If we assume a 12.5m distance between tear-off and impact point, and impact at 500 mph (223.5 m/sec) then acceleration of the foam fragment in the airstream, relative to the shuttle is A=V^2/2S = ~200g (Technically, this is a deceleration relative to stationary air, but the same number)

Length 12.5 m
Vel 500 mph
223.5 m/s
Acceleration 1997.6 m/s^2
203.6 g

Now, let's consider what the foam experiences as it is torn free in the swirling airstream:

A chunk of mass 1 kg is accelerated at 203.6g, which requires 203.6 kgf. It's surface area is 50 cm * 25 cm i.e. 0.1 sq metre, so the dynamic wind pressure on that surface must average 1629.0 kgf/m^2 or 15,981 kg/m^2 (That's why the foam get's torn off)

At the moment of Max-Q, the shuttle is at about 10.7 km altitude and external air pressure is ~0.3 bar.

At this point, dynamic pressure, q, is nearly 700 lb/sq ft (16,000 kg/m^2).

QED

So the dynamic pressure at max Q is sufficient to accelerate a foam fragment to 500mph in a distance of 12.5m (41 ft), given that the dynamic pressure is ~ 16 tonnes per square metre

From these figures, we can see why spacecraft made partially of foam aren't a good idea.

BTW, 1 kg at 500 mph delivers an impact equivalent to a baseball or cricket ball travelling at 3,500 mph