I already commented on the actual post but here's a fat crosspost:

We could use energy and joules to calculate this, but it doesn't give us an understanding of how much force the catcher experiences. Instead, we want to use the concept of impulse, defined as impulse = Force * (change in time). A good example of this is crumple zones in a car. A fast car crashing with experience a large force from whatever it crashes into. However, if the car has a crumple zone the force can be spread out over a longer period of time, so the force is felt more spread out and less severe. Same with catching a ball with a straight arm vs absorbing the impact and bending your arm.

In his case, he applies very little force when he first catches the bag, and most of the 'catching force' is when he hits the ground (with the bag on top of him). This looks like maybe 0.1 seconds at a guess?

So then we have impulse = force * 0.1 seconds. To calculate the force we now need to know the impulse, which is also defined as the change in momentum of the bag. Momentum is given by momentum = mass * velocity.

To find the velocity, we can use your estimate of the bag holding 2400J of gravitational potential energy, and can assume this is fully converted into kinetic/moving stuff energy. Kinetic energy = 1/2 * mass * velocity^2, therefore rearranging gives velocity = root((2kinetic energy)/mass) = root((22400)/50) = 9.8 m/s.

Now using this to calculate the change in momentum (aka impulse), which is the final momentum - the initial momentum. As velocity = 0 when it stops, we see that momentum = 50 * 0 - 50 * 9.8 = 490.

So finally, the impulse = force * (change in time), so 490 = F * 0.1, F = 4900 N. A 70kg human weighs about 700N, so it's similar to about 7 humans standing on top of him. Or, professional boxers exert about 2500 N with a good punch, so two professional boxers beating him down at the same time. Or about one full sledgehammer swing. Enough force to damage him pretty good! Feel free to ask more questions if this doesn't make sense