How to Calculate the Braking Momentum on a Wheel?

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The discussion focuses on calculating the torque produced by brake pads on a hollow-cylinder wheel model. The user seeks to express this torque, initially referring to it as momentum, which leads to clarification that torque is the correct term. Key equations for torque and moment of inertia are shared, emphasizing the relationship between force, distance from the axis, and angular acceleration. The conversation also touches on the effects of static friction and the importance of considering forces acting on the wheel, especially in the context of a vehicle's braking system. The user plans to explore how various factors, including temperature and friction, influence stopping distance.
  • #91
jbriggs444 said:
Right. But since you want to be able to express everything in terms of ##x## and get rid of ##\theta## and ##\omega##, you should be thinking about ##(x-x_0)\frac{R_b}{R_\text{ext}}##.

Maybe think about defining a variable name for the ratio of energy dissipated per distance travelled. And then make the intuitive leap: "That is another word for force".
Ok, I tried something but I'm kind of stuck now...
##\big[2F_BR_B\theta\big] = \big[E\big]##
So for "ratio of energy dissipated per distance traveled :
##\Big[\frac{2F_BR_B\theta}{(x-x_0)}\Big] = \big[F\big]##

But while the car travels to the positive ##x## values, ##\theta## gets negative do ##\Delta x = \frac{-\theta}{2\pi}R_\text{ext}##
So ##\frac{2F_BR_B\theta}{(x-x_0)} = \frac{-4\pi F_BR_Bx}{(x-x_0)R_\text{ext}}##

From here I see ##\frac{R_B}{R_\text{ext}}## but ##(x-x_0)## isn't at the right place...
 
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  • #92
ROOT0X57B said:
Ok, I tried something but I'm kind of stuck now...
##\big[2F_BR_B\theta\big] = \big[E\big]##
So for "ratio of energy dissipated per distance traveled :
##\Big[\frac{2F_BR_B\theta}{(x-x_0)}\Big] = \big[F\big]##
What I have in mind is something that you already almost have. Let's start over with the formula for kinetic energy.$$KE=\frac{1}{2}Mv^2 + n_\text{wheels} \frac{1}{2}I \omega^2$$Now we want to get that ##\omega## out of there and use ##v## instead. So we make the substitution of ##\frac{v}{R_\text{ext}}## for omega:$$KE=\frac{1}{2}Mv^2 + n_\text{wheels} \frac{I v^2}{2 {R_\text{ext}}^2}$$We should also take the opportunity to express that ##I## as ##\frac{1}{2}m_\text{wheel}{R_\text{ext}}^2##. After a bit of cancelling, that results in:$$KE=\frac{1}{2}Mv^2 + n_\text{wheels} \frac{m_\text{wheel} v^2}{4}$$Now this is a good equation. We can put it in a more familiar-seeming form:$$KE=\frac{1}{2}(M + n_\text{wheels}m_\text{wheel}/2)v^2$$If we use ##M_e## (equivalent mass) as shorthand for ##M + n_\text{wheels}m_\text{wheel}/2## then we can rewrite that formula as:$$KE=\frac{1}{2}M_ev^2$$
Now let's flip over to the work side of the equation. We start with the equation for work extracted from the car+wheels assembly as a function of rotation angle ##\theta##.$$W= -n_\text{wheels} F_B R_B \theta$$But we do not want this in terms of ##\theta##. We want it in terms of ##x##. Or, ##x-x_0## to be more precise. Fortunately, ##\theta = \frac{x-x_0}{R_\text{ext}}##. So we have:$$W = -n_\text{wheels} F_B R_B \frac{x-x_0}{R_\text{ext}}$$We can make that more familiar-looking by rearranging this as:$$W = -(n_\text{wheels} F_B \frac{R_B}{R_\text{ext}}) (x - x_0)$$If we use ##F_e## (equivalent force) as shorthand for ##n_\text{wheels} F_B \frac{R_b}{R_\text{ext}}## then we have:$$W = -F_e (x-x_0)$$.
That is what I had in mind when I suggested energy per unit distance.

