Well, they can defend it, but, then…there’s physics!

“Friction is the resistance to motion of objects in contact with each other. The standard friction equation detemines the resistive force of sliding friction for hard surfaces, when you know the normal force pushing the two surfaces together and the coefficient of friction for the two surfaces.

When applied to sliding friction of hard surfaces, the equation implies that friction is independent of the area of the surfaces in contact.

This equation can also apply to soft surfaces, rolling friction and fluid friction, but the coefficient of friction may depend on area, shape and viscosity factors.

Questions you may have include:

- What is the standard friction equation?
- What is the normal force?
- What is the coefficient of friction?

When a force is applied to an object, the resistive force of friction acts in the opposite direction, parallel to the surfaces.

The standard equation for determining the resistive force of friction when trying to slide two solid objects together states that the force of friction equals the coefficient friction times the normal force pushing the two objects together. This equation is written as

Fr = μN

where:

- Fr is the resistive force of friction
- μ is the coefficient of friction for the two surfaces (Greek letter “mu”)
- N is the normal or perpendicular force pushing the two objects together
- μN is μ times N

Fr and N are measured in units of force, which are pounds or newtons. μ is a number between 0 (zero) and ∞ (infinity).

Applies to static and kinetic

This equation applies to both static and kinetic sliding friction. Static friction is the friction before an object starts to slide. Kinetic friction is the friction when the object is actually moving or sliding.

Static friction and kinetic friction have different coefficient of friction values.

Independent of area for sliding hard surfaces

An interesting result of this equation is that in the case of sliding friction of hard surfaces, the friction is independent of the area of the surfaces. In other words, it is just as difficult to move a 1 square-cm object as a 1 square-meter object, if they both are pressed to the surface with the same amount of force.

This is not intuitive. You would think that there is more friction when the surfaces are larger, but the friction equation states otherwise. You can verify this fact with experiments.

Soft, adhesion, rolling and fluid

In situations where the surfaces deform or there is molecular adhesion, the friction is not independent of the areas in contact. In these cases surface area usually comes into play. This is also true for rolling and fluid friction.

When solid surfaces are soft and deform or when one material is a fluid, the shape of the solid object may be a factor.

Although the standard friction equation still holds, the coefficient of friction may have area, shape and other factors included in it.

OK

So, what this says is that the greater the force pushing down, which is mass times G, gravity the greater the force needed to move the surfaces past each other.

So adding weight does increase friction and therby traction. But as you notice once the system becomes kinetic (and in the case of cars, rolling) the equations change.

Keep in mind also that this is for determining friction between two surfaces. It has nothing to do with the inertia of the objects. Inertia comes into play also in this complex situation. You need force to overcome inertia, but the application of that force depends on the success of the friction between the tires and the road. If you make it to heavy you will not be able to move it because there will not be enough friction to overcome the inertia. This is what happens when your tires just spin on pavement, providing the engine has sufficient power to overcome enough of the friction to spin the tires.

So, there is a balance between weight, friction, inertia, and power. It will vary for every vehicle.

While adding some weight does work, a better approach would be to improve the coefficient of friction for the tires. This way you get better traction without the extra weight. Bottom line, invest in the best snow tires you can get and lighten the load. Provided you can afford it. Otherwise throw a bag of salt in the trunk and when you get stuck you can throw it under the wheels to try and increase the friction.”