Weird question in TBR Physics

[G1SG2]

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There was a question in TBR Physics (in the work/energy chapter) that asked for the speed of a roller coaster cart that barely completed the loop. The forces on it at the top of the loop would be gravity and the normal force, so N+mg=mv^2/r. However, their explanation says that if it barely makes it around the loop, the normal force is zero, and thus mg=mv^2/r, and v=sqrtg*r. That makes no sense? How can there be no normal force if it's making contact with the top of the track?!?! It's a contact/normal force!

😕

 

[capn jazz]

At the top of the rollercoaster, it's subject to gravity. If it didn't have a certain speed and thus kinetic energy/momentum, it would fall straight down. While the rollercoaster is "touching" the track, it's not exerting any weight in the upward direction - all its weight is pointing down along with gravity. So N = 0.

 

[minutemen11]

Normal force is greatest at the bottom of the loop and non-existent at the top. Normal F in this case is the force pushing upwards against YOUR weight, aka the seats on your butt. Thus, this force is greatest at the very bottom of the loop and is 0 at the very top. its 0 at the top because the seat isnt technically pushing "up" on your butt-youre not pushing down on the seat- and the only force at work is gravity (you dont fall because of centri'fugal' force.)

Also it is assumed that the normal force is 0 at the top but in reality there will be a very minimal normal force..

hope that helps, if it confuses you even more im sorry haha

 

[G1SG2]

At the top of the rollercoaster, it's subject to gravity. If it didn't have a certain speed and thus kinetic energy/momentum, it would fall straight down. While the rollercoaster is "touching" the track, it's not exerting any weight in the upward direction - all its weight is pointing down along with gravity. So N = 0.

Normal force is greatest at the bottom of the loop and non-existent at the top. Normal F in this case is the force pushing upwards against YOUR weight, aka the seats on your butt. Thus, this force is greatest at the very bottom of the loop and is 0 at the very top. its 0 at the top because the seat isnt technically pushing "up" on your butt-youre not pushing down on the seat- and the only force at work is gravity (you dont fall because of centri'fugal' force.)

Also it is assumed that the normal force is 0 at the top but in reality there will be a very minimal normal force..

hope that helps, if it confuses you even more im sorry haha

Okay, got it, thanks guys. So basically between the car and the tracks, there is no normal force at the top because the cart isn't really pushing down on the tracks for it to be pushing it back in the first place, and that's why the normal force is 0 at the top?

 

[Seraph 84]

Okay, got it, thanks guys. So basically between the car and the tracks, there is no normal force at the top because the cart isn't really pushing down on the tracks for it to be pushing it back in the first place, and that's why the normal force is 0 at the top?

The question asked for the speed that the coaster could be moving to just barely complete the loop. At the top and bottom of the loop, you'll have gravity and the normal force acting on the car. Since the direction of gravity never changes in the loop, the normal force on the car varies from very large to very small depending on where you are in the loop; their vector sum has to correspond with the acceleration vector at all times.

The normal force is zero in this problem because it asked for the minimum velocity that would be required to complete the loop, and that minimum velocity corresponds to a zero normal force and only gravity working to keep the coaster moving in a circle. If you were going much faster than the minimum velocity, then there would be both normal force and gravity at the top because now the vector sum must correspond to a greater acceleration.

 

[G1SG2]

The question asked for the speed that the coaster could be moving to just barely complete the loop. At the top and bottom of the loop, you'll have gravity and the normal force acting on the car. Since the direction of gravity never changes in the loop, the normal force on the car varies from very large to very small depending on where you are in the loop; their vector sum has to correspond with the acceleration vector at all times.

The normal force is zero in this problem because it asked for the minimum velocity that would be required to complete the loop, and that minimum velocity corresponds to a zero normal force and only gravity working to keep the coaster moving in a circle. If you were going much faster than the minimum velocity, then there would be both normal force and gravity at the top because now the vector sum must correspond to a greater acceleration.

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