### Overarch 2

Bricks are 20cm long and 10cm high. How high could an arch be built without mortar on a flat horizontal surface, to overhang by 1 metre? How big an overhang is it possible to make like this?

### Stonehenge

Explain why, when moving heavy objects on rollers, the object moves twice as fast as the rollers. Try a similar experiment yourself.

### Maximum Flow

Given the graph of a supply network and the maximum capacity for flow in each section find the maximum flow across the network.

# Differential Electricity

##### Stage: 5 Challenge Level:

Kirchoff's Voltage Law:

$V_0 = V_L + V_R + V = L\frac{dI_1}{dt} + IR + V$

Current conservation at node X:

$I_1 = I_2 + I_3$

$I_2 = I_C = C\frac{dV}{dt}$
$I_3 = \frac{V}{R}$

$I_1 = C\frac{dV}{dt} + \frac{V}{R}$

$\frac{dI_1}{dt} = C\frac{d^2V}{dt^2} +\frac{1}{R} \frac{dV}{dt}$

$V_0 = L(C\frac{d^2V}{dt^2} +\frac{1}{R} \frac{dV}{dt}) + (C\frac{dV}{dt} + \frac{V}{R})R + V = LC \frac{d^2V}{dt^2} + (\frac{L}{R} + CR)\frac{dV}{dt} + 2V$

The governing differential equation is:

$LC \frac{d^2V}{dt^2} + (\frac{L}{R} + CR)\frac{dV}{dt} + 2V = V_0$