Frictional force acts on an object being in contact with another object. While an object is moving on a surface, the surface exert frictional force on the object. Friction acts opposite to the direction of the object's motion.

Friction also depends on the kind of surface where the object is moving. Rough surface exerts more force than smoother surface. It also depends on how tightly the object is pressed into the surface - the amount of the normal force. This means that, heavier objects is more tightly pressed than lighter ones making them more difficult to push or pull. Friction is responsible for any motion along a surface - walking, runnning and moving vehicles - all these are possible because of friction. This also help in holding and keeping things together in place. Car and other vehicle brakes also depends on friction.

However, friction also has disadvantages. Objects may be torn out because of friction. Vehicle tires are replaced from tinme to time to prevent accident since friction causes the tire to become thinner and may burst out if not replaced on time. Machines efficiency may be reduced and will not function well because friction causes machines to dry up hence it has to be lubricated to lessen the friction.

Frictional force acting on the object may be computed using the formula

\(f=\mu N\)


                    \(f=\text{friction}\\ \mu=\text{coefficient of friction}\\ N=\text{normal force}\).

Friction is the force exerted by a surface to the object in contact with it. The coefficient of friction indicates the type of material or object and the normal force pertains to the pressing force.

There are two kinds of friction - static friction and kinetic friction. Static friction is the force applied to an object which is starting to move. Friction force keep an object at rest in its place. To move the object, a force greater than the static friction must be applied for it to start moving. As the object is in motion, kinetic friction is exerted by the surface to the object. It has two types - sliding friction and rolling friction. The type of kinetic friction depends on the motion of the object. Rolling balls and other materials in rolling motion involves rolling friction and the ones in sliding motion, sliding friction.

The equation for the static friction, \(f_s\) and kinetic friction, \(f_k\) are expressed as

\(f_s=\mu_s N\)      and      \(f_k=\mu_k N\).

Example 1.

A loaded cart is pushed by a force of 385 N. a.) If the cart is moving at constant speed, how much is the frictional force? b.) Solve for the normal force given the coefficient of friction is \(\mu_k=0.70.\)


a.) Moving at constant speed indicates that the forces are balanced giving a net force of zero. Given that the applied force is 385 N, thus the frictional force is also 385 N in opposite direction with the applied force.

b.) Since the cart is moving, kinetic friction is exerted by the surface to the cart's wheels. Given \(f_k=\mu_kN\), we derive the formula for normal force as


Example 2.

A cabinet weighing 980 N is pushed across the floor. What is the coefficient of friction if the floor exerts an 860-N force on the cabinet?


 From the formula of kinetic friction, the coefficient of kinetic friction can be solved as

\(\mu=\frac {f}{N}=\frac{860\; N}{980\; N}=0.90\).