Now you are almost home. You invoke the work-energy theorem and almost immediately write:$$\frac{1}{2}M_ev^2 - \frac{1}{2}M_e{v_0}^2 = -F_e (x-x_0)$$
 
  • #93
I'm sorry I missed that thread earlier.

If you are looking for a distance, you start with an equation that has a distance in it. You want to relate it to a force? We have a simple one that uses both, the definition of work:
$$\Delta E = F\Delta x$$
Where ##F## is an applied force, for a distance ##\Delta x##, that will require (or dissipate) an amount of energy ##\Delta E##. So:
$$ \Delta x = \frac{\Delta E}{F}$$
How can we relate this to a braking vehicle? Obviously, the force ##F## is the braking force. For the energy, we know the car will change speed during braking, thus changing its kinetic energy ##\frac{1}{2}mv^2##.

But what is the braking force? Well if we assume we are braking with the friction force created by all the tires supporting the car and that all tires have equal characteristics, then the braking force is ##\mu mg##. So:
$$\Delta x = \frac{\frac{1}{2}m(v_f^2 - v_0^2)}{\mu mg}$$
Or:
$$\Delta x = \frac{1}{2}\frac{v_f^2 - v_0^2}{\mu g}$$
That is the base in a very simplistic manner. The stopping distance required for the car speed to drop from ##v_f## to ##v_0## only depends on the tire-road coefficient of friction ##\mu##. We don't even know how the torque is applied to wheels, we just know that it creates a friction force; a friction force that has an upper limit set by ##\mu##.

But let's elaborate on the equation a little bit further.

First with the energy of the car. A car has also parts that rotate, like the wheels but also the braking system, driveshaft, transmission gears, etc. These parts will also reduce their RPM as the car slows down, which means more energy to dissipate. The good news is that it will be in proportion with the total mass of the car, thus introducing the concept of equivalent mass ##m_e## as shown in post #92. I also suggest this article for a deeper study on the subject. So, to resume, we get:
$$E = \frac{1}{2}m_ev^2$$
Where ##m_e = \lambda m##, and ##\lambda = 1 + \frac{\sum_x I_x g_{r\ x}}{mr_t}##.

##\lambda## depends on the summation of the product of the rotational inertia ##I_x## and gear ratio ##g_{r\ x}## with respect to the wheel for all rotating component ##x##, and the tire radius ##r_t##. (see the article for the derivation).

Second, let's look at the braking force. Yes, there is the tire-road friction force ##F_t##, but there is also the drag ##F_D## and the rolling resistance ##F_R##. So now ##F = F_t + F_D + F_R##.

Oh shoot! ##F_D## has the car speed in it (##\frac{1}{2}\rho C_D A v^2##). We now need to use the differential version and integrate:
$$F(v)dx = m_e vdv$$
Or:
$$\Delta x = \int_{v=v_0}^{v_f} \frac{m_e v}{F_t + \frac{1}{2}\rho C_D A v^2 + F_R} dv$$
(It's a fun one!)

Now, you can use the maximum tire-road friction force as ##F_t## to get the minimum braking distance required to slow down the car. But what if the braking system is unable to make enough torque to create that maximum friction force? Obviously, the distance will be greater. And now we're getting into the heart of your problem.

I'm going to let you digest this introduction and see where you want to go from here.
 
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  • #94
jack action said:
I'm sorry I missed that thread earlier.

If you are looking for a distance, you start with an equation that has a distance in it. You want to relate it to a force? We have a simple one that uses both, the definition of work:
$$\Delta E = F\Delta x$$
Where ##F## is an applied force, for a distance ##\Delta x##, that will require (or dissipate) an amount of energy ##\Delta E##. So:
$$ \Delta x = \frac{\Delta E}{F}$$
How can we relate this to a braking vehicle? Obviously, the force ##F## is the braking force. For the energy, we know the car will change speed during braking, thus changing its kinetic energy ##\frac{1}{2}mv^2##.

