physics question

"PartyPooper2012" (PartyPooper2012) 03/22/2019 at 09:39 • Filed to: None

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TV is on in background. I looked up and there was that beer commercial where they didn’t use corn syrup or whatever...

Scene in commercial was a huge barrel and few people pushing it up hill . Barrel is so big, it seems impossible for people to roll it over to go up so if I were to do this in real life without CGI or shenanigans, I would probably place a small car inside barrel and drive it forward. As the front of the car drove up on the higher part of the side of barrel, barrel would lower that part and roll forward...

What I am curious about though is besides the obvious problems of steering, would jerky motion cause the barrel to jerk back and roll backwards essentially killing everything in its path?

Azrek > PartyPooper2012 03/22/2019 at 09:54

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Then there is the philosophical question...

If they push shitty beer to the top of a hill...will people ascend the hill to drink it?

Sisyphus would likely prefer to push this than a rock.

Ash78, voting early and often > PartyPooper2012 03/22/2019 at 09:54

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Sounds like the only problem is ensuring adequate traction between the barrel and the hill. Otherwise you’re golden, Colorado.

/LMAO at corn-based adjunct lager pissing contest

WRXforScience > PartyPooper2012 03/22/2019 at 09:57

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That’s basically a car powered treadwheel, the car wouldn’t necessarily roll up and down the wheel. If you match the speed of the car with the speed of the wheel you can drive in place.

Barrels are designed to be rolled and there are even competitions for it: https://en.wikipedia.org/wiki/Competitive_barrel_rolling

I don’t normally watch regular TV, so I hav en’t seen the commercial in question; however, people are pretty good at moving stuff (see the Pyramids).

nermal > PartyPooper2012 03/22/2019 at 09:58

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Basically, this would happen :

Highlander-Datsuns are Forever > PartyPooper2012 03/22/2019 at 09:59

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You need to start with a force diagram. If the car is heavy enough and the center of mass is low enough it will overcome the barrel’s tendency to roll backwards. Once you reach a certain slope the barrel will roll backwards no matter the mass in the bottom. The steering is not too hard as a barrel has curved sides so if you drive to one side or the other it will lean and then go in that direction.

RamblinRover Luxury-Yacht > PartyPooper2012 03/22/2019 at 10:19

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Well, let’s see. Assuming a 5 degree grade, and 1000lb barrel, the force at the center of the barrel in x required to resist going downhill should be about 1000lbf*tan(5) = 87lbf. (The normal force should be about 1004lbf).

(If I’ve got this right)

To provide that force by pushing up on the barrel on the outside rim...

Let’s suppose they’re pushing at 45 degrees from flat.

That “retaining on slope” force has to equal F_p * 2 * cos 45 + F_p*sin 45 - because pushing up on the barrel (rotating it around center) is doing the work of a 2:1 lever, and pushing back on the barrel is directly providing the Sisyphian Force we’re looking for. Solving for F_p, we get 3*.707*F_p = 87lbf, or F_p = 41lbf.

So, to roll a barrel without rolling resistance that weighs 1000lb up a 5 degree incline should in theory require only > 41lbf. A 10,000lb barrel should be within the abilities of four people... maybe.

Obviously if the slope were greater it would be worse. You could probably double-check the figures by using an energy calculation in units of lbf*in in potential energy vs. work, but work done isn’t necessarily as useful or precise.

BigBlock440 > Azrek 03/22/2019 at 10:28

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Yes, yes they would.

punkgoose17 > PartyPooper2012 03/22/2019 at 10:30

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I think I finally got my force diagram right. The yellow equation is what you need to not roll down the hill.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > RamblinRover Luxury-Yacht 03/22/2019 at 10:34

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#OppoDoesMaths

BaconSandwich is tasty. > Azrek 03/22/2019 at 10:40

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Never underestimate desperate high school or university students.

RamblinRover Luxury-Yacht > PartyPooper2012 03/22/2019 at 11:03

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To answer the question you actually asked, it would depend somewhat on what the moment of inertia of a feckin’ yuge barrel is vs. the torque moment of a small car on the inside.

Say it’s a very small car only pushing very slowly. I’ve assumed a ‘98 Hyundai Accent, and lazily assumed a c.g. exactly centered on the wheelbase. A 2500lb weight set up like this, in a 20' barrel:

The couple to rotate the barrel by shifting the center of gravity is very slight. Rolling the car up to 24 degrees shifts the c.g. by 40". 8333lb-ft on a 20ft barrel can be opposed by as little as an 833lb force acting at the rim, as you can see. The balance is pretty fragile, so it comes down to whether the barrel has enough inertia to resist a shock - and then, having started rolling back , have enough momentum to deal with slinging the car .

