MOMENTUM: What is it really, and why it matters

[Ale Vallecchi]

We always hear Marshals and Experts recommend to use "momentum" while driving in the desert.

As this most favorite advice is given, or sometimes shouted through the radio (“BUILD YOUR MOMENTUM”, or “KEEP THE MOMENTUM”) heads bob in confirmation, and a murmur of "yes" and "understood" runs through the briefed convoy.

But how many really know what momentum is?

Do we confuse it with speed, acceleration, or what else?

Without going too much into the physics of it, let us see what we really mean with "momentum".

 

THE WHAT

Momentum is the product of the mass of a particle and its velocity.

It is a vector quantity: i.e., it has both magnitude and direction.

In other words, Momentum is the force that exists in a moving object. To make some examples, a moving truck has more momentum than a car driving at the same speed because the truck has more mass. Similarly, a fast car has more momentum than a slow truck. When you are driving, you and your vehicle have momentum force, based on the total weight of your vehicle and your speed. While you are driving, momentum keeps your vehicle moving, unless it is acted upon by something, such as your brakes, a fixed object (such as a tree), another vehicle.... or the road surface.

Enter our deserts' sand: this shifty, soft surface interferes with momentum.

On a flat track, sand will simply reduce the speed at which you are driving (as the vehicle will tend to sink in it and encounter more friction). The loss of momentum will make the car eventually come to a stop, much quicker than on a harder, more compact tarmac road. Much like a jet ski on water, the more momentum your vehicle has on sand, the smoother it will glide, or 'plane', across the surface. That is why it is important to keep your speed up on sand - without overdoing it.

 

THE WHY

On a dune, we will have to contend not only with the sand’s friction, but also with the infamous force of GRAVITY!! As we all know, on Earth all bodies have a weight, or downward force of gravity which Earth's mass exerts on them, proportional to their mass. The heavier the object (read car), the stronger the downward pull.

An object (car) placed on a tilted surface (dune’s slope) will tend to slide down the surface.

The rate at which the object slides down depends on how tilted the surface is: the greater the tilt of the surface, the faster the rate at which the object will slide down it. In physics, a tilted surface is called an inclined plane (in desert off-roading is called a …. dune). Objects are known to accelerate down inclined planes because of an unbalanced force. Determining the net force acting upon an object (car) on an inclined plane (dune) involves dividing the gravity into two perpendicular components. On inclined planes (dunes_ the force of gravity, in fact, will be equal to the sum of two components - one parallel to the inclined surface and the other one perpendicular to the inclined surface. The diagram below shows how the force of gravity has been replaced by the two components - a parallel and a perpendicular component of force.

Taking a leap from the purely mathematical explanation of these dynamics to a more intuitive representation, (see figure attached) you may think of the right momentum (the car’s mass times its speed) as producing F1, that is the force that “anchors” the car to the slope/dune, being greater than F2. However, removing, or significantly reducing the element of speed, leaves only the weight of your car in play, thus prompting it to slide down the dune.

Clearly, when momentum is lost, F1 will start to become smaller and smaller, and the component of gravity that will prevail will be F2, pulling the car toward the bottom of the slope.

When this happens, you’ll hear the second most shouted command by a Marshal or Expert: DON’T FIGHT GRAVITY (as you’ll never win).

And remember, you’d better not!!!

[Thomas Varghese]

Very clearly explained by @Ale Vallecchi. Just made me recollect my Eng Mechanics during my 1st year in college. It would be even better if @Ale Vallecchi can explain why the car will flip if it fights Gravity and starts to fish tail. I have understood up to the point DON"T FIGHT GRAVITY. This article will be complete if you can give the explanation to why the car will roll over if anybody fights GRAVITY. 

Edited February 1, 2021 by Thomas Varghese

[Frederic]

Fantastic article @Ale Vallecchi !

