Paul, why is it so hard to understand?

before entering the turn the mass has a speed of +40m/s to the RIGHT, after the turn the mass has a speed of +40m/s to the LEFT.
Instead of LEFT and RIGHT we are using signs:
+40m/s (to the RIGHT), -40m/s (to the LEFT).

The difference is a velocity of 80m/s.
Because velocity has a direction, its a vector.

As kallog stated several times: depending on how much time you allow for this velocity change of 80m/s the force applied to the mass can be anything.
You chose 1 second, so the mass (100kg) will get a force of 8000N applied.