But what is the braking force? Well if we assume we are braking with the friction force created by all the tires supporting the car and that all tires have equal characteristics, then the braking force is ##\mu mg##. So:
$$\Delta x = \frac{\frac{1}{2}m(v_f^2 - v_0^2)}{\mu mg}$$
Or:
$$\Delta x = \frac{1}{2}\frac{v_f^2 - v_0^2}{\mu g}$$
That is the base in a very simplistic manner. The stopping distance required for the car speed to drop from ##v_f## to ##v_0## only depends on the tire-road coefficient of friction ##\mu##. We don't even know how the torque is applied to wheels, we just know that it creates a friction force; a friction force that has an upper limit set by ##\mu##.

But let's elaborate on the equation a little bit further.

First with the energy of the car. A car has also parts that rotate, like the wheels but also the braking system, driveshaft, transmission gears, etc. These parts will also reduce their RPM as the car slows down, which means more energy to dissipate. The good news is that it will be in proportion with the total mass of the car, thus introducing the concept of equivalent mass ##m_e## as shown in post #92. I also suggest this article for a deeper study on the subject. So, to resume, we get:
$$E = \frac{1}{2}m_ev^2$$
Where ##m_e = \lambda m##, and ##\lambda = 1 + \frac{\sum_x I_x g_{r\ x}}{mr_t}##.

##\lambda## depends on the summation of the product of the rotational inertia ##I_x## and gear ratio ##g_{r\ x}## with respect to the wheel for all rotating component ##x##, and the tire radius ##r_t##. (see the article for the derivation).

Second, let's look at the braking force. Yes, there is the tire-road friction force ##F_t##, but there is also the drag ##F_D## and the rolling resistance ##F_R##. So now ##F = F_t + F_D + F_R##.

Oh shoot! ##F_D## has the car speed in it (##\frac{1}{2}\rho C_D A v^2##). We now need to use the differential version and integrate:
$$F(v)dx = m_e vdv$$
Or:
$$\Delta x = \int_{v=v_0}^{v_f} \frac{m_e v}{F_t + \frac{1}{2}\rho C_D A v^2 + F_R} dv$$
(It's a fun one!)

Now, you can use the maximum tire-road friction force as ##F_t## to get the minimum braking distance required to slow down the car. But what if the braking system is unable to make enough torque to create that maximum friction force? Obviously, the distance will be greater. And now we're getting into the heart of your problem.

I'm going to let you digest this introduction and see where you want to go from here.
If anything I suspect this will be a bit of an eye opener to the actual complexity of this problem. Perhaps, stopping on dry road will be complex enough for their project. You kind of cut to the chase, ( I think ) we were "teaching them to fish". But the cat is out of the bag now, so to speak.
 
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  • #95
erobz said:
You kind of cut to the chase, ( I think ) we were leading them to water. But the cat is out of the bag now, so to speak.
No, I began at the starting point: I want to know the braking distance. Now we can detail and complicate this equation as much as we want such that we reach a 156 hours project.
 
  • #96
jack action said:
No, I began at the starting point: I want to know the braking distance. Now we can detail and complicate this equation as much as we want such that we reach a 156 hours project.

jack action said:
If you are looking for a distance, you start with an equation that has a distance in it. You want to relate it to a force? We have a simple one that uses both, the definition of work:
ΔE=FΔx
Where F is an applied force, for a distance Δx, that will require (or dissipate) an amount of energy ΔE. So:
Δx=ΔEF
How can we relate this to a braking vehicle? Obviously, the force F is the braking force. For the energy, we know the car will change speed during braking, thus changing its kinetic energy 12mv2.

But what is the braking force? Well if we assume we are braking with the friction force created by all the tires supporting the car and that all tires have equal characteristics, then the braking force is μmg. So:
Δx=12m(vf2−v02)μmg
Or:
Δx=12vf2−v02μg
That is the base in a very simplistic manner. The stopping distance required for the car speed to drop from vf to v0 only depends on the tire-road coefficient of friction μ. We don't even know how the torque is applied to wheels, we just know that it creates a friction force; a friction force that has an upper limit set by μ.
Yeah... when I said that earlier, I was referring to the portion of your response above "will not take 156 hrs". I said if you can't produce this model ## \uparrow ## your not going to produce the other models.

