Why is there no dust on the Lunar Lander's footpads?

[Noldi400]

The whole trough concept reminds me of the "water brakes" they used on Col. Stapp's rocket sled back in the day.

I may not be an aerospace professional or, indeed, an engineer of any sort, but I knew who John P. "Death Wish" Stapp was before I was clear on Captain Kangaroo.

[Commander Cody]

the amount of blast force onto the moon while the apollo was landing it must of blew the moon dust to the sides. but at the same time some dust might fall down onto the pads, but mabye the image was not zoomed in enough to see the dust on the pads?

[Jason Thompson]

How would dust fall 'down' on the pads when it is being blown away by the engine? Remember it doesn't billow in clouds or linger around. It is blasted off at very high speeds laterally.

[cjameshuff]

And as the images I linked to show, some bits of dust did end up on the pads, probably bouncing off the leg and getting trapped in folds of mylar, or the trailing wisps of dust as the engine shut down.

[ka9q]

the amount of blast force onto the moon while the apollo was landing it must of blew the moon dust to the sides. but at the same time some dust might fall down onto the pads, but mabye the image was not zoomed in enough to see the dust on the pads?

You are basing this on your intuition, but your intuition is based on a lifetime of living on the earth, with 1 g gravity and 1 atmosphere pressure. Neither is true on the moon, so some things behave very differently there.

[Britmax]

the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.

The really interesting thing is how often you can find a real-world meaning to the units on some set of measurements.

Example: the fuel economy of a car. In the USA it's usually measured in miles per gallon. It could also be given as gallons per mile, as in many other countries it is given as litres per 100 km. A gallon is a unit of volume, so it has units of length cubed. (You compute volume by multiplying three length measurements, so the result has units of length cubed).

The mile, of course, is just a unit of length. So if you divide gallons by miles you get units of length squared. Does this have a physical meaning? Actually, it does! It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Edited to add: As an example, a car that gets 30 mpg would have to scoop up a trough of gasoline with a cross-sectional area of 0.0784 mm^2. That's a square 0.28 mm on a side. Doesn't seem like much, but it adds up.

The same works for electric vehicles, which are rated in units of miles per kilowatt-hour, or kilowatt-hours per 100 km. A kilowatt-hour is a unit of energy, which has basic units of kg m²/s², also known as the joule. (1 kWh = 3.6 million joules.) If you divide that by units of distance, you get kg m/s², which happens to be the newton, the unit of force. In other words, the mileage rating for an electric car is equivalent to the physical force needed to overcome drag and keep the car going. (This also includes some electrical and mechanical losses that appear as "virtual" drag in the final result.)

There are all sorts of other examples like these; physics can be a lot more intuitive than many people think.

I was having trouble understanding what you meant by trough but I think I have it.

Imagine a car is like a train running on a third rail. Except instead of a rail of steel carrying a current, it is a gully filled with petrol that miraculously doesn't evaporate. Instead of a contact shoe, it has a suction inlet that dips into the petrol and is shaped to spade the gully and as the car moves, the inlet sweeps the gully leaving it dry behind and sucks in the petrol it sweeps. The thirstier the car, the bigger this gully needs to be to satisfy the requirements. The cross sectional area of this gully is mathematically equivalent to the consumption.

In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.

[Jason Thompson]

In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.

Used to great comic effect in an episode of Dad's Army, where the end of the episode sees the entire platoon (after a series of comic mishaps, of course) line up trackside to salute a train carrying the King. The train used the scoop and trough system, and of course the final shot shows the whole lot of them getting absolutely drenched as they attempt to smartly salute the monarch....

[Britmax]

In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.

Used to great comic effect in an episode of Dad's Army, where the end of the episode sees the entire platoon (after a series of comic mishaps, of course) line up trackside to salute a train carrying the King. The train used the scoop and trough system, and of course the final shot shows the whole lot of them getting absolutely drenched as they attempt to smartly salute the monarch....

I remember that episode but haven't seen it lately. Those looking for a "return to topic" will enjoy the thought I had that that according to hoaxer logic, Jimmy Perry and David Croft must have made it all up because one of them wasn't in the real Home Guard during the war, as obviously that "conflict" was made up by the Jewish Lizards. Or the military industrial complex. And that.