Rolling Friction: Definition, Examples, Coefficient, Causes

When you push a vehicle, it stops rolling after some distance if the force applied equals zero. The force that resists the motion of this vehicle is called friction. Generally, friction can be considered the resistive force offered by the surfaces on which an object moves. It is a force that works in the direction opposite to the direction of the object’s movement. Due to friction, a car stays in a static position as long as the magnitude of the external force exceeds the magnitude of friction. As a result, the rolling of vehicles on the ground is resisted. 

Definition of Rolling Friction 

Rolling resistance is caused when a circular object rolls over the surface and slows down the motion. If the surface of a moving solid body is smooth, rolling friction decreases. The velocity of the point of contact between the two surfaces in relation to the ground will be zero. Hence, the rolling body keeps moving in this situation. Basically, rolling resistance is a combination of different frictional forces exerted by the circular object or ground on which it is moving. 

Analogous to sliding resistance, rolling friction can be lower than sliding friction. Rolling friction can be calculated as the force per unit weight of the moving object at a constant speed. 

When a train moves on a track, it slows down if there is greater rolling friction. On the other hand, when the force exceeds the value of rolling friction, the train moves at a higher speed. This is an example of how rolling friction can influence the motion of a body.

Coefficient of Rolling Friction

Rolling friction can be defined as an opposing force that specifies how great the rolling resistance is offered during the movement. It is an indicator that tells about the force between the two surfaces in contact with each other. Some of the coefficients of rolling resistance are as follows: 

• Bicycle tires – 0.0022
• Large truck tires – 0.0045
• Car tires – 0.0062

The coefficient is proportional to the width of the object moving and inversely proportional to its radius. Additionally, the strength of the surface and the depth to which it may sink also influences the coefficient. It is considerably more complex as compared to the coefficient of sliding friction. But it’s important to know about it because rolling resistance is coefficient times the normal force.

The equation of rolling resistance can be written as: 

Fr = μrN

Where,

μr is the coefficient of rolling friction 

Fr is the resistive frictional force when the body rolls

N is the normal force of the wheel on a surface it is in contact with 

The above equation can also be written as:

μr = Fr/N

Causes of Rolling Friction 

The main cause of this type of friction is the deformation of surfaces. When the deformation material rolls over the surface and its deformation energy is larger than the energy of recovery. Hysteresis is the reason behind the energy losses associated with tires’ rolling resistance and viscoelastic properties.

Also, this adhesive force between the two surfaces is based on the following factors:

• The pattern of a rolling body
• The surface quality on which it is moving 
• Load and Pressure 
• The diameter of the rolling circular object 
• The surface area of the circular object 
• The surface roughness

Examples of Rolling Friction 

• A moving car is said to experience rolling resistance on its wheels. 
• Rolling of the football, tennis ball, or any other ball.
• Less rolling friction allows tires of Heavy-duty trucks to move more quickly.
• A skateboard set on an inclined surface is said to have resistance caused by the rolling friction.
• While driving, bike wheels roll with less speed. There is greater friction between wheel surfaces and the road.
• When kicked across a grassy field, a soccer ball slows down due to increased rolling friction.
• A full-sized bowling ball with a particular size and the weight moves at a larger speed if there is less rolling friction.

Conclusion 

In a broad sense, rolling resistance happens due to deformation and some sliding force at the point of contact between the two surfaces. Therefore when an object is moving, it gets formed along with the point of contact on the surface it is moving. Also, remember any type of friction is always responsible for slowing a moving object down. The rolling friction occurs in the opposite direction to the moving objects. It occurs when two surfaces are slid or rubbed against each other. The above examples should be implemented to understand the concept of rolling resistance.