We were trying to explain to them that even stopping on dry road is significantly more complex than it first appears and there might not be a need to add extra complexities of "water drag" on the tires etc...

I was planning on asking them to think of extra possible discrete additions of complexity as the model progressed. The OP has not entered college yet.
 
  • #97
This is just pure speculation, but I'd imagine if we actually considered the full complexity of the automobile and all its systems one might spend a few years modeling stopping on a dry road! :smile:
 
  • #98
jack action said:
I'm sorry I missed that thread earlier.

If you are looking for a distance, you start with an equation that has a distance in it. You want to relate it to a force? We have a simple one that uses both, the definition of work:
$$\Delta E = F\Delta x$$
Where ##F## is an applied force, for a distance ##\Delta x##, that will require (or dissipate) an amount of energy ##\Delta E##. So:
$$ \Delta x = \frac{\Delta E}{F}$$
How can we relate this to a braking vehicle? Obviously, the force ##F## is the braking force. For the energy, we know the car will change speed during braking, thus changing its kinetic energy ##\frac{1}{2}mv^2##.

But what is the braking force? Well if we assume we are braking with the friction force created by all the tires supporting the car and that all tires have equal characteristics, then the braking force is ##\mu mg##. So:
$$\Delta x = \frac{\frac{1}{2}m(v_f^2 - v_0^2)}{\mu mg}$$
Or:
$$\Delta x = \frac{1}{2}\frac{v_f^2 - v_0^2}{\mu g}$$
That is the base in a very simplistic manner. The stopping distance required for the car speed to drop from ##v_f## to ##v_0## only depends on the tire-road coefficient of friction ##\mu##. We don't even know how the torque is applied to wheels, we just know that it creates a friction force; a friction force that has an upper limit set by ##\mu##.

But let's elaborate on the equation a little bit further.

First with the energy of the car. A car has also parts that rotate, like the wheels but also the braking system, driveshaft, transmission gears, etc. These parts will also reduce their RPM as the car slows down, which means more energy to dissipate. The good news is that it will be in proportion with the total mass of the car, thus introducing the concept of equivalent mass ##m_e## as shown in post #92. I also suggest this article for a deeper study on the subject. So, to resume, we get:
$$E = \frac{1}{2}m_ev^2$$
Where ##m_e = \lambda m##, and ##\lambda = 1 + \frac{\sum_x I_x g_{r\ x}}{mr_t}##.

##\lambda## depends on the summation of the product of the rotational inertia ##I_x## and gear ratio ##g_{r\ x}## with respect to the wheel for all rotating component ##x##, and the tire radius ##r_t##. (see the article for the derivation).

Second, let's look at the braking force. Yes, there is the tire-road friction force ##F_t##, but there is also the drag ##F_D## and the rolling resistance ##F_R##. So now ##F = F_t + F_D + F_R##.

Oh shoot! ##F_D## has the car speed in it (##\frac{1}{2}\rho C_D A v^2##). We now need to use the differential version and integrate:
$$F(v)dx = m_e vdv$$
Or:
$$\Delta x = \int_{v=v_0}^{v_f} \frac{m_e v}{F_t + \frac{1}{2}\rho C_D A v^2 + F_R} dv$$
(It's a fun one!)

Now, you can use the maximum tire-road friction force as ##F_t## to get the minimum braking distance required to slow down the car. But what if the braking system is unable to make enough torque to create that maximum friction force? Obviously, the distance will be greater. And now we're getting into the heart of your problem.

I'm going to let you digest this introduction and see where you want to go from here.
I think you considered the force to be tire-road friction instead of brake pads-disk friction
 
  • #99
ROOT0X57B said:
I think you considered the force to be tire-road friction instead of brake pads-disk friction
Yes, because that's what is actually governing the maximum braking force ##F_t## you can get.

But you can get a smaller braking force ##F_t## by modulating the brake pads-disk friction. So you can still extend the equation further by setting ##F_t = MIN(\frac{T_B}{r_t}, \mu mg)##, where ##T_B## is the braking torque from all wheels and ##r_t## the tire radius.
 
