Friction, Parallel Forces

FRICTION

Friction is one of the most common force in the field Physics. It palsy an omportant role in Kinematics (the study of motion). It is the resistance which opposes every effort to slide or roll one body over another. Its direction is parallel to the surface and opposite to the direction of motion. Friction is caused by association of ting projection on rubbing surface.

FACTORS THAT AFFECTS FRICTION
1. Does not depend very much on velocity.
2. Does not depend on the surface area.
3. It is when surfaces are smooth and hard.
4. Like surface is greater than between unlike surfaces.

EFFECT OF LOAD
Frictional force is dependent to its weight. In fact the greater the weight, the greater, the was the largest required force.

COEFFICIENT OF FRICTION

Frictional force is proportional to the normal force.

F= kNf

where: [k is the constant coefficient value (μ)]

Coefficient of friction (μ) is the ratio of the needed to overcome friction to the normal force, is constant for a given pair of surfaces regardless of the load.

Coefficient of Friction = Frictional force / normal force

STATIC AND KINETIC FRICTION

Figure 1. Forces on a block of wood

Static friction refers to the frictional opposition on at rest body must overcome in order to be set in motion.

fs = μs Nf

Kinetic friction refers to frictional opposition upon a body moving at a constant speed across its surface.

fk = μk Nf

Note: Kinetic friction is greater than static friction. However if there is no lubricant is present, the coefficient of static friction is greater than kinetic friction.


Sample Problem:

A 100 kg wooden crate is being pushed across a wooden floor with a horizontal force of 350 N. Find the net force acting on the crate? (μ=0.3)

Given:
F= 350N
m= 100kg
μ = 0.3

Find F net

Formula: F net = F applied – Frictional Force


Step 1: Solve the normal force

Nf = mg---------Nf = (100kg)(9.8m/s^2)------------Nf = 980N

Step 2: Compute for the Frictional force

f = μ Nf----------f = (0.3)(980N)-----------------------f = 294N

Step 3: Solve for the F net

Fnet = F applied – Frictional Force

Fnet = 350N – 294N-----------------------Fnet = 56N