Week 7, clip 11

Timepoint:  06:59.9

1) Using the equation for dynamical friction, f_D = ( G^2 M^2 \rho ) / ( v_M^2 ), let us figure out an estimate for the drag on a globular cluster passing through the Milky Way.  Consider a globular cluster with a mass M = 1e6 * M_sun moving in a circular orbit perpendicular to the disk of the galaxy at a radius around R = 8 kpc (and thus a velocity of 220 km/s).  

How large is the dynamic friction force when the globular cluster passes through a region with a stellar mass density of 1 M_sun / pc^3?  Give your answer in newtons, rounded to two significant digits.

Answers:
1) The accepted range is [2e22,3e22]

Explanation:
f_D = ( G^2 M^2 \rho ) / ( v_M^2 )

f_D = ( (6.673e-11)^2 (1.9891e36)^2 (1.9891e30) ) / ( (2.2e5)^2 (3.08e16)^3 ) = 2.5e22 newtons.

For comparison, the gravitational force exerted by the Sun on the Earth is about 3.6e22 newtons.