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### Course: Physics archive>Unit 4

Lesson 1: Circular motion and centripetal acceleration

# Loop de loop answer part 2

Figuring out the car's average speed while completing the loop de loop. Created by Sal Khan.

## Want to join the conversation?

• What would happen if he went too fast?
• If he went WAAYYY too fast he might break the loop-de-loop. Since the car pushes against the road with the same force that the road pushes against the car.
• Does anybody understand why you would use an "egg shaped"/oval track for this experiment rather than a perfectly circular track?
• There are two main reasons I can detect: One would be the degree of gravitational acceleration; since the acceleration due to gravity is a VERY significant factor a shorter distance (and therefore a shorter time spent with gravity = approx. -9.8m/s^2 Jhat) and greater arc degree is preferable to a longer distance and a smaller arc degree. Two is that the car's ability to maintain a constant speed is inhibited severely by the lack of friction over the top part of the circle/ellipse; so, once again, a shorter distance and greater arc degree is preferable to a longer distance and shorter arc degree. Which gives us a vertically orientated ellipse.
• Since the speed at the top is (approximately) twice the required speed, wouldn't he have normal traction?
• Yes there would be traction(friction); but only in the velocity at the top of the loop is greater than 7.7m/s. This is because you only need 7.7m/s at the top of the loop to stay on, any extra is being counteracted by the normal force (the normal force is adding to the ac rather than just the force of gravity), rather than the weight force. If you ignore friction and propulsion (if he is giving it gas while on the loop) then KEi+ PEi = KEf + PEf. There is no PEi so then we have Initial Kinetic energy is equal to the final (top of loop) kinetic energy and it's Gravitational potential energy. The formula is :
1/2mv^2 = mgh + 1/2mv^2
divide by m since it is in all terms yields
1/2v^2 = gh + 1/2v^2
substitute variables that we know from the video:
1/2(15.3m/s^2) = 9.81m/s^2(12m) + 1/2v^2
117.045 = 117.72 + 1/2v^2
-1(-.675 = 1/2v^2)
1.35 = -v^2
-1.16m/s = v
as you can see the car has to be providing a propulsion force during the loop de loop or else it would have a negative velocity, which would mean falling off of the loop.
• does mass has anything to do about it??
• The mass would have an impact if we didn't already have the speed. If we were given newtons of force, we would use the mass to calculate the acceleration. And yes, it does affect the gravity which isn't taken into account.
(1 vote)
• At in the video, "frames" is used. Does anyone know why it is called "frames" and not kilo-second or something? Thanks.
• This has to do with the way that video is stored on a computer (and how it was originally stored on film).
Back then, video was a series of pictures (or "frames") that were projected one after another on a screen -- very much like a flip-book. In order for the video to not be jumpy like a flip-book can be, they often used 30 frames per second (fps), which gave the impression of real life.
In modern video formats (.mp4, .avi, etc.) there is still this concept of frames, though it gets a bit more complicated because they cut some corners to make the file smaller, such that there might not actually be 30 pictures stored for each second of video.
• What happens when car moves on a plane horizontal circular road... what is the EFFECT OF FRICTION on the horizontal track, if the car starts from rest from a point on the circular track?
• Well, in order to move on a circular road car requires an inward force or force from outside which is ofcourse due to centripital acceleration, but in this case there is no force acting on the car to change its direction so the force required comes from the friction between tires and road if there is no friction, there is no thing which moves it in circular path so in this case the car continue to move in straight line.
• how would you experience an acceleration greater than that of gravity, like when someone says that, for example, you were experiencing 4g's.
• you can search for "g-force training"
(1 vote)
• How do you determine the 'margin of safety'? As Mr. Khan said, you can't go too fast, or too slow.