# Calculate drag force

1. Sep 1, 2012

### steve978

1. The problem statement, all variables and given/known data

A tennis player hit a serve of 251 km.h-1 in a match. Calculate the Drag Force acting on the tennis ball. Diameter of ball is 6.5cm, density of air is 1.3 kg.m-3 and air viscosity is 1.8x10-5 kg.m-1.s-1.

2. Relevant equations

Drag Force is: Fd= $\pi$.d2.CD.pf.u2.1/4

3. The attempt at a solution

My attempt is to find Reynolds number: Rep = (pfutdp)/u = (1.3)(903,600,000)(0.065)/0.000018
and then use a graph of drag coefficient against Rep to find Cd. However my number for Re is 4.24E+13 which is far too large to be correct. Is there something i am doing wrong?

2. Sep 1, 2012

### TSny

Hello steve978.

Check your conversion of km/h to m/s.