Thames & Kosmos Physics Workshop handleiding

Niet gecategoriseerd

Thames & Kosmos

19 pagina's

PDF beschikbaar

Handleiding

DID YOU KNOW?

The Kilogram

Prototype

If you want to measure something, you

have to have something else to compare

it with, a so-called unit of measure.

5 kg, for example, corresponds to 5

times a 1 kg unit. The kilogram is the

unit of measure for mass. In order for

everyone in the entire world to be able

to calibrate their scales exactly alike,

they have to have a determination of a

unit of 1 kg from a source that always

stays the same. For over a century, that

source has been the kilogram prototype

kept in Paris since 1889: a cylinder made

of heavy platinum-iridium metal, 39

mm high and 39 mm in diameter. For

unknown reasons, however — perhaps

as a result of cleaning — the prototype

has lost a tiny bit of its mass.

Scientists have been looking for a

diﬀerent yardstick for mass, one that

would be universally valid and would

never change. At the moment, there

are at least two candidates to replace

the kilogram prototype in Paris. One

possibility is to use a silicon crystal

containing a precisely known quantity

of atoms. Another suggestion is to

use the electromagnetic force needed

to balance a 1-kg object as a unit of

measure. If either of these ideas is

successfully developed, then perhaps

the kilogram prototype can enjoy a

well-deserved retirement in a museum.

Now let’s drop a hammer onto the moon’s dusty surface. It falls a lot more slowly

than it would on Earth — as does the dust that it kicks up when it lands. While the

mass of an object remains the same on all moons and planets, its weight depends on

the gravity of the celestial body it is on or near. So an object’s mass makes it heavy,

but to diﬀerent degrees in diﬀerent places.

Weightlessness

In empty space, far away from any sizeable celestial bodies, things are practically

weightless. There, we would weigh almost nothing at all. The same thing would

happen between two celestial bodies: Wherever their powers of araction cancel

each other out, the result is a state of weightlessness. When astronauts go there, their

space ships provide them no solid ground beneath their feet, leaving them to ﬂoat in

mid-air. If they want to drink something, they have to suck it out of a bole through

a straw. Why? Because the liquid can’t ﬂow down into their mouths from a cup. It has

no weight. But both the liquid and the astronaut drinking it have the same mass that

they would have on Earth. The mass of a body is measured in kilograms (kg).

Where Is it Pulling Us?

Where is Earth taking our experimental potatoes? In what direction is it pulling

them? The answer: to its center. More precisely, it is pulling them towards its center

of gravity or center of mass. Does Earth always pull with the same force? No,

not quite. The farther we are from its center of mass, the weaker its gravitational

araction on us. Deep in a mine, its araction is stronger than on Earth’s surface,

while we weigh more on Earth’s surface than we do in a ﬂying airplane. But even on

the surface of Earth, there are diﬀerences. At the North or South Pole, an object is just

a lile bit heavier than it would be, say, at the Equator. Earth is a lile ﬂaened at

the poles, and its surface there is thus a lile closer to its center. These diﬀerences

are so slight, however, that they “carry lile weight.” In any case, there is another

reason why Earth’s gravity is weaker at the Equator, which is that its rotation results

in a greater centrifugal force there. To ﬁnd out more about that, see p. 94.

Where Is the Center of Gravity?

On two legs, you’ll stand more safely than on one. On your hands and knees, you

will be even more stable. Why? Just like Earth, any object has its own center of

gravity, the location of which determines its equilibrium, how stable it stands, and

how easily it tips. But how can you ﬁnd an object’s center of gravity? Let’s build a

center-of-gravity locator.

A picture of the kilogram prototype

kept at the Bureau International des

Poids et Mesures, in Paris, France.

An astronaut after the ﬁrst moon landing in 1969. While the ﬂag seems to be ﬂuering in the

wind, in reality it is not, because there is no atmosphere on the moon and therefore no wind.

KEYWORD: CENTER OF GRAVITY

Center of gravity is the point at which the entire weight of a body may be

considered as concentrated so that if supported at this point the body would

remain in equilibrium in any position.

KEYWORD: WEIGHT

Weight is the force with which a body is aracted toward Earth or a celestial

body by gravitation and which is equal to the product of the mass and the

local gravitational acceleration.



Bekijk gratis de handleiding van Thames & Kosmos Physics Workshop, stel vragen en lees de antwoorden op veelvoorkomende problemen, of gebruik onze assistent om sneller informatie in de handleiding te vinden of uitleg te krijgen over specifieke functies.