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### Course: Physics archive>Unit 4

Lesson 1: Circular motion and centripetal acceleration

# Loop de loop question

Asks students to find the minimum speed necessary to complete the loop de loop. Created by Sal Khan.

## Want to join the conversation?

• does the average velocity change at every differnt time and place on the loop de loop?
• In a sense, yes because velocity includes direction. However, the magnitude should be the same. For example, if the car was going at say 50mph, and we said that straight up is positive and straight down is negative, than the velocity of the car on the far right of the circle would be +50mph, while the velocity of the car on the far left side would be -50mph. The velocities are thus different. However, the absolute values of + and - 50 are both 50. I have literally just forgotten the point of this. OH YEAH your original question! So yeah the velocity changes but when dealing with the equations in this section with those omegas and thetas and stuff, the velocity should be kept the same. So no? It's fine if you just use the same number throughout. Sorry I wrote so much. I'm bored.
• what would happen if he slammed the brakes while he was upside down?
• The car would probably fall
• Just wondering how to find the largest value the radius can have in order to complete the loop.
• Did Sal tell us what was the radius of the track ??
(1 vote)
• What would the energy of the car be at the top of the loop de loop?
(1 vote)
• When i make sonething move along aloop in space with some certain initial speed( which is not changed)is the speed while going through loop is constant(assuming it is not affected by any force)?
(1 vote)
• If there were multiple loop de loops together on say, a roller coaster, how would the moving roller coaster car be able to go at top speed on the tracks without falling off of the tracks?
(1 vote)
• What is the loop de loop?

## Video transcript

What I want to do in this video is show you a very cool video, then pose a question. But I want to make sure that everything is up and up, in terms of the copyright laws. So the video I want to show you is from the show called the fifth gear, that appears on channel 5, I believe in the United Kingdom. I think I'm okay on good copyright grounds to claim fair use here, because obviously the point of this is I'm going to ask a math question about the vide were going to see and so fair use laws the purpose and character of use including whether such a commercial nature or non profit educational purposes we're definitely the later non profit educational purposes we are actually we aren't trying to in any way replace the work being used I'm just going to show a little snippet of it so we can use it as the basis for a problem this is kinda the meat of fair use law the viewer may fairly cite from the original work if it is design if his design be really and truly to use the passages for the purposes for fair and reasonable criticism in this case it's fair and reasonable math problem or physics problem on the other hand it is as clear he thus cites the most important parts of the work. with the view not to criticize but to supercede the the use of the original work and substitute the review for it such a use will be deemed in law a piracy so in no way do I want to supersede it. You should go look it up, go watch the show. this is not good enough I don't even have the sound on it's going to be a small window over here where I'm just going to show this really really the part where they actually do a loop de loop. now with that out of the way this is where you can find more about fifth gear which is on channel 5 so now let me show you the video I'm going to have the sound down and so this is the video over here this is a pretty neat video this guy is going to try to do a loop de loop and he's actually going to succeed at doing the loop de loop and what I want you to do, the question I want to pose is look at this video and get a stop watch out or whatever else you need out a ruler whatever else you might need to do and try to figure out what we're going to try to figure out two things we're going to find out the minimum velocity the car needs to do the loop de loop and we're also going to try to figure out what his actual velocity was over the course of the loop de loop so let's actually just watch this video so there he goes he's starting he's accelerating it looks like he's just crossed he's pushing almost forty miles an hour at least I'm assuming that's miles per hour this is a British show and then he hits the loop de loop and then he finishes it and he's done so let's think about for the next video let's think about if we can figure out what the minimum what the minimum velocity or i should say the magnitude of the velocity or the speed had to be to actually complete the loop right over here and then we can actually get a stopwatch later on to see what his actall velocity was