yes, it sounds off topic and a joke. but sadly no, my question really is physics related.
i have a picture of me sliding in the torrential rain today, downhill, its a one handed frontside slide (not sure how to get it up here if you want i can send it to you)
for a photo competition/extra credit i need to explain the physics behind it. I have a lot of other and more important school work to do, such as studying for finals, and i was wondering if anyone who had a background in physics or any general knowledge could help me write a very brief description!
i take physics but so far we've done communication and electricity and starting motion in a couple of weeks, but obviously gravity and newtons laws apply in this pic
i've taken phyics for two the past two years, studied many many topics, newton's laws and laws of motion definitely apply there, but it's kind of just common sense.
as a skateboarder going downhill, you have lots of momentum (aka p=mv)
there is also a frictional force between the road, and the weight of yourself, your hand and your skateboard. rain and water reduce friction of the road, which any longboard slider can tell you, this can be useful and make sliding that much more over the edge than usual.
when you try to change your velocity and it's direction, initiating a slide, your wheels lose traction because they are slide wheels and fail to continue to grip the road because of your are heading in a certain direction, and naturally you want to stay moving in that direction, your board doesn't really want to continue going downhill while you are doing god knows what, so your wheels break free and you slide, overwhelming "u" or the frictional force of the road and yourself, until your slide ends of course.
there's also friction between grip tape and your shoes that prevents you from slipping off your board, and allows you to keep the board with you during slides, even though it doesn't really want to, i guess you could say, it would be more content if you just rode it down the hill and didn't force it against the grain.
when you slide you use the friction of the road to your advantage, to slow yourself down, but you do it in style
when you continue in the direction that your board naturally wants to ride due to what a skateboard is and how it turns, it regains grip on the road and you can control it again, which is even harder to due in the rain due to less friction.
hopefully that helps you, i'm not the best at physics, but i'm pretty good when its involving motion. hopefully it's not too confusing.
placing your hand on the road takes weight off your board, therefore reducing the normal force on your wheels. since the normal force is directly proportional to the friction force (F_f="mu"*N), this obviously reduces the force needed to break traction
also, when you thrust your hips up to initiate this type of slide, you are temporally accelerating your mass upwards. this force (f=m*a) counteracts the downward gravitational force, and in a sense makes you "lighter"... therefore... less normal force = less friction
wet pavement reduces the coefficient of friction between the asphalt/wheel interface
before the slide, you have the relatively low rolling friction.... during the slide, this changes to the much higher *kinetic* friction
by conservation of energy, when sliding you slow down (ie, lose kinetic energy) because this energy is being transformed into other forms: heat on gloves/wheels, the screeching sound of your wheels, work done by frictional forces (W=F_f*d), etc.
during your slide, there is a shearing force on your wheels which is what causes them to wear so qucikly
when you try to change your velocity and it's direction, initiating a slide, your wheels lose traction because they are slide wheels and fail to continue to grip the road because of your are heading in a certain direction, and naturally you want to stay moving in that direction, your board doesn't really want to continue going downhill while you are doing god knows what, so your wheels break free and you slide, overwhelming "u" or the frictional force of the road and yourself, until your slide ends of course.
even when you rotate your board, its center of mass will still continue in the same direction. inertia my friend. your board wants to keep going to downhill along its original path. it doesn't have a mind of its own
before the slide, you have the relatively low rolling friction.... during the slide, this changes to the much higher static friction
Actually, rolling without slippage (pre-slide) is governed by static friction, while the slide itself is either purely kinetic friction (when the wheels are sliding completely perpendicular to the fall line) or a combination of static and kinetic (e.g. the gradual transition between pure roll and pure slide during a coleman).
Also, the force needed to overcome static friction tends to be higher than the force needed to maintain motion with kinetic friction.
Edit: Hope I didn't come off as a dick - that was just Mechanical & Aerospace Engr. 101 taking over
Actually, rolling without slippage (pre-slide) is governed by static friction, while the slide itself is either purely kinetic friction (when the wheels are sliding completely perpendicular to the fall line) or a combination of static and kinetic (e.g. the gradual transition between pure roll and pure slide during a coleman).
Also, the force needed to overcome static friction tends to be higher than the force needed to maintain motion with kinetic friction.
Edit: Hope I didn't come off as a dick - that was just Mechanical & Aerospace Engr. 101 taking over
yeah i'll agree with on the kinetic friction when sliding.... it's late and i didn't really think that one through
however, there really are three types of friction relevant to this topic: static, kinetic, and rolling. my dynamics and physics books are still packed away, but some googling validates my point
Rolling Friction : "You are absolutely right that there is no relative motion at the point of contact, so it is not kinetic friction. It is also not static friction since the wheel is moving, though the point of contact is not; if there were no rolling friction, the wheel would continue forever on a perfectly level surface.
Rolling friction is due mainly to the deformation of the wheel. Consider a car wheel. The tire is a little flat on the bottom. Therefore, for it to roll, a force must be exerted on the tire to deform it. If it were perfectly elastic, the tire would push forward as it reforms to its round shape. It is not perfectly elastic, however, as is shown by the fact that it gets warm as it rolls along."
and...
Friction : "A rolling wheel requires a certain amount of friction so that the point of contact of the wheel with the surface will not slip. The amount of traction which can be obtained for an auto tire is determined by the coefficient of static friction between the tire and the road. If the wheel is locked and sliding, the force of friction is determined by the coefficient of kinetic friction and is usually significantly less.
Assuming that a wheel is rolling without slipping, the surface friction does no work against the motion of the wheel and no energy is lost at that point. However, there is some loss of energy and some deceleration from friction for any real wheel, and this is sometimes referred to as rolling friction. It is partly friction at the axle and can be partly due to flexing of the wheel which will dissipate some energy. Figures of 0.02 to 0.06 have been reported as effective coefficients of rolling friction for automobile tires, compared to about 0.8 for the maximum static friction coefficient between the tire and the road."
^^^ these coefficients were what i was talking about^^^
edit: i'm also a ME.... looking to transfer to a school that offers aerospace/astronautical
even when you rotate your board, its center of mass will still continue in the same direction. inertia my friend. your board wants to keep going to downhill along its original path. it doesn't have a mind of its own
haha, exactly, your board theoretically doesn't "want" to go into a 900 flatspin, so you slide.
great job explaining the friction guys...possible mechanical/ocean physics major over here i forgot about the different types to be honest...that was like two years ago for me.
i know this is off topic but does anyone else find einstein's theory of basic relativity really cool or i am just that strange.
physics help
i forgot about the different types to be honest...that was like two years ago for me.