In fact, the only difference is that this is a downward curved path with the center of this circular curve below the slide rather than above. (Again, the gray C represents the rider’s path on the slide and the circular path of his or her body, and the point is the center of the circle.) This means that the centripetal acceleration is also downward and toward the center of the circle.
Since the acceleration has switched directions, the force should be normal (N) less From the radial component of the force of gravity, which pulls a person down towards the Earth. What happens when the vertical force decreases?
Remember that in order to make the rider move in a circular path, there must be a net force pointing to the center of the circle, which is a downward direction for a downward-curving slide. Since the force of friction is always tangential to a sliding path, this net radial force, which we call centripetal force, is made up of the normal force (pushing away) and a component of gravitational force (pull toward the center).
If the jockey’s speed is slow enough, you don’t need very much gravitational force to move it in a circle. The element of gravitational force alone can be enough to make this happen. The normal power from the chip could be just a small value being pushed away.
If the rider’s speed increases too quickly, the force of gravity alone will not be enough to produce a circular motion. You will need normal strength to also Drag toward the center of the circle. But the slides don’t do that: they just move away. This means that the sliding human will not actually move in a circle, but instead along an equivalent path as it leaves the surface of the slide and becomes airborne—at least for a short time, until it hits the slide again. This is what happened to the slide riders in Detroit.
Let’s model a person’s motion on a slide that is curved downward. I’ll start with the rider at the top of the curve. You can see that at some point a person flies off the trajectory and becomes a freely falling projectile:
A person’s speed when starting their journey matters. If someone starts the downward curve at a fast enough speed, they will fly off course – but the exact value of the speed that will cause the person to go off course depends on the starting and ending angle of the slide curve.
If you want to keep the riders on the slide, you need to increase the coefficient of friction between them and the slide. In the end, the Michigan Department of Natural Resources, which operates Belle Isle Park, posted a video to Facebook explaining the updates they made: “We cleaned the surface and started spraying a little water on the slide between rides to help control speed,” they wrote. They also urge the riders to lean forward – which one of the park employees demonstrates in the video.
Why water? Water is actually rather viscous, so adding a little more can increase friction due to its cohesive nature. (Of course, adding just enough to create a complete waterslide may reduce friction and make the rider faster – but this can take Many More water.)
Leaning forward can help ensure that each passenger’s weight is on the beanbag on their legs. The sacks are made of burlap, which is rough and provides some friction – and because all riders are required to wear these sacks, this makes for a more consistent surface recognizable than any clothing the riders wear. Asking them to bend forward makes sure that the burlap is in contact with the slat, not the material that a person’s shirt is made of – which will happen if they lean back.
If park operators want to get a little more creative, another option is to have the riders glide while wearing something other than burlap bags — perhaps something with a little bit of rubber to increase the friction reaction. It is also possible that the coating layer increases the coefficient of friction.
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