FRICTION

Friction is when two objects rub or slide over one another. Friction slows motion down. It keeps objects from moving. Smooth surfaces make less friction. A rough surface makes more friction.

Friction can be helpful. One example of friction is when you rub two sticks together to make a fire. Another example is when you use your bicycle brake to slow down.

Friction can also cause problems. Too much friction can make heavy objects hard to move. Too much friction can also wear things down. Too little friction can also cause problems like on wet, slippery roads.

Friction can cause problems. When objects rub against each other, the surfaces are worn away. Friction with the ground causes the heels and soles of your shoes to wear away.

Box cars have a brake system that uses friction between the rear tires and the brake pads to slow down or stop the box car.