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  • #100
jack action said:
Yes, because that's what is actually governing the maximum braking force ##F_t## you can get.

But you can get a smaller braking force ##F_t## by modulating the brake pads-disk friction. So you can still extend the equation further by setting ##F_t = MIN(\frac{T_B}{r_t}, \mu mg)##, where ##T_B## is the braking torque from all wheels and ##r_t## the tire radius.
Okay, seems a lot clearer...
How can I have ##T_B## now?

EDIT : It seems like @jbriggs444 answered it earlier in the detailed post
 
  • #102
Okay, as I want to be able to simulate the evolution of the distance over time, I will try to figure out ##\text{d}v##

I worked by myself using all your stuff and a bit with my teacher who validated the main part

I use french physics notations, but it may not be a big deal (##J_\Delta = I## for you and I think that's it)

I stopped here last time :
$$\displaystyle \frac{1}{2}M({v_0}^2-v^2) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$

As ##v = v_0 + \text{d}v## (a small step of speed):
$$\displaystyle \frac{1}{2}M((v+v_0)(v-v_0)) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$
$$\displaystyle \frac{1}{2}M((2v_0+\text{d}v)(\text{d}v)) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + \text O(\text{d}v^2) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$

Because ##\displaystyle\omega = \frac{v}{R_\text{ext}}## :
$$\displaystyle Mv_0\text{d}v + 4 * \frac{1}{2}J_\Delta\bigg(\frac{{v_0}^2}{{R_\text{ext}}^2}-\frac{{v}^2}{{R_\text{ext}}^2}\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + 4 * \frac{1}{2}J_\Delta\bigg(\frac{1}{{R_\text{ext}}^2}\bigg({v_0}^2-v^2 \bigg)\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + 4 J_\Delta\bigg(\frac{v_0 \text{d}v}{{R_\text{ext}}^2}\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$


From here, I have not shown my physics teacher

##\displaystyle v = {R_\text{ext}} \frac{\text{d}\theta}{\text{d}t}## so ##\displaystyle \text{d}v = {R_\text{ext}}\frac{\text{d}\theta}{\text{d}t}## so ##\displaystyle\text{d}\theta = \frac{\text{d}v\text{d}t}{R_\text{ext}}##

As ##\theta = \theta_0 + \text{d}\theta## (a small step of rotation) :
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B\bigg(\theta_0 + \frac{\text{d}v \text{d}t}{R}\bigg)$$
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B\frac{\text{d}v}{R}\text{d}t + 2F_BR_B \theta_0$$

So now with a little bit of python programming, I can simulate the distance traveled over time by little incrementations of time ##\text{d}t##
 
  • #103
jack action said:
Second, let's look at the braking force. Yes, there is the tire-road friction force ##F_t##, but there is also the drag ##F_D## and the rolling resistance ##F_R##. So now ##F = F_t + F_D + F_R##.
Actually, I do not count the drag as I deal with relatively slow speeds and my wheel is a perfect cylinder so there is no rolling resistance either
jack action said:
##T_B## is the friction torque produced by your braking system (what you actually started with, in this thread). You might even add a braking force from the engine or electric motor (regenerative braking).
I consider the motor to be inactive as it's the braking time and regenerative braking is none of my topic
 
  • #104
ROOT0X57B said:
So now with a little bit of python programming, I can simulate the distance traveled over time by little incrementations of time
This is a case of constant acceleration. That means that there is a SUVAT equation for this. No need for any differential equations:$$s=v_0t + \frac{1}{2}at^2$$.
Sure, one can do it with Euler integration and Python or your choice of language. But with an analytic solution so close at hand, there seems little point in resorting to numerical methods.

Having a canned solution against which to compare a numerical result might give you a little bit of experience seeing the behavior of different numerical approaches. Here is an article to give you a taste.

http://faculty.olin.edu/bstorey/Notes/DiffEq.pdf

Things get interesting if ##F_B## is a function of ##v## (first order differential equation) or of ##x## (second order).
 
