Montshire Minute: Friction

Originally aired during the week of December 1, 2003

Monday

Friction is a force that slows down moving things. Galileo was the first scientist we know of that really identified what friction was. When he wasn't dropping objects off the top of the leaning tower of Pisa, Galileo was doing experiments rolling balls down inclined planes. He noticed that a ball picked up speed if rolled down a ramp, and it lost speed when it rolled up a ramp. Finally, he asked himself the question "If the ball rolled down a plane and onto a completely level surface, how far would the ball travel?" The answer, theoretically, was - forever! Any moving body, even a moving body rolling across a seemingly flat plane, will roll forever, an effect Galileo called inertia. Friction is the force that interferes with this motion. In this case, friction caused by the slightly uneven surface of the tabletop creates a drag on the rolling object, which will eventually come to a stop.

Tuesday

A batted baseball traveling through the air in a high altitude place like Mexico City, will travel further than a ball at sea level. The thinner air means less air-resistance - less friction. Air resistance caused by friction is also a friend to falling cats. Come again? Well, cats that live in high rise apartment buildings have been known to loose their balance, and fall out of a window. According to Kathy Woolard in her book How Come? only five percent of cats that fall out of windows from the 7th to the 32 story die. The mortality rate is twice as high for cats that fall six stories or less. What gives? Well, the cat knows that increasing its surface area leads to greater friction or air resistance. So, falling cats right themselves and spread their legs to form a sort of kitty parachute. The uprushing air has a bigger area to push against. Join us Sunday, December 7, for our Science Sunday activity Rollways, where we experiment with how things roll, drop and move. It will be offered at 1 p.m. and 3 p.m.

Wednesday

When two surfaces rub against each other - let's say its a comb and your hair - the result is friction. Now those hairs of yours - they may appear to be smooth and slinky, like the hairs of TV models on shampoo commercials. But at a microscopic level, everyone's hairs are kind of cracked and lumpy and - well, NOT very smooth. So the hairs catch on the bristles of the brush. This friction causes electrons, those tiny negatively charged particles orbiting around the atom's nucleus, to jump from hair atoms to the brush. So the brush gets a negative charge from the electrons it picks up. Meanwhile, your hair has a positive charge, since it lost some of those electrons. And you know what they say about opposites - they attract! So, your hair sometimes stands up on your head as you wave the brush above it. Now there's a real hair raising experience caused by friction.

Thursday

All you need to do to create friction is rub your hands together. Go on, try it. Rub really hard. Can't do it for too long, can you? Why? Well, because your hands feel like their on FIRE, right? By rubbing your palms, you're making friction. Friction takes the energy of your moving hands and turns it into heat energy. When any two things rub together, like your car's tires and the road, friction acts between them to slow you down. Friction is also described as a force - the force that is NOT with you. The direction of the friction force is in the opposite direction of the motion. If you try to slide a big, heavy crate across the floor... push really hard now... you reach a point where the crate finally begins to move - this is the point where the applied force in one direction becomes greater than the friction force against you.

Thursday

Crumple a sheet of aluminum foil into a little ball. Then, use a pencil to make a tunnel through the ball. But make the hole in a "v" shape - in other words, make sure the tunnel has a little elbow in it, instead of being perfectly straight. Then, push a piece of string through the hole. You may need a pair of tweezers to do this part. If you hold both ends of the string loosely, the foil ball should slide easily up and down. Now, if you pull hard on the ends and tighten the string, what happens? I'll bet the ball will move very slowly, or stop altogether. The movement of the ball is "foiled" because the string is rubbing against the v-shaped tunnel, creating friction. The ball and string work a little bit like the brakes on a bicycle. The brake pads rub against the wheel rim, slowing the bike down. The more you squeeze the brakes, the greater the friction! Join us Sunday, December 7, for our Science Sunday activity Rollways, where we experiment with friction. The program will be offered at 1 p.m. and 3 p.m.