VI
If the broomstick is then struck violently with another stout
stick, the former will be broken, but the glasses will remain
intact. The more energy there is thrown into the blow the better
the experiment answers." (Young England, January 1882.)
Before leaping over a ditch we run towards it; descending
from and getting into a carriage in motion; cleaning dusty books
by striking them against each other; dusting a coat by beating
it with a stick.
§ 17. Rolling Friction less than sliding friction; friction
lessened by rubbing the surfaces with fatty substances. A heavy
weight suspended by a rope may be put in motion by a finger
(which of the three impedements are in action in this case
and which not?). Friction sometimes increased, e.g. when a drag is
applied to a wheel. The necessity of friction. 128.
Friction is the greater
1) the rougher the surfaces in contact,
2) the larger the surfaces,
3) the more solid the substances of the bodies exposed
to friction,
4) the heavier the moving body.
§ 18. In case the surfaces are smooth only No. 3 & 4 have
to be considered. Friction is an impediment to motion, because
the moving body has either to break or to ascend elevations on
the way, the one being resisted by cohesion, the other by gravity.
If the surfaces are very smooth or one of them is the surface of
water, motion has to overcome adhesion. How strongly this
adhesion may act as an impediment to motion, We may ex
perience in shoving forward a piece of a looking-glass on an
other. As to the resistance of the air or any other medium
the impediment will be the greater,