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  • #105
jbriggs444 said:
This is a case of constant acceleration. That means that there a SUVAT equation for this. No need for any differential equations:$$s=v_0t + \frac{1}{2}at^2$$.
Sure, one can do it with Euler integration and Python or your choice of language. But with an analytic solution so close at hand, there seems little point to doing so.
I don't want to solve a differential equation, I want to simulate it with programming :
I will set ##v_0## and the others parameters and will calculate ##v## and ##x## at each point by increments of time small enough so that ##\text{d}v## doesn't change a lot, this will give me a plot of distance traveled over time.
 
  • #106
ROOT0X57B said:
I don't want to solve a differential equation, I want to simulate it with programming :
I will set ##v_0## and the others parameters and will calculate ##v## and ##x## at each point by increments of time small enough so that ##\text{d}v## doesn't change a lot, this will give me a plot of distance traveled over time.
Fair enough. But my mind boggles at the length of the equations you are producing when your main loop should be something simple like:

Vanilla Euler:
10 x = x + v * delta_t
20 v = v + a * delta_t
30 t = t + delta_t
40 print t, v, x
50 go to 10

More accurate:
10 x = x + v * delta_t + 1/2 * a * delta_t^2
20 v = v + a * delta_t
30 t = t + delta_t
40 print t, v, x
50 go to 10

If ##a## is the key input that you need for your main loop then ##a## is the constant that you should be using your equations to solve for. It is dead easy to solve for. ##a=\frac{F_e}{M_e}##. It falls straight out of Newton's second law.
 
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  • #107
Yep, but as sadly it's mandatory for me to explain and detail everything, also I could be questioned about anything I might talk about
 
  • #108
Ok, I worked a bit but I'm stuck with a little problem now

I have $$\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
Let ##M_{eq} = \displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)## (equivalent mass)
Let ##E_B = 2F_BR_B## (energy equivalent for braking)

So I have $$M_{eq} v_0 \text{d}v = E_B \theta$$

My code loop is like
10 t = t+dt
20 theta = theta + d_theta ?
30 dv = some_calculus_here
40 v = v + dv
50 x = x + v * dt

How can I manage to express ##\theta## with ##\text{d}v## so that I no longer have to handle it?

I know ##\displaystyle v = {R_\text{ext}} \frac{\text{d}\theta}{\text{d}t}##
##v = v_0 + \text{d}v##

But I'm stuck here because I don't know how to calculate ##\text{d}\theta## in function of ##\text{d}v## as ##\text{d}v## depends of ##\text{d}\theta## too
 
  • #109
ROOT0X57B said:
Ok, I worked a bit but I'm stuck with a little problem now

I have $$\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
Let ##M_{eq} = \displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)## (equivalent mass)
Let ##E_B = 2F_BR_B## (energy equivalent for braking)

So I have $$M_{eq} v_0 \text{d}v = E_B \theta$$

My code loop is like
10 t = t+dt
20 theta = theta + d_theta ?
30 dv = some_calculus_here
40 v = v + dv
50 x = x + v * dt

How can I manage to express ##\theta## with ##\text{d}v## so that I no longer have to handle it?

I know ##\displaystyle v = {R_\text{ext}} \frac{\text{d}\theta}{\text{d}t}##
##v = v_0 + \text{d}v##

But I'm stuck here because I don't know how to calculate ##\text{d}\theta## in function of ##\text{d}v## as ##\text{d}v## depends of ##\text{d}\theta## too

It appears like you didn't let the angel change in your linearization. You introduced a small change in ##v## but you did not introduce a small change in ##\theta##

ROOT0X57B said:
Okay, as I want to be able to simulate the evolution of the distance over time, I will try to figure out ##\text{d}v##

I worked by myself using all your stuff and a bit with my teacher who validated the main part

I use french physics notations, but it may not be a big deal (##J_\Delta = I## for you and I think that's it)

I stopped here last time :
$$\displaystyle \frac{1}{2}M({v_0}^2-v^2) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$

As ##v = v_0 + \text{d}v## (a small step of speed):
$$\displaystyle \frac{1}{2}M((v+v_0)(v-v_0)) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$
$$\displaystyle \frac{1}{2}M((2v_0+\text{d}v)(\text{d}v)) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + \text O(\text{d}v^2) + 4 * \frac{1}{2}J_\Delta({\omega_0}^2-\omega^2) = 2F_BR_B \theta$$

