Frictional force is applied in the opposite direction to the force that causes objects to move when they are in contact with each other.
It is proportional to the force applied to the object and can be expressed by the following formula.
μ is the coefficient of friction and is determined by the material of the object.
Before the object starts moving, the static friction coefficient is used, and after it starts moving, the kinetic friction coefficient is used.
Generally, the coefficient of kinetic friction is smaller than the coefficient of static friction, so the friction force is smaller and the object moves more easily once it starts moving.
In the above, the relationship between the force F applied to the object and the acceleration a is as follows.(reference:Newton's laws)
■Tire friction
Consider the frictional force when an object has tires and the driving force moves the vehicle forward.
＜When the tires are moving forward without slipping＞
Static friction is what allows the tires to move forward without slipping.
For the forward force, the static friction force is not subtracted from the driving force, but the following rolling resistance is subtracted.
Static friction coefficient and rolling resistance coefficient are different; rolling resistance coefficient takes into account not only loss due to friction but also energy loss due to tire deformation.
However, the rolling resistance coefficient is still much smaller.
Note that the rolling resistance coefficient varies greatly depending on the tire material and road surface conditions, so rolling resistance is generally determined by actually measuring it in a car under certain conditions.
＜When the tires are slipping＞
Tire slipping occurs when the driving force exceeds the static friction between the tires and the road surface.
Once it slips, it will continue to slip as long as the driving force exceeds the kinetic friction force (dynamic friction is smaller, so once it slips, it will be more likely to slip).
Therefore, in order for the car to move forward without slipping, the driving force must be in the following condition.
During a slip, the reaction force of dynamic friction is transmitted to the road surface and becomes the force that propels the vehicle forward.
4WD vehicles are less prone to slipping than 2WD vehicles. This is because the driving force is distributed to each tire, making it difficult for the driving force to exceed the static friction of each tire.
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