# How slow can slow motion get?

Hi,
ever been wondering how slow can slow motion get? If you hang for a longer time on YouTube eventually you might notice the channel The Slow Mo Guys or Smarter Every Day, both of them feature „quite often“ slow motion videos, meaning videos that have many frames per second (FPS). If you then slow the frames down you will get very slow video showing detail of whatever you are filming, whether it is flame tornado or AK-47 under water.

The slowest I ever watched is this one: https://www.youtube.com/watch?v=xbuvcQrAOSk

It has FPS of 343 915. That is a lot and in the video they are cracking glass container with high temperature difference. The video is so dark because in the short amount of time, not much light can get into the camera and that is often the limit for such high speed filming. You can also see that even though the video is so slow, the crack still propagates through the glass in surprising speed.

But this is not the limit, you can go much slower but it requires whole different technique. With 1 trillion FPS you can actually see light traveling through medium, it looks pretty impressive but how can it be done?

The scientists use a laser to lighten up what they want to take photo of. They take a 1 dimensional pictures, basically line of pixels[1], couple hundred times and then with a smart mirror they move along the object that they photograph. This technique is called femto-photography and the event on stage has to be performed many times before one gets the picture.

To put it in different words, you take a titanium-sapphire laser. Lighten up the object for a short time. Capture the photons that bounce back to you. Repeat over and over again and with some mathematical reconstruction techniques you got your result:

Interestingly enough this kind of device is able to see around corners and has very high potential for use in many technical fields if we are able to make it work faster and in smaller devices.

Post based on TED talk.

Dragallur

[1]It is not really line of pixels as you can read in the next paragraph. You are getting back couple of photons and through the mathematical analysing you get the picture.

# There is no centrifugal force

Hi,
why do you feel the push in carousel? Or when you are in car turning and you feel pushed on some person which can feel pretty awkward. This is all because of centrifugal force right?

# YOU have been living in a lie

Your whole life was lie if you ever thought that there is something like centrifugal force.

So where is the problem? What causes us to fly from circle? There are three forces acting on you on carousel, gravity of Earth, the bench up so you do not fall through it and also towards the middle so you do not fly away. But still you feel the push from the circle, why if there is no force?

Your body wants to stay in the movement (it has some inertia) which is straight in direction and also in speed. This is Newton’s first law. But it does not because the bench goes in circle and you hold it so you go in circle too but your body does not want to do it so it would rather fly away in straight line which would be tangent to the movement.

Above you can see that the line TP is what your movement would actually look like relative to the circle which is the carousel.

At every point of the movement you want to go tangent to the circle but you are holded by the force which has the tag of centripetal force. This is just a tag, all kinds of forces can be centripetal, gravity for orbits, friction of wheels for car in turn, ball on string or the bench of the carousel on which you are sitting.

Lesson: do not use the word centrifugal force anymore, it is not there and laws of physics would not work if it would be there.

Check out the video below showing when the speed gets way too big and the centripetal force simply is not able to hold it together anymore (here the centripetal force are the bonds holding the CD together):

Dragallur

Thanks my physics teacher!