs above using your function of $v(t)$, but they don't look right, particularly for $a$. This is for my own understanding only. $\endgroup$
    – Mihael
    Aug 23 '19 at 14:21
  • $\begingroup$ @probably_someone Not all of the power $P$ goes into increasing the kinetic energy of the bike. Some of it is lost in overcoming friction. $\endgroup$
    – gandalf61
    Aug 23 '19 at 15:55

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