How to 'flatten' a set of values

Good day! I'm not sure if it's because I can't find the right term for it or because I just can't search. But anyway, I can't find a solution to this either on excelforum.com or on Google.

Everything I find on Google has something to do with a trendline and even though that's exactly what I want, I need something else. To be more precise, I need the data behind the line, not the visual line itself. Also, the trendline shifts unwanted on the x-axis.

In this spreadsheet (XLSX) of mine you can find a set of data that came from a datalogger. It logs the speed of a wheel, I gave it quite a yank and then let it spin till it stopped. What I want to reveal is the deceleration and therefore I need the velocity.

The speed of the wheel is measured by a frequency sensor that measures what the frequency is of holes in a disc that pass by. The disc has 30 holes and therefore, 30 Hz is one rotation per second.

The nice thing about this is that I can really precisely measure the velocity of the wheel. IF I can actually manage to take away the 'errror' effect of the sensor itself. You see, that's where the problem lies. The sensor has some kind of deviation that makes the output data 'wobble' up and down. This effect is bigger when the measured frequency is bigger.

I'd like to 'flatten' this data so I get a nice and smooth curve. Because when I do, I get a nice and smooth deceleration graph too! Can someone explain me how to do this, give me an example, a website/tutorial or whatsoever? I'm really struggling here and since this case is hard to Google I just thought I'd put it here.

The things I've tried are:

Lineair interpolation, although Excel can't do that, I tried it manually but it's very time consuming when you have over 3000 values.
=TREND is some kind of interpolation I discovered. Maybe I'm doing it wrong but if I try to apply this I still get a wobbly line.
=AVERAGE between 10 values. But I feel like this isn't a nice way since it will just create a "staircase" graph.

Here are some illustrations of the case (Dutch titles, don't worry, the XLSX is English)

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As I told in my first post, I would like to have the data, not the trendline itself. So the data is important to me, since I will use it in another piece of software. I simply need to flatten out my data so that I get a clear acceleration.

The acceleration (the red graph) seems to be very jerky now and that's because the velocity is not nice and smooth. The smoother my velocity data is, the better the acceleration will become visible.