[Thomas Varghese]

10 minutes ago, Thomas Varghese said:

 

 

Edited February 1, 2021 by Thomas Varghese

[M.Seidam]

11 hours ago, Thomas Varghese said:

Very clearly explained by @Ale Vallecchi. Just made me recollect my Eng Mechanics during my 1st year in college. It would be even better if @Ale Vallecchi can explain why the car will flip if it fights Gravity and starts to fish tail. I have understood up to the point DON"T FIGHT GRAVITY. This article will be complete if you can give the explanation to why the car will roll over if anybody fights GRAVITY. 

It’s the same principle applied to explain it . Flip or the multiple flip ( roll over and over and over ) on incline or flat . In flat there is outward force ( off set by the incline and not shown in ale’s diagram ) so the only thing what prevent the car from flip is its weight and velocity ( momentum ) ... when you are doing sidy in a pretty inclined curve  you should keep the velocity constant in a steady given direction . If you slow down , then direction has to change toward and not against FII ( in the diagram ) so the trajectories are parallel or more let say convergent on one line and not divergent ) otherwise the mass of your car will come in to parallel with FII and here is exactly where you flip over because no matter how powerful is your engine to offset ( fight ) this is of no use 

see below this ( I have posted before also in the thread of what went wrong ) 

 

 

12 hours ago, Ale Vallecchi said:

We always hear Marshals and Experts recommend to use "momentum" while driving in the desert.

As this most favorite advice is given, or sometimes shouted through the radio (“BUILD YOUR MOMENTUM”, or “KEEP THE MOMENTUM”) heads bob in confirmation, and a murmur of "yes" and "understood" runs through the briefed convoy.

But how many really know what momentum is?

Do we confuse it with speed, acceleration, or what else?

Without going too much into the physics of it, let us see what we really mean with "momentum".

 

THE WHAT

Momentum is the product of the mass of a particle and its velocity.

It is a vector quantity: i.e., it has both magnitude and direction.

In other words, Momentum is the force that exists in a moving object. To make some examples, a moving truck has more momentum than a car driving at the same speed because the truck has more mass. Similarly, a fast car has more momentum than a slow truck. When you are driving, you and your vehicle have momentum force, based on the total weight of your vehicle and your speed. While you are driving, momentum keeps your vehicle moving, unless it is acted upon by something, such as your brakes, a fixed object (such as a tree), another vehicle.... or the road surface.

Enter our deserts' sand: this shifty, soft surface interferes with momentum.

On a flat track, sand will simply reduce the speed at which you are driving (as the vehicle will tend to sink in it and encounter more friction). The loss of momentum will make the car eventually come to a stop, much quicker than on a harder, more compact tarmac road. Much like a jet ski on water, the more momentum your vehicle has on sand, the smoother it will glide, or 'plane', across the surface. That is why it is important to keep your speed up on sand - without overdoing it.

 

THE WHY

On a dune, we will have to contend not only with the sand’s friction, but also with the infamous force of GRAVITY!! As we all know, on Earth all bodies have a weight, or downward force of gravity which Earth's mass exerts on them, proportional to their mass. The heavier the object (read car), the stronger the downward pull.

An object (car) placed on a tilted surface (dune’s slope) will tend to slide down the surface.

The rate at which the object slides down depends on how tilted the surface is: the greater the tilt of the surface, the faster the rate at which the object will slide down it. In physics, a tilted surface is called an inclined plane (in desert off-roading is called a …. dune). Objects are known to accelerate down inclined planes because of an unbalanced force. Determining the net force acting upon an object (car) on an inclined plane (dune) involves dividing the gravity into two perpendicular components. On inclined planes (dunes_ the force of gravity, in fact, will be equal to the sum of two components - one parallel to the inclined surface and the other one perpendicular to the inclined surface. The diagram below shows how the force of gravity has been replaced by the two components - a parallel and a perpendicular component of force.

