# Man's physical universe

94 THE UNIVERSE A VAST SYSTEM OF PARTS

The "spring" '

tides are the highest because they occur when the

pull of the sun and moon are acting together, in other words, when the

sun and moon are on the same or opposite sides of the earth.

The "neap" tides occur when the gravitational pull of the sun exerts

the maximum neutralizing effect on the gravitational pull of the moon.

The physiographic features of the earth's surface modify the intensity

of the tides, delaying their arrival at different points.

Because of the presence of the continents, the tides in the Atlantic,

Pacific, and Indian oceans are secondary tides produced by the primary

tides in the Southern ocean.

The height of the tides varies from one foot to forty feet.

Weight and Mass Are Proportional to,

Other.

but Not Identical with, Each

It was pointed out at the beginning of this Section that all

bodies

in the universe attract each other with a force that is called gravitation.

A special but very familiar example of this force is the force exerted

on objects at the earth's surface. This is called their weight. This

force, like every gravitational force, is proportional to the mass of the

body, as stated in the expression of the general law of gravitation.

is also proportional to the mass of the earth and inversely proportional

to the square of the earth's radius, but these quantities do not vary

much. It is true that a body weighs slightly less on a mountain top

than it does at sea level, because there is a greater distance between

the centers of gravity. But the force of gravity at the surface of the

moon is only one-sixth of that for the same body at the earth's surface

because the moon is so much lighter than the earth. On the moon,

then, the weight of a body will be one-sixth of its weight on the

earth.

It is obvious, however, that there is something about an object that

does not change, though its weight does. This unchanging property

of matter is its mass, as already referred to in connection with the

general law of gravitation. It is properly measured by the reaction of

the body when it is acted on by a force, as will be discussed below in

connection with Newton's second law of motion. In practice, the

masses of two bodies are compared by their weights, to which they are

proportional. The bodies are placed on the two pans of a balance, such

as the chemist uses, and adjusted until the weights are equal. Then,

by Newton's general law of gravitation, the masses are also equal.

Balances are thus a device for comparing masses. Many food markets

It

'

" Spring" is a name in this case which has no relation to the spring season of the year.

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