Crew dragon falcon 9 first stage shut down around 6700 km/hr and starlink shut down around 7900 km/hr, but around the same time. Is this because the crew dragon weighs more?
Actually it is the opposite. MECO for crew dragon took place sooner as it was lighter. Starlink missions are usually on the edge of the capabilities of Falcon 9 rockets. Each Starlink satellite has a mass of about 260 kg1. Reusable Falcon 9 has a payload limit of 15600 kg. At each Starlink mission, they launch 60 satellites.
60sats*260kg = 15600kg
As we can see the whole Starlink payload is a lot heavier. So why the lighter missions have MECO sooner? The reason behind this is simple physics. The less is the mass of the object, the higher is the acceleration with the same force applied (F=ma). As we can see with lighter missions acceleration is higher, as a result, they reach target speed sooner.
MECO - Main Engine Cut Off