CK-12 Physical Science - For Middle School

http://www.ck12.org Chapter 13. Forces

FIGURE 13.13

The ball bearings in this wheel reduce fric-

tion between the inner and outer cylinders

when they turn.

greater is the fluid friction resisting its motion. The very large surface area of a parachute, for example, has greater
air resistance than a skydiver’s body.

Lesson Summary

• Friction is a force that opposes motion between two surfaces that are touching. Friction occurs because no
  surface is perfectly smooth. Friction is greater when objects have rougher surfaces or are heavier so they press
  together with greater force.


• Types of friction include static friction, sliding friction, rolling friction, and fluid friction. Fluid friction with
  air is called air resistance.

Lesson Review Questions

Recall

1. What is friction?


2. List factors that affect friction.


3. How does friction produce heat?

Apply Concepts

4. Identify two forms of friction that oppose the motion of a moving car.