Smoosh him flat from toe to hat
Spoiler alert: he doesn't survive.
172,500 Newtons is a lot of force. On Earth, the average human exerts about 600N of force on the ground. To exert 172,500N, you've gotta weigh nearly 20 tons. (38,700lb, or 17,500kg) And you wanna cram that into... Two fists? A back of my hand estimate (yes, literally) says that I have a punch area of 2 inches by 3 inches. Twice that yields 12 $in^2$, which transates roughly to .3048 $m^2$. This yields a pressure of 565,000 $Pa$, which is three or four atmospheres of pressure.
Delivered horizontally to a 600$lb$ knight over a period of 1 second, it would accelerate them from rest to 634.2 $m/s$, roughly mach 2. An immediate sonic boom would result, and they would be flung out of continued melee reach.
Delivered upward? 64.4Gs of force. Wikipedia says that:
A typical person can handle about 5 $g_0$ (49 $m/s^2$) (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before losing consciousness, but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle a sustained 9 $g_0$ (88 $m/s^2$)
Needless to say, a 'typical medieval knight' isn't going to have g-force training.
But wait! There's more!
I assume that your knight is being uppercut to the chin in typical anime fashion.
Were this to be the case, the whiplash from this event would be severe, likely causing severe organ failure or death.
If that weren't enough, a bit more math shows that, using the same punch time and knight mass, but orienting up would launch the knight 20 km up into the atmosphere. On Earth, this is well into the stratosphere, coming with its own host of issues, namely the insufficient atmospheric pressure and the extreme cold (circa -60 degrees C,) to say nothing of the landing.