Wall of death
If the bike makes an angle
q with the vertical
we have: tan
q= v
2/rg
Aircraft banking

If
the plane banks at an angle of
q with the horizontal we have:
tanq= v2/rg
Cyclist cornering

If the cyclist
leans over at an angle of
q with the vertical we have:
R = mg F = mv
2/r and tan
q= F/R
The coat hangar
Take an
ordinary wire coat hanger, bend the frame into a square and bend the hook until it faces the
opposite corner of the square. File the end of the hook flat. Now balance a 1p coin on the hook
and spin the coat hanger in a vertical circle with your finger in the top of the frame. The centripetal
force of the hook on the coin will keep the coin balanced on it although it may be upside
down!
Strength of rotating objects - an extension topic
This proof
enables you to find the maximum tension of a wire before breaking in terms of its angular
velocity.
Let the mass per unit length be
r. The m =
rL where L is the length of the wire.
Considering the forces acting
on a short length of wire:
(T + dT) - T = -
rw2dm giving dT = -
rw2dr since dm =
rdr
Integration with
limits gives T = ½(m
w2)(L -r
2/L)
giving the maximum tension of ½
m
w2L when r = 0. That is the wire will break at the point of attachment.
Maximum tension before breaking (T) = ½mw2L when r = 0
Example problem
A wire of mass 0.4 kg and length 2.0 m and cross sectional area 8x10-6 m is swung round in a horizontal circle. If the breaking stress is 8x109 Pa calculate the maximum frequency of rotation before the wire snaps.
Using the formula and the figures given above: f = 63.6 Hz
Useful
data
Passenger jet – maximum legal banking angle = 27.5
oDaytona race track
= 31
o at 150 mph
Aston Martin test track =
11
o for r = 516 m v = 70 mph
A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB