# Physics misconception

by T@P
Tags: misconception, physics
 P: 270 I have a question: if a roller coaster is gowing around a track, what is keeping it from falling down at the top? I know there is no "force" pulling it up, but how could i write an equation to measure that pull (acceleration)?
 Emeritus Sci Advisor PF Gold P: 11,155 Sure it's falling down, but the track has a greater curvature that the trajectory of the car falling at that speed. So, it stays on the track.
 P: 270 true, but is there a way to calculate the force acting up on the roller coaster at the apex of its revolution?
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P: 41,475
Physics misconception

 Quote by T@P true, but is there a way to calculate the force acting up on the roller coaster at the apex of its revolution?
I assume you are talking about a coaster going upside down in a loop-the-loop fashion? If so, realize that the coaster is undergoing centripetal acceleration downward. So identify the forces on the coaster and use Newton's 2nd law.
 P: 270 The forces at the top would be mg down and the normal force also down. How does it stay up...?
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P: 41,475
 Quote by T@P The forces at the top would be mg down and the normal force also down. How does it stay up...?
Inertia---It's moving! It "stays" up the same way a ball on the end of a string stays up if you were to swing it overhead. (It doesn't really stay up there---it goes around and back down.)

If there were no forces acting on the coaster, it would just keep moving in a straight line. If only gravity acted, it would assume a parabolic trajectory. But the track also exerts a force making it go in a tighter circle. Assuming the coaster is not attached to the track (of course it is!), if the speed is too low, it will fall off the track!
 P: 270 I understand the idea, but what keeps it up would be the velocity? but isnt the velocity perpendicular to the force of gravity and therefore have no effect on it? Im sorry if I appear slow and thank you for your help, but i simply dont understand how I could draw a freebody diagram of an object at the top of a roller coaster and see that it does not fall down?
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 Quote by T@P I understand the idea, but what keeps it up would be the velocity? but isnt the velocity perpendicular to the force of gravity and therefore have no effect on it?
In a loop, the acceleration is perpendicular to the velocity and the acceleration force is what you are looking for
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P: 41,475
 Quote by T@P I understand the idea, but what keeps it up would be the velocity? but isnt the velocity perpendicular to the force of gravity and therefore have no effect on it? Im sorry if I appear slow and thank you for your help, but i simply dont understand how I could draw a freebody diagram of an object at the top of a roller coaster and see that it does not fall down?
But it does fall down! It is accelerated downward due to the forces on it. (Keep in mind that a force is need to change a velocity, not cause a velocity.) It doesn't fall straight down, since it does have a sideways velocity.

And why would you think gravity would not affect something just because it is moving sideways? (True, it would not affect the horizontal speed, but it would certainly change the vertical.)

The free body diagram is easy. The only forces, as you noted, are the weight and normal force which both act down. And the acceleration is down, so it all makes sense. Note that if the curve of the track is stronger (smaller radius) than the parabolic arc of a free projectile, that the track must push down on the coaster to force it to follow the track--that's the normal force.
 P: 270 I think I understand. Thank you doc al and russ_watters for your help! :)
 P: 1 Are there "types" of physics misconception? i can't seem to find anything about it... tnx!
P: 2,251
 Quote by T@P The forces at the top would be mg down and the normal force also down. How does it stay up...?
it doesn't stay up. it is falling as well as moving forward.
 P: 29 This is the same as a centrifuge. Spins everything around and around at high speeds, heavier masses going more vertical, lower mass sinking to the bottom. This is from the centrifugal force giving off a greater force than that of gravity. I think thats right anyways.
 Emeritus Sci Advisor PF Gold P: 11,155 Note: The original question, which is nearly 2 years old, has been answered to the satisfaction of the OP. The current quesstion is about misconceptions in physics.
 Emeritus Sci Advisor PF Gold P: 11,155 As for some common physics misconceptions, a few that come to mind, going by threads here are: i. Action Reaction Confusion: Horse pulls cart; so cart pulls horse equally ; there should be no motion ii. Trainstopping: ball thrown at train, head on; at some point of time, ball has zero speed in ground frame; so train must too iii. SR: Several concepts in special relativity are widely discussed though poorly understood; notable among these are time dilation and simultaneity iv. Antimass: antimatter has negative mass...!

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