Unit 2 Week 2 - Friction and Forces

Four types of friction

static, sliding, rolling, fluid

Fluid friction

a friction force that opposes the motion of an object through a fluid (liquid or gas)

Free body diagram

a physical model that represents the forces acting on a system; should be used for every force problem to help identify equal forces and net force.

Weight

the force of gravity on an object, proportional to mass

Normal force

the perpendicular contact force exerted by a surface on another object, without which the object would fall through the surface.

Friction

A force that opposes motion between two surfaces that are in contact

What causes friction?

Friction results from the sticking together of two surfaces that are in contact.

Microwelds

areas where surface bumpers stick together, are the source of friction

Friction is dependent on

1. The force pushing the surfaces together (Fn= Fg)
2. Physical properties of the surfaces (coefficient of friction: u - mu)
3. Generally, independent of surface area

Coefficient of friction

the ratio of the magnitude of the force of friction between two objects in contact to the magnitude of the normal force with which the objects press against each other

Friction equation

Ff = μFn, Force of friction = coefficient of friction * Normal force

Kinetic friction

the force that opposes the movement of two surfaces that are in contact and are moving over each other

Static friction

a friction force that acts on objects that are not moving

Relationship between kinetic friction and static friction

Static friction is more than kinetic friction

Why is static friction greater than kinetic friction?

The fissures and peaks can interlock more deeply, when the object is not in motion. When the object is in motion the peaks and fissures do not have time to deeply interlock.

Why does wet ice have the least amount of friction?

The water acts as a lubricant on an already slippery surface.

Why are wheels often covered in rubber?

rubber has some of the highest coefficients of friction

How to reduce friction?

- Lubricants fill the microscopic voids
- Make surface ridges as small as possible (sanding or polishing surface)
- Use wheels or ball bearings

Another term for air resistance is:

Fluid friction

System

A group of interacting, interrelated, or interdependent elements or parts that function together as a whole. Often used to calculate net force.

Tension

stress that occurs when forces act to try to stretch an object