If my calculator is working correctly, that’s 126,000 miles per hour. Wonder what the mass of the comet is.
Since C/2013 A1 is a hyperbolic comet and moves in a retrograde orbit, its velocity with respect to the planet will be very high, approximately 56 km/s. With the current estimate of the absolute magnitude of the nucleus M2 = 10.3, which might indicate the diameter up to 50 km, the energy of impact might reach the equivalent of staggering 2×10¹º megatonnes! This kind of event can leave a crater 500 km across and 2 km deep. Such an event would overshadow even the famous bombardment of Jupiter by the disintegrated comet ShoemakerLevy 9 in July 1994, which by some estimates was originally 15 km in diameter.I don't know what kind of forming that would do to Mars, but it won't be terraforming!
All that is said above is based on the current measurements, and will of course be refined as more data comes in. In any case, even now we can say that the close approach will happen. The current orbit uncertainty allows for a collision scenario, but the possibility of this is small.
Based on its magnitude I saw an size estimate of 50 km. If it is a sphere its volume would be 1.33*3.14*(25^3) or about 65,000 km^3. Assuming its mass is the same as water, we get 6.5*10^13 metric tons (I think). With little atmosphere to impede or ablate the comet, if it hits, it may be quite the show. I’ll leave it to someone else to do force and energy calculations.