Taking a leap from the purely mathematical explanation of these dynamics to a more intuitive representation, (see figure attached) you may think of the right momentum (the car’s mass times its speed) as producing F1, that is the force that “anchors” the car to the slope/dune, being greater than F2. However, removing, or significantly reducing the element of speed, leaves only the weight of your car in play, thus prompting it to slide down the dune.

Clearly, when momentum is lost, F1 will start to become smaller and smaller, and the component of gravity that will prevail will be F2, pulling the car toward the bottom of the slope.

When this happens, you’ll hear the second most shouted command by a Marshal or Expert: DON’T FIGHT GRAVITY (as you’ll never win).

And remember, you’d better not!!!

@Ale Vallecchi excellent post  I have always loved applied physics , but not at times of university sure 😆

[Thomas Varghese]

@Mohamed Seidam my only doubt is because somewhere I read the car flips over because the sand starts to build up like a wall at the point of contact of the tires which are at the lower level. My understanding is a body over turns because the center of gravity point exceeds the critical angle. Now again my subsequent doubt creeps in my mind. If its the center of gravity point exceeding the critical angle which causes the flip isn't it more safe to keep the car at stock height rather than lifting it? 

I think the Marshals here can clear this doubt very easily as they know the answers to most of the reasons which cause a flip over and must have seen it all. 

Does lifting a car beyond its stock height really increase the chances of a flip over? Of course lifting must be helping in crossing over ridges and criss crossing. 

[M.Seidam]

7 minutes ago, Thomas Varghese said:

@Mohamed Seidam my only doubt is because somewhere I read the car flips over because the sand starts to build up like a wall at the point of contact of the tires which are at the lower level. My understanding is a body over turns because the center of gravity point exceeds the critical angle. Now again my subsequent doubt creeps in my mind. If its the center of gravity point exceeding the critical angle which causes the flip isn't it more safe to keep the car at stock height rather than lifting it? 

I think the Marshals here can clear this doubt very easily as they know the answers to most of the reasons which cause a flip over and must have seen it all. 

Does lifting a car beyond its stock height really increase the chances of a flip over? Of course lifting must be helping in crossing over ridges and criss crossing. 

The proper constant Speed is the key the higher the incline the higher the speed ( thing of a roll coaster ) . Regarding lift kit ( center of gravity ) should be offset by proper spacers. That’s why you see the army hammers already wide in platform and tires spaced properly . Regarding sand wall build beside tires that’s an additional favoring reason ( imagine on flat tarmac when drift attempt vigorously immediately flip is imminent and happens ) 

that’s what I think to best of what I know . 
 

sure more experienced leader comment is of more than a mine of gold to be taken in consideration 🙏🏻

[Thomas Varghese]

5 minutes ago, Mohamed Seidam said:

The proper constant Speed is the key the higher the incline the higher the speed ( thing of a roll coaster ) . Regarding lift kit ( center of gravity ) should be offset by proper spacers. That’s why you see the army hammers already wide in platform and tires spaced properly . Regarding sand wall build beside tires that’s an additional favoring reason ( imagine on flat tarmac when drift attempt vigorously immediately flip is imminent and happens ) 

that’s what I think to best of what I know . 
 

sure more experienced leader comment is of more than a mine of gold to be taken in consideration 🙏🏻

@Mohamed Seidam now my most important doubt. If you know all this applied physics why did you opt to become a doctor? 😅

[M.Seidam]

33 minutes ago, Thomas Varghese said:

@Mohamed Seidam now my most important doubt. If you know all this applied physics why did you opt to become a doctor? 😅

That’s why I chooses to be an orthopedic ( bone ) doc as it needs ton  of “physics “ 😅

[DANIEL NOGUEIRA]

This article reminds me of my sports, I skateboard on bowls,  and the speed will determine how far and how high I can go, same on surfing, I can only ride the wave and do turns depending on my speed,  or I will sink... it's all in the centrifugal force to keep the gravity in your favor...