Posted on 18 March 2011 at 13:30h

During break some of the staff were talking about a video they had seen on the web about a guy called Professor Splash who belly flops into 30cm of water from about 9m (new record is now 11m). I thought this might be a good example of motion to discuss with my SL class. First they calculated his velocity on hitting the water and got about 13ms^-1.  Then, assuming the acceleration to be constant found the deceleration through 30cm of water to be 300ms^-2.  Wow 30g. He's a big guy so assuming his mass is 100kg then the force required to slow him down is 30,000N which if the area of his front is 1m² (he has a bit of a belly and a big chest) the the pressure is 30 kPa or 3Ncm^-2 . Well that's not much I think I'll give it a go. Of course there were rather a lot of approximations there including the old "constant acceleration" one.

Watching the video carefully you can see a couple of interesting features. Just before hitting the water he puts his hands out giving an extra 50cm of deceleration just before hitting the water. I guess his hands will be pushed against his body just before it makes contact with the water, his body is angled forwards so that even though his hands hit first he still hits the water flat. Having such a small amount of water in a plastic paddling pool might mean he is able to displace the water sideways instead of upwards but I haven't quite got my head around that one yet. Apparently the amount of water is important as is its temperature, I can't see the temp being so important as the change in density of water at different temps is actually not that much (1000.00 kgm^-3 at 4°C and 995.68 kgm^-3 at 30°C density calculator).

 I almost forgot, probably the most important feature, the mattress under the pool.