Category Science

Automatic wrist watches stop functioning if not worn even for a day. How does wearing them on our hand make them function?

    Automatic wrist watches, like any other watch, also need energy for functioning.  This comes from the up, down and twisting movement of our hand.

There is a small pendulum-like component which rotates around a fulcrum due to our hand’s movement. The movement of the pendulum (clockwise and anti-clockwise) is used to wind a spiral spring. This action is similar to winding it manually. The energy stored in a fully wound spring is adequate to run the watch for a day or two. Within this time, if the watch is not worn, there is no recharging of the spiral spring. Hence the watch will naturally come to a stop.  

Unlike winding type watches, automatic watches have a different mechanism: a rotor which can rotate freely (like a free wheel in a bicycle).

 Our hand movements make the rotor rotate and wind a coil spring. The rotor frees itself to the original position after the winding. As a result the rotations of the rotor further tighten the spring. The watch works as the spring releases very slowly.

There is also a release mechanism by which the rotor is freed completely when the coil spring is wound to a preset maximum.

If the watch (spring) is kept idle for some time, there is no rewinding of the spring, and so it stops functioning.

Why does not the cycle travel backwards when we pedal it in the reverse direction?

It is the free wheel which prevents a bicycle from moving backwards when we pedal in the reverse direction. In the positive drive, the rollers are pressed against a spring pad. As a result the rollers are wedged between the driving member (outer disc with the chain on it) and the driven members (inner axle connected to the rear wheel). So the force applied on the outer disc is made to rotate the wheel. The mechanism is such that when pedaled backwards, the outer disc simply slides over the axle and the drive is disconnected. This s called freewheeling and the mechanism is called freewheel or over-running clutch.

Why does it seem difficult to cycle up a steep hill than to push the bike up at the same speed?

A bicycle is unusual among machines because it has a mechanical advantage (MA) of less than one – typically between 1/3 (in low gear) and 1/8 (in top gear). This helps to convert effort to speed but also means that forces acting on the machine are multiplied by the reciprocal of the MA.

 When riding on the level, the forces of wind and road resistance are small so not much effort is needed, but when going uphill the force of gravity, is also multiplied up, making the journey more difficult.

 On steep hills it may therefore become easier to walk or run than to cycle. When you push the bike up a hill you tend to be doing about 4 or 5 kilometers per hour whereas while riding, even slowly it is at least 8 or 9 km/hr. Also while cycling up a hill our legs move at about half the rotational speed involved in walking, which means we will have to put in twice the amount of work.

Mountain bikers overcome this problem by having extremely low gears on their machines.

Cycling in a bike with low gears at 4 km/hr is surprisingly easy, so it need not be more difficult.

      

Why don’t water and oil mix?

    A phenomenon called polarity prevents oil and water from mixing. All molecules carry electrical charge which is distributed uniformly or non-uniformly over the length of the molecule. In polar compounds, the positive and negative charges are concentrated at the two ends of the molecule. When such substances are mixed together, the positive and negative regions of their molecules attract each other and as a result a clear solution is obtained. Water is a polar substance and mixes freely with other polar substances. Oil molecules, on the other hand, are non-polar. When polar and non-polar substances are mixed together, the mutual attraction of polar molecules separates out the non-polar molecules and the two substances do not mix.   

Why is salt soluble in water but not in oil?

     The solubility of a substance depends on the nature of bonding in both the solute and the solvent. Salt is an ionic substance; that is, its atoms are held together by opposite electrical charges. Water molecule also has electrical charges – the hydrogen atoms are positively charged and the oxygen atom negatively charged. When salt is put in water the charges on the water molecule pull the oppositely charged atoms in salt apart, thus dissolving it. Oil molecules, on the other hand, do not carry any charge and so does not have any effect on the ionic bonds in salt when put in it. As a result, salt does not dissolve in oil.

                                    

Why is difficult to tie a knot in a silk lace?

            A knot in a lace is held together by the friction between the surfaces of the lace. Greater the friction more stable the knot will be. Since there is enough friction between cotton fibres in a lace made of cotton, it is quite easy to tie a knot. But silk being a smooth fibre, a knot tied with a silk lace becomes loose and opens up easily due to lack of friction between the fibres.