For procedures, see your mechanical laboratory manual.

Title: Friction Experiment

Part A

Aim

1. To determine the coefficient of friction between the mild steel plane and the various materials of the slider trays.

2. To determine the angles of friction between these surfaces.

Theory

Friction

Friction is defined as the force resisting the relative motion of solid surfaces, fluid layers, and materials elements sliding against each other. It is said to be a 'non-conservative' force; it is the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid. Friction appears in most mechanical systems, where there is motion or tendency for motion between two physical components because all surfaces are irregular at the microscopic level. When contacting surfaces move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy or heat.

Coefficient of friction

The coefficient of friction "µ" is a dimensionless number that is defined as the ratio between friction force and normal force. It is a measure of the amount of friction existing between two surfaces. A low value of the coefficient of friction indicates that the force required for sliding to occur is less than the force required when the coefficient of friction is high. The value of the coefficient of friction is given by

The coefficient of friction is inversely proportional to pressure and proportional to velocity.

Friction can be divided into Static and Kinetic (Dynamic) Friction.

Static Friction

Static friction is the tangential force that must be overcome to initiate the sliding of one object over another. It is the friction between two or more solid objects that are not moving relative to each other. It is considered to arise as the result of surface roughness features across multiple length-scales at solid surfaces. The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force. The coefficient of static friction μs can be measured experimentally for an object placed on a flat surface and pulled using a known force. The coefficient of static friction is related to the Normal Force FN of the object on the surface when the object just begins to slide. It is typically denoted as μs.

Kinetic Friction

Kinetic friction is the force resisting movement when one body is in motion relative to the other. It is also called "dynamic friction" or "sliding friction". Kinetic friction is used to describe the changing resistance in this slip process. It is the tangential force that must be overcome to maintain sliding. The coefficient of kinetic friction is typically denoted as μk and is usually less than the coefficient of static friction for the same materials.

Part B - Friction on an Inclined Plane

Aim

1. To determine experimentally the force parallel to the plane required to drag a body up or down on an inclined plane.
2. To compute the experimental results with those determined theoretically, using the values of coefficients of friction as determined in the friction experiment.
3. To repeat for several different sets of surfaces in contact.
4. To observe the effect of rolling.

Theory

Empirical laws that describe friction between two surfaces:

1. The ratio of the maximum frictional force and the normal force is a constant and equals the coefficient of friction and depends only on the nature of the two surfaces in contact. i.e. (Frictional Force) / (Normal Force).
2. The coefficient of kinetic friction is lower than the coefficient of static friction.
3. The coefficient of friction is independent of the area of contact.
For surfaces at rest relative to each other μ = μs, where μs is the coefficient of static friction. This is usually larger than its kinetic counterpart. For surfaces in relative motion μ = μk, where μk is the coefficient of kinetic friction.

Rolling Friction

Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. It is sometimes called rolling resistance or rolling drag. When a force or torque is applied to a stationary wheel, there is a small static rolling friction force holding back the rolling motion. It is mainly caused by non-elastic effects.

Applications

1. Printing Control mechanisms
2. Cams are used in Automatic machines
3. In machine tools

References

1. Friction, sciencedirect.com
2. Static and Kinetic friction, sciencedirect.com
3. Friction, wikipedia.org