Because ##\displaystyle\omega = \frac{v}{R_\text{ext}}## :
$$\displaystyle Mv_0\text{d}v + 4 * \frac{1}{2}J_\Delta\bigg(\frac{{v_0}^2}{{R_\text{ext}}^2}-\frac{{v}^2}{{R_\text{ext}}^2}\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + 4 * \frac{1}{2}J_\Delta\bigg(\frac{1}{{R_\text{ext}}^2}\bigg({v_0}^2-v^2 \bigg)\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle Mv_0\text{d}v + 4 J_\Delta\bigg(\frac{v_0 \text{d}v}{{R_\text{ext}}^2}\bigg) + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B \theta$$


From here, I have not shown my physics teacher

##\displaystyle v = {R_\text{ext}} \frac{\text{d}\theta}{\text{d}t}## so ##\displaystyle \text{d}v = {R_\text{ext}}\frac{\text{d}\theta}{\text{d}t}## so ##\displaystyle\text{d}\theta = \frac{\text{d}v\text{d}t}{R_\text{ext}}##

As ##\theta = \theta_0 + \text{d}\theta## (a small step of rotation) :
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B\bigg(\theta_0 + \frac{\text{d}v \text{d}t}{R}\bigg)$$
$$\displaystyle\bigg(M + 4\frac{J_\Delta}{{R_\text{ext}}^2}\bigg)v_0 \text{d}v + \text{O}(\text{d}v^2) = 2F_BR_B\frac{\text{d}v}{R}\text{d}t + 2F_BR_B \theta_0$$

So now with a little bit of python programming, I can simulate the distance traveled over time by little incrementations of time ##\text{d}t##
 
  • #110
Yep, I can't figure out how to get things to work together
 
  • #111
ROOT0X57B said:
Yep, I can't figure out how to get things to work together
Well, the RHS linearizes as:

$$ 2 F_b R_b d \theta $$

and

$$ v = R_{ext} \frac{d \theta}{dt} = R_{ext} \frac{d \theta}{dx} \frac{dx}{dt} = R_{ext} \frac{d \theta}{dx} v $$

This implies:

$$ d \theta = \frac{1}{R_{ext}} dx $$
 
  • #112
Thanks !

Now I have
$$\displaystyle M_{eq}v_0 \text{d}v = 2F_B \frac{R_B}{R_{ext}}dx$$
But I need ##\text{d}v## to calculate ##\text{d}x## and ##\text{d}x## to calculate ##\text{d}v##
I am probably missing something...
 
  • #113
ROOT0X57B said:
Thanks !

Now I have
$$\displaystyle M_{eq}v_0 \text{d}v = 2F_B \frac{R_B}{R_{ext}}dx$$
But I need ##\text{d}v## to calculate ##\text{d}x## and ##\text{d}x## to calculate ##\text{d}v##
I am probably missing something...

$$ dv = a \, dt $$
 
  • #114
Newton's Second Law ?
 
  • #115
ROOT0X57B said:
Newton's Second Law ?
I think @jbriggs444 mentioned:

$$ a = \frac{F_e}{M_e} $$

And you were already intending to use that. I'm getting mixed up on what approach you want to take.

As it stands you have something like:

$$ \frac{dv}{dx} = K $$

What are you after? ##x(t)## ?
 
  • #116
I want to calculate ##\text{d}v## for each ##\text{d}t## so that I can follow to evolution of ##v## and ##x## over time (I don't care about ##\theta##)

My goal is to use a code such as below
(For your understanding, "frein" = "brake", "plaquette" = "brake pad", "roue" = "wheel", "pneu" = "tire")

Do not take care of the expression in dv = ..., it's messed up
code.png


When I run the code, I get this kind of graph (this one is clearly wrong) :
image.png
 
  • #117
I am not going to try to explode that screen image of code to be able to read it. Try pasting it in a [code] block here. For example:
Code:
t = t + dt
 
  • #118
okay, did not really know how to do it...
 
