I did some simple calculations based purely on conservation of momentum. If projectile, with mass = m_{p} and velocity = v_{p}, collides with a tank with its own mass m_{t} and velocity v_{t}, and the two objects stick together and wind up moving at final velocity v_{f}, then to conserve momentum:

m_{p}v_{p} + m_{t}v_{t} = (m_{p} + m_{t}) * v_{f}

If the tank is stationary, v_{t} = 0. And since the mass of the shell is negligible compared to the mass of the tank, this all simplifies to:

v_{f}_{ = }m_{p}v_{p} / m_{t}

Suppose an E 100 shoots its 15cm KwK 44 L/55 AP shell at a Sheridan. The data:

Shell mass = 43kg

Shell velocity = 865 m/sec

Sheridan mass = 15,200 kg

Barring friction, the Sheridan would wind up moving at 2.4 m/sec.

But if the target is another E 100 at 130,000 kg, the target would only move at 0.29 m/sec.

Because the targets in fact have extremely high friction, they don't actually move, they just rock.

Now, there are some big assumptions that undermine this math. I've assumed the tank completely stops the shell that hits it. If the shell goes clear through the target, or bounces off, then it retains lots of momentum and the target doesn't rock as much. I've ignored the fact that in penetrating the armor, the shell converts much of its energy to heat. So the numbers I calculated are probably high, maybe by a factor of two or more.

But still, I think they justify why the target should rock when it's hit by a big, fast projectile.

Bear in mind that the mass of the projectile scales with nearly the cube of the bore diameter. (The 2.5th power of the bore is reasonably close, and this doesn't apply to APCR or APDS, just normal AP.) So the impact of smaller-diameter rounds scales down very quickly. Against the Sheridan, a 90mm M3 gun firing the M82 APC round (11 kg @ 810 m/sec) produces a final velocity of only 0.59 m/sec, and a 76mm M1A1 firing the M62A1 APC round (7 kg @ 790 m/sec) only produces 0.36 m/sec final velocity. So guns below about 120mm may not produce much of a visible effect against most targets.