Force is calculated as velocity * mass. Since velocity in Construct physics objects is separated into X and Y components, I believe you would just add them together. Don't quote me on it, but I'm pretty sure.
But since leftward and upward velocities give negative values you would have to use abs() to make sure you're only adding positive values.
Finally, velocity is returned as a floating point value so you'll probably want to round() or ceil() it into a more usable number. The final expression would look like so:
[code:3gyjlp4s]round((abs(Sprite[Physics].VelocityX) + abs(Sprite[Physics].VelocityY)) * Sprite[Physics].Mass)[/code:3gyjlp4s]
Here's a .cap showing the collision in action:http://dl.dropbox.com/u/529356/usetheforce.cap
Throw the red ball at the green ball with the mouse. The force of the collision is shown in the text box. Notice that the force can get pretty high rather quickly, so you may want to calculate your final damage as a percentage of the total force instead. Also note that On Collision will keep triggering as long as the two physics objects are rubbing together, so you may want to give a bit of a grace period to your collisions so things aren't drained of all their health by getting pinned or something.
Oh and if I'm doing this totally wrong then someone please speak up