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This question has been bugging me for years, but I could not find a satisfactory answer myself.

When one is cornering on a bicycle, one leans into the corner in order to generate the centripetal force necessary to do the actual cornering. By leaning, the center of mass is lowered, so the potential energy is decreased.

But where does this energy go? I cannot think of any work that is done in the direction of the movemement of the center of mass.

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Summary

The decrease in your potential energy is compensated by the increase in your rotational kinetic energy.

Setup

First let's analyze a similar scenario in another context to build the intuition needed to understand the original problem. Imagine a horizontal rod spinning around a stationary axis and you are hanging at the farther end of the rod. Let's say that now you want to get closer to the axis of rotation and thus you try to pull yourself closer to the axis, however you feel an opposing force, the centrifugal force (a pseudo force encountered in the rotating frame of reference whose magnitude is $m\omega ^2 r$). So you need to do some work against this force to get closer to the axis.

Angular Momentum

But the question arises, where does this work done go? The answer is that this work done contributes to the change in kinetic energy. How? See, when you changed your position, one thing remained constant throughout the change, which is the angular momentum. This is famously stated as the law of conservation of angular momentum. It implies that the final angular momentum is the same as the initial angular momentum, that is

$$\mathrm d (I \omega)=0\quad \Rightarrow \quad I_{\text{initial}}\omega_{\text{initial}}=I_{\text{final}}\omega_{\text{final}}\tag{1}$$

where $I$ is the moment of inertia and $\omega$ is the angular velocity. In the above case, since you came closer to the axis, the moment of inertia decreased and thus the angular velocity increased while keeping the angular momentum constant

Energy

From now on, I will denote "final" as "f" and initial as "i".

However, the energy was not conserved during this change, which was expected since you did some non zero positive work to get closer to the axis. So, the change in the energy is

$$\Delta E =\frac 1 2 I_\text{f}\omega^2_\text{f}-\frac 1 2 I_\text{i}\omega^2_\text{i} \tag{2}$$

Since $I_{\text{i}}\omega_{\text{i}}=I_{\text{f}}\omega_{\text{f}}$, thus $(2)$ simplifies to

$$\Delta E =\frac 1 2 I_\text{i} \omega_\text{i}(\omega_\text{f}-\omega_\text{i})$$

Now since $\omega_\text{f}>\omega_\text{i}$, thus $\Delta E>0$. This positive energy change, which was manifested as increase in the rotational kinetic energy, is due to the work done by you against the centrifugal force.

The bicycle turn case

You'd have already observed the analogy and the connection between the above example and the bicycle turning case. In the turning case, the friction is almost always acting along the center of curvature of the turning circle, so we can safely conserve the angular momentum of the cyclist about the center of curvature (in reality, there would be a tangential component of friction which would reduce the angular momentum and the rotational kinetic energy, however it's negligible for a fully inflated tire and we can ignore it). And again, in this case as you lean, you get closer to the axis passing through the center of curvature, and the moment of inertia of the system decreases, so the final angular velocity increases and so does the final rotational kinetic energy. And thus, you speed up while turning around the circle.

Difference

However, there is a subtle difference in the bicycle case, there is a torque due to gravity, however this torque does not stop us from conserving angular momentum since it is perpendicular to the angular velocity. This case is analogous to a particle moving in uniform circular motion. The gravity's torque only changes the direction of the angular momentum and does not change its magnitude, thus as long as we are only concerned with the magnitude of the angular momentum, which we are, we need not worry about the gravity's torque. However, if you are interested, then you can refer to Wikipedia's page on precession for further reading. So it is precession which makes your bicycle go around a corner/turn after you tilt it and it is the law of angular momentum conservation which makes your bicycle speed up once you turn it. This scenario is very similar to its translational analog, a ball (connected by a thread) moving uniformly in a 2D circle initially and then you reduce (change) its radius

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I'm amazed that I have never asked this question myself. The most likely explanation is that you speed up a little when you lean over to turn (assuming you're not using the brakes).

Especially at low speed, I have often felt like leaning over to do a tight turn seems to cause the bike to speed up, but it never occurred to me that this was a real effect. It would be a good intro physics experiment to try to measure this using a speedometer attached to the wheel and some way of measuring the angle of lean.

Except at low speed, the effect would likely be small because the change in potential energy is not large compared to the kinetic energy of a bike.

