What are the mechanics of a dunk tank?

Get a big plastic tank filled with water. You can use an industrial sized barrel, storage tank or some sort of above ground pool.

Take a large board, wide enough to fit across the tank, and cut out a rectangular piece from the middle for the seat.

Attach the seat back into the support board with heavy duty hinges at the back so that it can swing down freely.

Attach a heavy duty door latch of the type used in public restrooms to the underside of the seat so that, when the latch is closed, the seat can't swing down. Use screws and epoxy to attach and reinforce it.

Horizontally mount a bicycle wheel on the far side of the tank. It should be mounted on the same side that the latch is going to be on. Mount it in such a way that it will spin freely.

Create a target. You can use a simple piece of wood, a caricature, or anything else. The point is to make something for people to throw at.

Nail the target to the end of the lever arm.

Attach a large lever arm to the bicycle wheel. This is the board that the target is going to be mounted on, so it should reach out far to the side of the wheel.

Cover the seat board with something comfortable to sit on. This is going to be the seat, so you want to use something padded. Foam attached with contact cement would be an excellent choice, but even a thin layer of cloth or vinyl would work.

Attach the longer board across the top of the tank so that the other board swings downward when released. Depending on what the tank is made of, you could use epoxy, nails, brackets or any combination.

Attach a string from the bicycle wheel to the latch so that, when the lever arm swings back, the latch pulls out and releases the seat.

Fill the pool with water...

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.