# Homework Help: Calculate the magnitude of the net force

1. Jul 10, 2012

### ninaw21

1. The problem statement, all variables and given/known data
Calculate the magnitude of the net force on an air parcel of mass 1.0x10-3kg which is at a temperature 250K and which is in the US standard atmosphere at an altitude of 4.0km. Take the lapse rate as 6.5K/km and the surface temperature as 288K

2. Relevant equations
Force = Mass x Acceleration
Acceleration = Speed / Time

3. The attempt at a solution
?

2. Jul 10, 2012

### Staff: Mentor

Can you say more about this problem? What is a "lapse rate"? The net force on the package from what?

3. Jul 10, 2012

### Staff: Mentor

The lapse rate is the rate of decrease of atmospheric temperature with altitude. In this problem, the temperature of the air parcel is different from the temperature of the surrounding atmospheric air, since the parcel has not yet equilibrated. First, you calculate the pressure of the atmospheric air at 4 km, knowing that the pressure at the surface is 1 atm., and assuming that the atmospheric air column is in static equilibrium (and knowing how the temperature, and thus the density, vary with altitude). Once you know the atmospheric pressure, you can get the density at 4 km, and compare it with the density of the air parcel at the same ambient pressure, and at the temperature of the parcel. From this, you can get the net buoyant force on the parcel.