91 Science Friction-Mid

Science Friction FOCUS Question How is friction related to heat energy?

Warm Up with Friction It's a cold day, and you forgot your mittens. You rub your chilly hands together. Suddenly they feel warmer. Where did that warmth come from? The warmth in your hands came from heat energy generated by friction. Friction is a force. It acts between objects that rub against each other. Friction also causes moving things to slow down. It changes the energy of a moving object from motion into heat.

What Is Friction? You use friction every day. When you walk, run, jump, and stop, your feet touch the ground and create friction. If you wear skates with wheels, you have less friction with the ground. Zoom! Friction happens between all kinds of matter— solids, liquids, and gases. Matter makes up everything around you. All matter is made of tiny particles. When things rub together, the edges of the particles catch. This is friction. Friction happens whenever matter touches. As particles catch and pull apart again, they move back and forth. The movement of particles is heat.

Factors of Friction It's harder to walk through water than through air. If you run through water. it only gets harder. Water has more particles in the same amount of space than air does. More particles mean more friction. As speed increases, so does friction. Mass, or the amount of matter, creates more friction. It is easier to push an empty box than a full one. The size of a surface can also affect friction. It is easier to push a skinny box than a wide box. A rough surface causes more friction. It's easier to push a box across a smooth floor than across a carpet.

Fire from Friction Friction changes motion into heat. Lots of friction makes lots of heat If wheels are moving fast enough, the tires will start to smoke and burn (see cover). At times you can get enough heat to start a fire! Matches use friction to create a flame. The head of a match catches fire at a fairly low temperature. The match lights when it is rubbed quickly across a rough surface. This friction gives off heat, so the match catches fire. Some people can light a fire by rubbing two sticks together. Wood must be at a much higher temperature than a match head before it will catch fire. It takes a lot of skill—and a lot of patience—to light a fire by using friction Safety! Never light matches without the help of an adult.

Machinery and Friction Machines can create lots of friction and, as a result, lots of heat. When machines get too hot, their parts can short-circuit, melt, or even catch fire. Many machines have ways to prevent friction. Some have tiny parts called ball bearings between their moving parts. The bearings roll past each other. Rolling causes a lot less friction than sliding. Other machines use lubricants, or slippery materials, to prevent friction. Oil and grease are lubricants. Some machines need friction to do their jobs. A saw can't cut wood without rubbing against it, but the motion heats the blade. When machines get too hot, people cool them using water, fans, or air conditioners.