# Friction - Key terms

### ACCELERATION:

A change in velocity.

### COEFFICIENT OF FRICTION:

A figure, constant for a particular pair of surfaces in contact, that can be multiplied by the normal force between them to calculate the frictional force they experience.

### FORCE:

The product of mass multiplied by acceleration.

### FRICTION:

The force that resists motion when the surface of one object comes into contact with the surface of another. Varieties including sliding friction, static friction, and rolling friction. The degree of friction between two specific surfaces is proportional to coefficient of friction.

### FRICTIONAL FORCE:

The force necessary to set an object in motion, or to keep it in motion; equal to normal force multiplied by coefficient of friction.

### INERTIA:

The tendency of an object in motion to remain in motion, and of an object at rest to remain at rest.

### MASS:

A measure of inertia, indicating the resistance of an object to a change in its motion—including a change in velocity.

The ratio of force output to force input in amachine.

### NORMAL FORCE:

The perpendicular force with which two objects press against one another. On a plane without any incline (which would add acceleration in addition to that of gravity) normal force is the same as weight.

### ROLLING FRICTION:

The frictional resistance that a circular object experiences when it rolls over a relatively smooth, flatsurface. With a coefficient of friction much smaller than that of sliding friction, rolling friction involves by far the least amount of resistance among the three varieties of friction.

### SLIDING FRICTION:

The frictional resistance experienced by a body in motion. Here the coefficient of friction is greater than that for rolling friction, but less than for static friction.

### SPEED:

The rate at which the position of an object changes over a given period of time.

### STATIC FRICTION:

The frictional resistance that a stationary object must overcome before it can go into motion. Its coefficient of friction is greater than that of sliding friction, and thus largest among the three varieties of friction.

### VELOCITY:

The speed of an object in a particular direction.

### WEIGHT:

A measure of the gravitational force on an object; the product of mass multiplied by the acceleration due to gravity.