Solve for Velocity (without Mark Rober's $89 Blanket)

So, you want to calculate the velocity of something, but don't want to spend $89 to do it (or $229 from the scalpers on e-bay... since it's currently sold out in the official Mark Rober store)? Congratulations! You're probably smart enough to have figured this out for yourself, but you can make one yourself for the cost of a ho-bo dinner if you've got a free night (which I'm guessing if you're reading this you're the type of person that has a free night like me.... ¯\_(ツ)_/¯ )

Required:

• Writing Device (sharpie, watercolors, ashes.... we won't ask where you got them from)
• Display Device (cardboard, blanket, that rear projection TV you regret carrying downstairs)
• Measuring Device (calipers, drafting ruler, phone with known pixel density and a grid)

Recommended:

• Tape
• Drywall T
• Multiple colors
• (and phone with slo-mo record if you actually want to use it [ideally with a tripod])

So... In case you hadn't heard... Amazon offers great prices and ships (for free with prime membership) directly to your door. In my case doing subscribe and save with 1 month supply of size 6 diapers, 1 month supply of size 3 diapers, 1 month supply of toilet paper, 1 month supply of paper towels.... you get it... got me the HUGEST box I've ever gotten from Amazon this week (they even have an option to 'ship in the fewest boxes possible' nowadays. Maybe we should make this a game...)

Ideally the goal of this project is to get what you need for free or minimal cost to you so you can spend your money on making things go fast instead of fancy blankets.

Ok, so I'm going to use Mark Rober's exact sized squares (2.19 inches) for my poor man's velocity blanket. This will make it so you can count each individual square to get 1 MPH of velocity for your calculations. You can multiply by 1.6 to get km/h, but alternatively I'll give you convenient square sizes to get native measurements in your preferred units.

Also, depending upon where you live and how good your camera is there's a decent chance your record and playback frame rates may differ from Mark's. First and foremost, there's 3 main framerates cinamatic (24 FPS), NTSC (30 or 29.97 FPS - Japan/USA), or PAL/SECAM (25 FPS - rest of the world). Usually your camera will record in multiples of these values, and at the end of the day all we need is the ratio of these numbers.

You may also prefer your measurements or speed in different units (standard vs. metric). I'll give the most common I think you'll need here, but you could solve for any others if desired:

8x slo-mo:

• 2.19 in squares: 1 square = 1 MPH
• 1.50 in squares: 1 square = 1 FPS (foot/sec not frame/sec)
• 3.47 cm squares: 1 square = 1 km/h
• 12.5 cm squares: 1 square = 1 m/s

10x slo-mo:

• 2.78 cm squares: 1 square = 1 km/h
• 10.0 cm squares: 1 square = 1 m/s

Make your squares twice as big for 4x or 5x slo-mo modes, or see table for other variations.

The equation if you want to solve for something else is:

(Playback Speed (FPS) / Record Speed (FPS)) * ((# of units you'll draw in (inches/cm) in your speed measurement (miles/km) / # of seconds in your unit of time measurement)

For example, if you want to see how many Big Mac's (in height) your hand moves every minute at maximum velocity while eating fries from the golden arches and blazing away on a phantom flex4k camera then viewing at a cinematic experience you would use:

(24 FPS / 1000 FPS) * (9.5 cm / 60 s) = .0038 cm sized boxes. Not practical... but possible?

Now we're ready to get crafty.

I highly recommend using calipers, a drywall t-square, and making a template to get this going in the right direction if you're going to use an inconvenient square size (like 2.19 inches). What I did was take some gaffer's tape, and wrap it around my t-square. I then folded it so it was exactly 2.19 inches edge to edge minus the distance I found from the edge of my marker to the square. Then I just had to slide my t-square so that the left edge of my tape lined up with the left edge of my previous line, and draw vertical lines along my cardboard. I then repeated this for the horizontal lines, and added sharpie to every other square like a chess board.

Note 1: Ideally you would take all measurements from the same reference so that you didn't accumulate any errors by repeating your template. If you've chosen a convenient measurement like 10 cm boxes, or 3 inch boxes this will be much easier, since most meter sticks don't have markings every 2.19 inches. To be true to the original I didn't use one of these more convenient numbers, and lived with any accumulated errors I made for myself.

Note 2: There really is no need to do the horizontal lines in my opinion. If you're going to be counting the vertical (up/down) component you'll need to use the quadratic equation to solve for your actual velocity defeating the ease of use of this 'blanket'. This will speed up your project significantly! Again - I tried to stay true to the original

Note 3: Mark's blanket had bolder lines every 5 boxes. I instead went back and used different colors to mark the uncolored boxes with x's and o's (to count by 5's or 10's). If I'd had more foresight I would have just colored the boxes different colored boxes from the start. Also, consider your color choice. blue and black look pretty similar on cardboard, but red definitely shows up differently.

Note 4: If you're interested in speed, I think the ABSOLUTE FASTEST WAY to do this project would be to find 1.5 inch or 3 inch painters tape and just slap it directly to the surface in vertical lines then use feet/sec as your velocity units. You could even put it on the wall in your house without worry! Google calculator can quickly get you into your preferred velocity measurements from there.

