English longbow (picture 1 above), and other
bows are
classified by the tension in lbs on pulling the bow string to its maximum
tension.
In old 'weight' units this equates to typically 50 lb
(23 kg)
to 150 lb (68 kg). (1 lb = 0.454 kg).
Imagine these 'weights' hanging from
the bow string.
You can measure this with a spring balance
by attaching it to the bow string and pulling it back and noting the reading
(picture 2).
So the tension in the bow string is
equivalent to hanging on it 'weights' of typically 23 kg to 68 kg.
In terms of weight equalling the tension
force in the bow string you multiply the mass by 9.8 N/kg (due to gravity).
weight (N) = mass (kg)
x gravitational field constant (g)
Therefore the tension in the string is
typically 224 to 667 N, and the bow and arrow is now an elastic potential
energy store!
To reduce the force of friction (drag
force) between the
arrow and air, the arrow shaft is thin, as are the flight feathers, and a sharp metal point at the front end
(picture 3).
When the bow is bent by drawing back the string,
the string is
in tension, AND there are forces of both tension and compression in the
bow structure.
On the inner curved surface of the bow you
get compression as the layers are pressed together.
On the outer curved surface of the arched bow
you get the tension as the outer layers are stretched.
When you let go of the arrow. all this
stored
potential energy is released and converted into kinetic energy of the fast moving arrow.
The drawn bow and arrow are an elastic
potential energy store.
The arrow becomes a kinetic energy store,
much of it is retained on flying back down to Earth.
As the arrow flies upwards it loses
kinetic energy and increases its gravitational potential energy store.
As it falls, the GPE is converted back
to kinetic energy, and the arrow can be as penetrating as when first
fired.
History note (not required
for GCSE physics, not sure about GCSE history!):
The rapid firing of many longbows
was a major factor in the English winning the Battle of Agincourt
(1415) in the 100 years war between France and England. The French
archers uses a crossbow that fires a bolt - deadly and very
effective, BUT the bolt must be drawn back by a mechanical winding
system to build up the store potential energy in the bolt mechanism. With no
mechanism to deal with, just muscle power, a skilled English archer
could pull back his bow and fire arrows at ten times the speed of
the French crossbow men. No contest! Shakespeare built the whole
thing up in his play "Henry the Fifth", he didn't know much
about physics
but he was pretty good with words!
Any object standing on a
surface in a gravitational field involves normal contact forces.
The object has weight due to
gravity, so the object presses down on the surface e.g. a stationary book lying on a table or you standing on the floor or
sitting on a chair.
The atoms of the table are
compressed and push back on the surface against the weight of the
book.
The two normal contact forces are
equal, but acting in opposite directions - no change in motion!
If the they were not equal, the
book would either move upwards or sink into the table!
Its the same argument for you
standing on the floor.
Your body weight force acts down
on the floor and the compressed atoms of the floor push back up with
a force equal to your weight - otherwise you would move up or down!
When a spring is stretched you have 'in tension'
force as the spring tries to recover to its original shape acting
against the weight or force applied to stretch it e.g. when you use a
spring force meter in the laboratory or a spring balance for weighing
things.
(5) Normal contact forces