Thames & Kosmos Physics Workshop handleiding

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Thames & Kosmos

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stable

unstable

indiﬀerent

wind energy

center

point of

sail

vehicle’s center

of gravity

base of support

tipping

line

wind energy

center

point of

sail

vehicle’s center

of gravity

tipping

line

Stable

Equilibrium

Unstable

Equilibrium

When Is a Body in Equilibrium?

It’s a tricky business to try to keep your balance while siing on top of a large ball.

But it’s not a problem siing on top of a large box. There are three ways a body

can be in equilibrium. Its state of equilibrium at any given moment depends on

what would happen if it were moved. If its center of gravity were to rise, then its

equilibrium is stable, if it were to drop, then it is unstable (wobbly). If its center

of gravity remains at the same level when the object is moved, then its state of

equilibrium is said to be indiﬀerent.

When Is a Body Stable?

A body is stable if its center of gravity is located vertically above its base of support.

Then, it is in a state of stable equilibrium.

If its center of gravity lies vertically above its tipping line (the edge of the base),

then it will wobble or tilt. The slightest movement will make it tip over. But its

steadiness will also depend on the force needed to push it oﬀ balance, as well as on

the base on which it supports itself. In the case of a car, the four wheels resting on

the pavement form a rectangle. That is its base of support.

Tipping Force and Center of Gravity

A car will be most stable as it steers through a sharp curve when its center of gravity

is as low as possible and its wheels are as wide apart as possible. Then the tipping

force — that is, the force trying to throw the car from the curve (the centrifugal

force, see page 94) — has lile chance to get the upper hand.

The four-part rule for stability also applies to other vehicles, including the sail

car that you will be building yourself (instructions on page 14). In the case of a sail

vehicle, the tipping force is the wind, which exerts itself against the center of the sail

and tries to push the car past its tipping point.

The weight of the sail car, making its impact at the vehicle’s center of gravity

(located “in the air”), lies vertically above the triangular base of support and keeps

the sail car on the ground — its equilibrium is stable (upper picture).

If the wind is strong, the vehicle rises up on one of its rear wheels, the base of

support shrinks toward the tipping line (lower picture), and the equilibrium becomes

unstable. A really strong wind will tip the vehicle over. Modern sail cars reach

speeds of up to 160 km/h. But what exactly is meant by speed?

An object’s stability is greater if... ...its weight is greater.

...its base is larger.

...the force tipping it is weaker.

...its center of gravity is lower.

The balance or equilibrium of the motor box (with the baery in it) is stable when it rests ﬂat

on its boom; when it rests on its corner edge, it is unstable; and when it rests on its corner

edge and shaft, it is stable again. What about when the baery is removed?

KEYWORD: EQUILIBRIUM

Equilibrium is the state of a body or physical system at rest or in unaccelerated motion in which the

resultant of all forces acting on it is zero and the sum of all torques about any axis is zero.

Earth Attracts Us

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