# Homework Help: Drag Equation

1. Jun 1, 2010

### cmorency

In the drag equation FD =1/2ρ x v^2 x CD x A
FD is the force of drag, which is by definition the force component in the direction of the flow velocity,
ρ is the mass density of the fluid,
u is the velocity of the object
A is the reference area, and
CD is the drag coefficient

How would you find A of a sphere 1.65cm in diameter
And what would mass density be if we are measuring air resistance and not water drag? The air temperature and presure is at SATP (1 atm, 25 degrees celsius) if that helps?

2. Jun 1, 2010

### PhanthomJay

The area of the sphere is the projected frontal area exposed to the fluid, that is, the area of a circle with a 1.65 cm diameter. You can look up the mass density of air at that pressure and temperature in any table.

3. Jun 1, 2010

### cmorency

but it says per kg so would i need to determine the amount of air it travells through?

4. Jun 1, 2010

### PhanthomJay

I don't know what you mean by 'per kg'. Density is mass per unit volume. Mass density is in kg/m^3. You don't need to know the mount of air it travels through.

5. Jun 1, 2010

### cmorency

Thanks. I got it.