Technically I'd say we're done building for this Instructable. You may find that you want to mount this for your project. I think I'll end up using it in the following methods: Taped to a flat surface, zip tied to a chain link fence, and mounted to either pvc or 1x3 so I can mount it to other objects and use it at different angles.

Technicalities aside you'll want to actually use this to measure the speed of something. Therefore, I would recommend getting your phone (or possibly an upgrade?), and a tripod.

Note 1: I use the pixel 4a, which only cost $350 when I got it new over a year ago. It's not a top end phone, but can still record at 240 FPS (8x slo-mo). Your phone likely has a slow-mo mode of some sort, but you'll have to rely on your manufacturer for official specs. Tripods can be picked up for $20 on amazon, but really you're not going to move thaaat much in 1/8 of a second.

Note 2: There are even several Huawei phones that can push 1920 fps if you're willing to drop the resolution. I personally have been waiting to see what Sony does with the next RX100 camera (viii). The vii and ZV1 shoot up to 1000 FPS, but have a bigger sensor than a phone. I wouldn't rely on higher FPS settings that use frame interpolation. Going beyond these will me a multi thousand dollar endeavor and can cost more than your house...

• Just set up your speed checker behind your firing range with it as close as possible to your projectile. Then set up your phone as far away from your subject as possible while keeping good visibility.

Note 3: The further apart your checkerboard is from the shot the more you will suffer from 'parallax distortion', where because you're eyes and camera are looking at an angle the object will look further to the left than it actually is when it's on the left side, and further to the right than it actually is while it's on the right side artificially increasing your velocity record. Putting the camera further away also reduces this effect. This is why your passenger asks why you're going 3 below the speed limit when you're actually going 2 over...

• Next, select the appropriate slo-mo mode hit record and fire away!

Note 4: It's best to have a buddy. This is why 'slo-mo guys' work as a team. High speed film eats up a lot of storage, and takes a lot of time to review. Sometimes you only get .5 seconds at real world time to film your experiment depending on the camera because it can fill up your memory so quickly.

• When you stop your recording you'll want to trim the video to be a little more than 1 second long with the whole thing (or majority if your phone requires it) displayed in slow motion.

Note 5: This will help reduce your storage space, and review time, but more importantly (if you're going to do the last step on your phone) it will let you have greater precision when selecting frames on your scroll bar for your video.

Finally, you'll need to count how many squares your projectile moved in 1 second of 'slo-mo time'. If you're playing it back using a video editor (I use Corel VideoStudio) all you need to do is advance 1 second on your timeline and count how many squares the object moved. If you're doing this on a video player like your phone's you'll need to move the slider back and forth to hone in on 1 second so you can do your count.

Once you've counted your squares that's how fast you were going in your chosen units of speed. To make it a velocity you'll need to specify what direction it was traveling too.

Congratulations, you've done it! If you're like me, this project didn't cost you anything at all, and now you have what looks like a giant finish line! I did end up throwing away the sharpie I used when I was done. It faded throughout the whole project, but possibly worse I rubbed off most of the felt from the tip of the marker coloring that much. I don't know how old that was when I started though.

Mark's blanket is a '40 MPH blanket', but mine was nearly 10 feet long, and had 54 '1 MPH squares' at 240 FPS. He recommended using 2 or more blankets for higher speeds. I would actually recommend just counting how many squares you moved in an easy multiple of the slo-mo time (like .5 seconds, or .2 seconds then multiplying your answer by 2 or 5 respectively.) At 240 FPS record, and 30 FPS playback .2 seconds would be 6 frames. With 54 squares I could measure up to (54 x 5 =) 270 MPH using my setup. The precision goes down, but it's just easier this way.

You might also have noticed like me that he didn't even use the blanket in the video where he sells the blanket (probably because of the angles, or the fact they weren't finished), but you can be proud knowing you outsmarted the master. (In all fairness I probably would have just bought one for the cred on my own videos, but 'sold out' meant 'get creative' not 'you're defeated')

Note 1: You could alternately use the 'stick method' used by Destin on Smarter Every Day to calculate velocity where you count how many fractions of a second it takes to travel a known distance (like 6 foot spacing on your fence posts). At 240 FPS if it traveled 6 feet in 49 frames you would have (6 feet / (49 frames / 240 FPS) = ) 30 feet/sec

Note 2: What you're really calculating is the average velocity over that period of time or distance. In the real world you'll be experiencing external forces on your system, which will cause it to be either accelerating or decelerating at the same time as you're measuring. In this case shorter time measurements will likely give more accurate results as there could be significant changes in velocity in very short times.

If you want to see what high speed device I'll be using this for later this month please consider subscribing To Nerd Snipe on YouTube here!

Also, if you did a faster/better/prettier version let me see it down below. What I would really like to see is a rainbow where each line is a different color that repeats every 7 rows with high quality acrylic paint (maybe sharpies on white poster board?) OR I really want to see somebody use tape along a cinderblock wall as a quick temporary solution. OR use multiple poster boards taped accordion style to make it super long (or short depending on the activity of the day); the ideas are flowing now!

Thanks!