#baryallen

You can't run up the side of a building.

Like, okay, suspension of disbelief and all that, but you really can't run up the side of a building. Maybe if they'd handwaved it with "His Speed Force propels him up the side of the building", but they had Cisco do actual math, which implied he was using real-world physics to find out how fast he'd need to go to run up the side of a building.

But you can't run up the side of a building.

It's really basic physics. First year high school physics stuff. Vectors of force. When you run on solid ground, your feet push up and forward against gravity, and gravity pulls you back down so you don’t fly off into space. You rotate that 90 degrees upwards, the forces don’t all rotate with you.

So what happens if he tries to run up the side of a building? Well, step one, he runs towards the building at hundreds of miles per hour. Step two, splat. Or, step two, crash through the windows on the side of the building.

But let's suspension-of-disbelief away that splat/crash and assume that his speed force lets him magically change that vector immediately from forward to upwards. That's fine. He can ignore inertia to stop really fast and turn really fast, so that's in bounds. So say his forward momentum just flawlessly bends to upwards momentum.

He takes his first step in that new direction, pushing off the "ground" (i.e., the side of the building). Gravity isn’t pulling him towards the building, though, which means that the force vector of his foot pushing against wall isn’t countered by anything. So he's got his foot pushing him away from the building, momentum pushing him upwards, and gravity pulling him backwards. There are zero forces pushing him towards that building.

If he's trying to run up the side of the building, what's going to happen is he's going to fly in a big arc off the side. Then... Splat.

Previous episodes have shown Barry as 100% capable of running up lots of flights of stairs in far less time than it took him to have the "How can I get up to him?" discussion with the team. He'd have to break a window when he got there, but I feel like a chair chucked at Mach 1 would easily shatter some glass of the office underneath the window washer. Then just reach out and grab the guy and pull him inside.