(Makes things hard to move) Friction Friction is the force that opposes the motion between two surfaces that are in contact. (Makes things hard to move)

Traction is defined as friction between a drive member (wheel) and the surface it moves upon. It is the amount of force a wheel can apply to a surface before it slips. A rolling wheel is in static contact with the ground if it is not slipping.

On the microscopic scale all surfaces are rough. When two surfaces are in contact with each other the high points on one surface temporarily bond or lock with the high points of the other surface.

There are two types of friction. Static friction is when there is no relative motion between the two objects. “Such as a box sitting motionless on the floor.” Kinetic (sliding) friction is when there is relative motion between the two objects. “Such as when you manage to start to push that same box across the floor.” **Kinetic friction is “always” a lesser value then static friction.**

For the case of a spinning tire, there is relative motion between the tire and the ground therefore kinetic friction. If the tire is not spinning there is no relative motion between the tire and the ground therefore static friction. This explains why it is better to try no to spin your tires when your car is stuck in the winter, because static friction is larger than kinetic friction.

Friction depends on two things 1. The nature of the surfaces in contact, every different pair of surfaces will act differently with respect to friction. Every surface has a different amount of “grippeness”. This grippeness value is called the coefficient of friction. The symbol for the coefficient of friction is Mu (μ).

Note how the grippy surfaces have higher coefficients of friction that the slippery surfaces.

The second thing that the force of friction depends on is: 2. The force pushing the two surfaces together. This force is called the normal force (fn), it is equal to the weight of that object. A heaver object will have a larger normal force. The force of friction can be found using the following formula

Example The static coefficient of friction for rubber on dry asphalt is roughly 0.67, find the force of friction between a car tire and the road if the car has a weight of 1500 lb.

Example Find the force of friction in between Calvin’s toboggan and the snow if the coefficient of friction between wood and snow is 0.3, and the normal force for the toboggan is 47 lb.

Increasing Traction Since traction is dependent on the friction of the wheel and the surface, you must maximize this friction in order to increase traction.  Friction is dependent on 2 things 1) the coefficient of friction (between the wheel and the surface), and 2) the normal force (the weight of the robot pressing the wheel to the surface). To increase traction, you must either increase the coefficient of friction (more grippy tires) or increase the normal force on the wheel (add more weight).