How does a match stick catch fire when struck ?

Match sticks have phosphorous in them Phosphorus comes in two forms - red and white. They are just in two different crystalline forms. White phosphorus catches fire easily. Red phosphorous is relatively safe. Red phosphorous can be converted to white by a small force such a force of friction.

To strike a match we need a striking surface and a matchstick. The striking surface is rough. It has powdered glass and red phosphorous. Match head has antimony trisulphide and potassium chlorate placed on a wooden stick with glue. Potassium chlorate is the oxidizing agent.

For obtaining fire, we need at least three agents - heat, oxygen and material to sustain the fire. The act of striking converts the red phosphorus to white by friction and heat. White phosphorous ignites due to the presence of oxygen in air or in the matchstick head (potassium chlorate). This starts the combustion of the match head.

Place two flat and smooth glass surfaces together. Try moving them apart. Since they are flat and smooth, you would expect friction between the surface to be negligible. Then why do you find it difficult to slide them past each other ?

It is a misconception that smoother surface will have less friction. If two smooth surfaces are in contact with each other, the exact opposite happens. Larger amounts of areas of the two surfaces are in contact with each other. This increases the frictional force between the two surfaces. When two glass surfaces are placed one over the other, it is very difficult to take them apart. It is a general practice to place another material between two glass sheets, the material could be plastic or newspapers, etc.

To reduce friction between two surfaces, some amount of smoothness is needed but not a completely smoothened surface.

Why is difficult to walk on sand ?

Sand grains are small in size. When we stand on a beach, there is less uniform frictional force between the soles of the feet and the sand grains. The sand grains roll, as our feet press against them while walking. This rolling makes our feet slip as we walk on the sand. That is why it is difficult to walk on sand, the friction is less here.

Check the soles of your shoes. Why do they have a roughened surface ?

The purpose of having rough surface underneath our shoes is to have sufficient friction and grip between our shoes and the surface where we are standing. The purpose of each type of shoe is different. A tennis shoe will have a more rough sole than a walking shoe. A pair of shoes used for football or hockey or even jogging will all have different types of soles. The main purpose is to obtain as much friction as possible from the surface while doing any of these activities.

Most of the shoe soles are made of synthetic material, or rubber but not wood. Why so ? The reason for this is that in addition to friction the phenomena of adhesion-cohesion have also to be considered. The shoe sole should hive you the right amount of friction but so that an applied force by your feet should be able to overcome the frictional force easily. A wooden sole will not give correct adhesion between the sole and the floor.

What is a shooting star ? Why does it shine ?

All of us have seen "shooting stars" in the night sky and have felt fascinated by them. But shooting stars are not really stars! When any small rock or any other space debris, crosses Earth's orbit while it is revolving around the Sun, enters Earth's dense atmosphere, it burn up occurs due to friction with the air molecules. This burning makes the object shine and glow, which we call as a shooting star. Its origin is not from any star.

But where do these rocks come from?

There are at least three sources of rocks from the heavens. The first of the reasons is the easiest to understand. Many man made objects like satellites, spacecraft are orbiting the Earth. There are now more than a hundred of these. Such objects are continuously creating what is called space debris or space junk. When a space debris falls into the atmosphere, it burns up, causing a shooting star like effect. The second reason is the debris left behind by a passing comet. A comet is an orbiting body in the Solar System that has a nucleus or head and a tail.

A comet is made up of ice and dust. In its path around the Sun, a comet leaves behind dust and ice. If Earth enters this debris filled area, then many shooting stars are seen to suddenly rain down in the sky. This phenomenon is termed as a meteor shower.

The third reason is because of the presence of asteroids in the Solar System. If we look at the Solar System, besides the nine planets there is an asteroid belt between Mars and Jupiter. These asteroids are made up of solid rocks. Due to the competing gravitational pulls of the Sun and the Jupiter, sometimes an asteroid may leave its orbit and become wayward. There are thousands of orbiting asteroids. About 300 large asteroids have their orbits intersecting with the Earth!! Much smaller asteroids called meteoroids are constantly straying and entering into Earth's atmosphere all the time. These meteoroids burn up on friction with the atmosphere and give the appearance of shooting stars.

Thus shooting stars are not stars; they are space rocks or debris, which shine due to friction with the Earth's atmosphere. The friction gives rise to heat and thus these objects glow and look like stars.