The bottom of lakes and ponds, which usually hold still water, is usually covered with sediments formed of clay material. Clay contains chemical substances such as sodium bentonite which have the property of expanding when wet. The loss of moisture, therefore, makes this layer of clay shrink resulting in cracks on its surface. Thus, in summers when lakes and ponds go dry, the surface of their beds shows large cracks.
For any living being to get electrocuted, a current of sufficient voltage has to pass through its body, i.e. there should be completion of the electric circuit. Birds perching on live electric transmission lines are not killed because they are in contact with only on wire and no current passes through them. But if the bird sitting on a live-wire simultaneously comes in contact with the ground or touches another live-wire, the electric circuit would be complete and the bird will be killed by the current flowing through it. Similarly a person hanging on to a single live-wire will not be electrocuted unless he touches the ground or the earth wire.
Earth rotates from west to east causing the Sun, the Moon, stars andother planets rise from
east and set in the west. On the surface of the eartheach point is moving eastward with
respect to an astronaut in space.
Thus if a rocket is launched at a small angle eastward vertically the speed of earth gets added to
the rocket’s speed. This reduces the amount of fuelrequired. If the rocket is inclined along any
other direction, the speed of the earth gets subtracted from the speed of rocket and will
necessitate burning of more fuel.
In addition, in case of any failure during launch the rocket will fall back to the earth.
Hence if it is launched from the east coast, the rocket will fall into the sea only. If it is launched from the west coast, in case of failure, the rocket will fall on the land endangering human habitats. Thus choosing eastward inclination is scientific but choice of eastern coast concerns safety.
In normal telephone exchange systems, a pair of wires extends each telephone instrument to the telephone exchange.
Normally, copper conductors are used to give loop to indicate the exchange that the
telephone handset had been lifted for making a call or for receiving a call. In advanced countries, Optical Fibre is also used for extending the loop.
Down to milligrams one can use well-designed physical balances-indeed one can even o below one milligram if proper isolation from vibrations and other disturbances is ensured. But for going lower one needs to use other methods. We have to remember that our objective is to measure the mass, which is the quantity of material and not the amount of gravitational force. Mass is a measure of inertia.
Continue reading “How are minute quantities (like milligrams, micrograms, or nanograms) weighed?”
It is true that the moon makes one rotation around the earth in 27.3 days. A lunar month is the number of days between two new moon days. After one new moon day the moon does arrive at the same position with respect to the fixed stars after 27.3 days.
But it is not aligned with the earth and the sun because during this period the earth has moved some distance around the sun, indeed by about 270. So the moon has to move some more to come to a position between the sun and the earth.
A simple calculation shows that this additional traverse would take about 2.2 days. Therefore the next new moon comes after 29.5 days and not 27.3-the lunar month is longer than the period of rotation of the moon by about Two days!