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    $\begingroup$ I had considered that myself, but I cannot find a way to explain how that would work, since the directions of the forward velocity and the movement of the center of mass are orthogonal. $\endgroup$
    – slingeraap
    Apr 24 '20 at 15:14
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    $\begingroup$ @slingeraap The bike is turning, so there is no single "forward". If while moving north, you lean left, you increase the westward speed. The bike then moves west, so this is consistent. $\endgroup$
    – BowlOfRed
    Apr 24 '20 at 17:46
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Judging from the last line of your confusion, I think you have a slight confusion about the nature of energy. Energy is a scalar quantity and not a vector quantity. The total energy has to be conserved, but not in the directions.

A simple demonstration is a block sliding down an inclined plane. The potential energy decreases in the direction of the field (i.e. straight downwards) but the kinetic energy increases in a direction inclined to the field. Still, kinetic energy and potential energies are conserved.

All that really matters is that the sum of $E_\mathrm k$ and $E_\mathrm p$ should be conserved at the end of the day (or second, or minute). Now if the velocity of the bicycle increases, it will compensate for the decrease in height of the centre of mass. But as @taciteloquence noted, this decrease is probably very less.

For safe turn,

$$\theta=\tan^{-1}\left(\frac{v^2}{Rg}\right)$$ change in height $$\Delta h=h\left(1-\cos\left(\tan^{-1}\left(\frac{v^2}{Rg}\right)\right)\right)$$ change in potential energy $$mg\,\Delta h=mgh\left(1-\cos\left(\tan^{-1}\left(\frac{v^2}{Rg}\right)\right)\right)$$

For realistic values of $v=5.6\ \mathrm{m\ s^{-1}}$, $R=4\ \mathrm m$ and $h=1\ \mathrm m$, you will get at most, an increment of $0.43\ \mathrm{m\ s^{-1}}$ which is probably unnoticed. This is further reduced if friction is present to help you.

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    $\begingroup$ 1.5 m/s is more than 25% of the speed of the bicycle. Do you really think that amount of increase would be unnoticed? It seems much too large. With your numbers I get an increase of 0.45 m/s, less than 10% of the original speed (but still more than I would have expected!) $\endgroup$
    – brendan
    Apr 25 '20 at 23:24
  • $\begingroup$ @brendan I 'm so sorry. I was in a hurry , so got the calculations wrong. By the way, 0.45 m/s is the increase in speed when we neglect friction. With friction it would be reduced further. I'll try to add that too. Thanks for pointing out the error $\endgroup$
    – Elendil
    Apr 26 '20 at 5:00
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I am a bit late to the party, but in addition to all the other answers, I have to add that the force that produces the acceleration is friction between the wheels and the ground. In the initial (straight line) and final (turn with constant radius) states, friction cannot produce work. However, in the transition between these two states, a lot of things happen. Just presume that you are first going north in a straight line. Then, you lean into a turn and do a 360 degrees turn, effectively returning to going north, but still turning. As mentioned in the other answers, you are now going faster than you were originally. Assuming negligible rolling resistance and air resistance (which would slow you down), the only other force exerted in the horizontal plane is friction. There is no choice that this friction, that has been manipulated by leaning the bike, is the source of your acceleration by Newton's second law. Conservation of angular momentum provides a good way of looking at it, and is very convenient, but in the end, Newton's second law must be true. To get an acceleration, you need a force.

Of note, countersteering is the most efficient way to enter into a turn on a bicycle. By countersteering, you effectively turn the handlebar left to turn right, which allows you to easily lean into the turn. Once you lean at the right angle, you turn the handlebar back in the direction you want to turn. When you countersteer, your handlebar points away from where you are going. This produces forces perpendicular to your wheels that can accelerate the bicycle.

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Never thought about it before interesting.I think,as you lean,there is producing moment about center of mass which gives you angular acceleration for short time interval.And as turning radius gets shorter,required centripetal acceleration becomes higher which will be balanced by friction force from ground.That friction force is what equilibrate that moment about center of mass preventing you from fall. But at that tiny interval,produced moment is perpendicular to angular momentum of wheels. It possible that wheels behave like gyroscopes and by the perpendicular acting torque,angular momentum vector changes its direction but we dont experience angular acceleration because its being absorbed by frictional forces.And that work is being done by that lost potential energy.By the way,not only direction,but also magnitude is changing because torque doesnt change its direction while angular momentum vector does.If theta is angle between moment vector and momentum vector,then MCos(theta) will give you magnitude change per time.You can find increment in angular momentum by integrating [MCos(theta)*dt] just from my point of view.

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