I = MR^2. For the sake of argument, let’s say the barrel weighs 10,000lb. I = 1000000 lbm ft^2.

Assuming the barrel has been started rolling at 62.8 ft/min, that’s an angular momentum of 1000000 lbm ft^2/min. More interesting, however, is the kinetic energy, because that can be used to get the scale of the impulse force required to stop this thing. That is 500000 lbm ft^2/min^2, or 138 lbm ft^2/s^2, or 4.28 lbf*ft . Let’s conservatively double that, because of the additional angular momentum contributed by the car’s linear momentum (not sure how sound that is, but including it).

So, to make the barrel stop, or at the least lurch, would require all of 10lbf*ft of energy. To make it overcome the torque of the car would be much harder, however. If when bumped the car continued going until it raised to a 45 degree angle, the car’s c.g. would be raised 20".

2500lbf*1.66ft = 4166 lbf*ft, which is over 30 times as much energy as it would take to make it wobble a little.

However, that’s probably all it would take to start the car slipping.

To answer the question in simple terms - I don’t think jerky motion would ever start the barrel rolling back, *as such*, because the car can keep pushing energy into the system waaaaay in excess of how much the barrel would resist once it had started rolling back. The barrel just doesn’t have that much angular momentum. However, interestingly, it’s comparatively easy to push the barrel back against the car’s best efforts, or to avoid being crushed by merely pushing on the barrel.

RamblinRover Luxury-Yacht > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 03/22/2019 at 11:05

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I posted another one answering his actual question, in addition to “is it realistic to push a huge barrel with just a couple of dudes?”.

RamblinRover Luxury-Yacht > punkgoose17 03/22/2019 at 11:19

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My analysis wasn’t quite complete, as it didn’t have a static forces diagram of the hill angle. Yours helps establish what the max possible to climb in the first place is... and how high the car would be at the start of any “bumps”, which makes for a shorter distance to be lifted to overcome the struggling car, hence lower perturbation energy.

PartyPooper2012 > Highlander-Datsuns are Forever 03/22/2019 at 11:54

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Well yes.. .but the hard part would be to determine where you need to go. You are in a barrel after all and cant see much else. 

PartyPooper2012 > RamblinRover Luxury-Yacht 03/22/2019 at 11:59

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The jerky motion i was referring to would have been caused by accelerating and stopping due to.... fear factor.

So say you accelerate hard and then get scared and stop. the barrel would roll forward for a bit longer but then rear of the barrel would have the car up so it would tend to roll backwards and as you said... there isnt much needed to roll backwards and once you start.... it’s like Pringles

PartyPooper2012 > Azrek 03/22/2019 at 12:03

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Well... to conserve the energy, wouldn’t it make more sense to let the beer flow downhill?

Surely some sort of plumbing would be needed as to avoid beer contamination, but once the beer is up, there is little to no reason to climb hill. 

RamblinRover Luxury-Yacht > PartyPooper2012 03/22/2019 at 12:33

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The biggest single factor is what punkgoose’s static forces diagram covers - the slope of the hill. If the hill is relatively mild, 5-10 degrees, there will be a brief tank-slapper and then a stop, but if the hill is way steeper, the car won’t necessarily maintain traction.

The scenario in which the car stops, the barrel keeps rolling, and the car shifts to the back of the barrel isn’t that major in terms of the forces involved, because the barrel won’t tend to keep rolling very hard from its own momentum , and the car can pretty easily go back to being ahead of the center of gravity, which isn’t a big stabilizing force at all, but is way bigger than the angular momentum of the barrel . The only point at which it’s a major risk is when it’s really steep, because the margin of safety allows for a minor force to have effects way out of proportion, and the gravity trying to make the barrel roll w ould be way bigger than the resistance to “lifting” of the car and the angular momentum both. Although the car skidding on the inside of the barrel would slow it down a good bit.

Azrek > PartyPooper2012 03/22/2019 at 12:56

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So you are saying after pushing the keg up the hill you are going to make a water slide or plumbing? SO much work!

PartyPooper2012 > Azrek 03/22/2019 at 13:25

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yeah.... but imagine going on a beer slide? Beer park? Splish Booze

WARNING: THOSE VOMITING WILL BE THROWN OUT OF PARK AFTER THEY PAY TO REPLACE ALL BEER AND CLEAN FACILITIES

Azrek > PartyPooper2012 03/22/2019 at 13:43

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Yes and no...so am I doing this in my shoes or socks? Or freshly showered? Hmm...I am going against this suddenly...

PartyPooper2012 > Azrek 03/22/2019 at 14:30

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Yeah... its gonna be good for the first 3 minutes. Then someone will pee in it and game over.

hmmmm Gotta be a way... gotta be a way...