A Guide to Physics Problems. Part 2. Thermodynamics, Statistical Physics, and Quantum Mechanics

96

Equating the amount ofheat required tomelt amass of ice with the

heatavailable in themetal, we have

This mass exceeds theamount of icefrom part (a), so all of itwould melt.

4.12 Hydrogen Rocket (Moscow Phys-Tech)

Find theamount of water vapor produced in the reaction

One mole of hydrogenyields onemole ofwater, or inmass

Since is themass of fuelintake persecond, is the mass ofwater

ejected from theengine per second. If the water vapor density is this

rate may be expressed as

where is the velocity of the gas ejectedfrom theengine.Therefore,

Express the density as

From (S.4.12.4) and(S.4.12.5), we then have

The mass ejected persecondfrom the engine provides themomentum per

second which will be equal to theforce supplied by the

engine. Apart from this reactiveforce, there is astatic pressurefrom the

engine providing a force so the total force

In real life thesecond term isusually small(Pis not veryhigh), so the
force by an engine is determined by the reactive force.

SOLUTIONS