  • #119
ROOT0X57B said:
okay, did not really know how to do it...
Sorry, I was just about to edit the instructions into my previous post.

Start by turning off BB code rendering. You can click on the "[ ]" icon above your editing window to do so. That will let you see the "BB codes" in your post. Try it with this post
Code:
t = t + dt
You should see...
[CODE]
t + dt
[/CODE]

You can also go to Info => Help => BB Codes (https://www.physicsforums.com/help/bb-codes/) for more information than you may care for.
 
  • #120
ROOT0X57B said:
I want to calculate ##\text{d}v## for each ##\text{d}t## so that I can follow to evolution of ##v## and ##x## over time (I don't care about ##\theta##)

My goal is to use a code such as below
(For your understanding, "frein" = "brake", "plaquette" = "brake pad", "roue" = "wheel", "pneu" = "tire")

Do not take care of the expression in dv = ..., it's messed up
View attachment 302283

When I run the code, I get this kind of graph (this one is clearly wrong) :
View attachment 302284
EDIT: it is maybe not that wrong!

I compared the result (stop from 50km/h in 0.5s in 6 meters) with the official numbers : 14 meters
BUUUT, I did not consider the reaction time (which they do)
If I add to the result about 0.8 seconds of driving at 50 km/h (reaction delay is 1s, I take 0.8 to get the time there is little braking), I get 15 meters.
Not that far...

Anyway, here is the code in the image :
[CODE lang="python" title="Code in the image (comments may be wrong)" highlight="43"]# Simulation de la distance de freinage d'une voiture en ville
# Aubin SIONVILLE
# TIPE 2023

# Cette simulation utilise les données d'une Citroën C1 de 2008
# Données disponibles à
# https://www.lacentrale.fr/fiche-technique-voiture-citroen-c1-(2)+1.0+68+confort+3p-2008.html

from utils import *
from matplotlib import pyplot as plt# Définition des constantes
masse = 830 # Masse de la voiture
v0 = kmh_vers_mps(50) # Vitesse initiale : 50 km/h
rayon_pneu, rayon_interieur, largeur_bande = ref_roue_vers_dimensions('155-65R14')
rayon_frein = rayon_interieur * 0.8 # Approximation de R_B

pression_freins = bar_to_pascal(100) # Pression de freinage
surface_plaquette = 0.0057390 # Surface d'une plaquette de frein : 5739 mm² = 0.005739 m²

masse_roue = 6 # Masse de la roue : 6 kg
moment_inertie_roue = (masse_roue * (rayon_pneu**2 - rayon_interieur**2)) / 2 # Moment d'inertie de la roue

x = 0 # Position initiale
v = v0 # Vitesse initiale
t = 0 # Temps initial
theta = 0 # Angle initial

# Il faut un pas de temps pour que dv << v
dt = 0.0001

# On utilise la formule
# (masse + 4 * moment_inertie_roue / rayon_pneu**2) * v0dv = 2(pressions_freins * 2 *surface_plaquette)rayon_frein * theta

points_vitesse = [v0]
points_distance = [0]
points_temps = [0]

while v > limite_zero:
t = t + dt
theta = theta + x / rayon_pneu * dt
dv = dt * (pression_freins * 2 * surface_plaquette * rayon_frein * theta) / (masse + 4 * moment_inertie_roue / rayon_pneu**2)
v = v - dv
x = x + v * dt

points_distance.append(x)
points_vitesse.append(v)
points_temps.append(t)

points_vitesse = [mps_vers_kmh(v) for v in points_vitesse]

fig, ax = plt.subplots()
ax.plot(points_temps, points_distance, color='b')
ax.set_xlabel('Temps (s)')
ax.set_ylabel('Distance (m)', color='b')
ax.tick_params('y', colors='b')

ax2 = ax.twinx()
ax2.plot(points_temps, points_vitesse, 'r')
ax2.set_ylabel('Vitesse (km/h)', color='r')
ax2.tick_params('y', colors='r')

plt.title('Simulation de la distance de freinage d\'une voiture en ville')
plt.show()[/CODE]
 

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