Super+Vision+Beetles+(Zane+Allen,+Morgan

toc =Definitions= =
 * Rotation - Object turns around on internal axis
 * Revolution - Object turns around an external axis
 * Axis - Straight line around which rotation takes place
 * Static Friction Force - Newton's law says that an object should move unless there is a second horizontal force on an object opposite in direction to your force and equal size.
 * Kinetic Friction Force - The force exerted on one surface by the other when the surfaces are in relative motion

= =Explanations= Centripetal Force - The Real Force

Whenever an object moves in a circular path we know the object is accelerating because the velocity is constantly changing direction. All accelerations are caused by a net force acting on an object. In the case of an object moving in a circular path, the net force is a special force called the �86�centripetal force�116� (not centrifugal!). Centripetal is Latin for "//center seeking//". So a centripetal force is a center seeking force which means that the force is always directed toward the center of the circle. Without this force, an object will simply continue moving in straight line motion. Imagine swinging a rope with a mass attached to the end, around in a circle above your head: Compare each of the following animations to the original one in the top left. Look carefully for what has changed in each one. ||

The formula for centripetal force is where m represents the mass of the object, v is the speed (magnitude of the velocity) and r is the radius from the center of the circle to the object. A centripetal force ends up being a net force and a net force always causes an acceleration in the direction of the net force. So if the force is center seeking (centripetal) then the acceleration is also centripetal. The formula for centripetal acceleration is. [Notice that if you multiply this by mass (m) you get the formula for centripetal force...that's because a net force is equal to mass times acceleration.] It is conceptually better to think about the Centripetal force that is calculated from the formula as a requirement. If you meet the requirement, then you have circular motion at the radius and speed used in the formula. If you do not meet the requirement, then the object moves into a larger curve (which requires less force) or defaults into straight line motion (going off on a Tangent). || || ||  ||
 * [[image:http://www.regentsprep.org/Regents/physics/phys06/bcentrif/lrsmlv.gif width="259" height="284" align="center" caption="lrsmlv.gif (13932 bytes)"]] ||
 * * The smaller the mass, the smaller the centripetal force //(shown by the red vector labeled as the force of tension in the rope, FT )// you will have to apply to the rope. ||
 * [[image:http://www.regentsprep.org/Regents/physics/phys06/bcentrif/lrlmsv.gif width="259" height="284" align="center" caption="lrlmsv.gif (23137 bytes)"]] || [[image:http://www.regentsprep.org/Regents/physics/phys06/bcentrif/srlmlv.gif width="259" height="285" align="center" caption="srlmlv.gif (14927 bytes)"]] ||
 * * The smaller the velocity of the object, the less centripetal force you will have to apply.
 * The smaller the length of rope (radius), the more centripetal force you will have to apply to the rope.
 * Notice that the centripetal force and the centripetal acceleration are always pointing in the same direction. ||
 * [[image:http://www.regentsprep.org/Regents/physics/phys06/bcentrif/cutrope.gif width="259" height="284" align="center" caption="Cut the rope. (13k)"]]
 * If you let go of the rope (or the rope breaks) the object will no longer be kept in that circular path and it will be free to fly off on a tangent. ||
 * In this animation, the "sticky" or adhesive forces from the mud to the tire tread are large enough to be the centripetal force required to keep the mud in a circular path as the tire spins. ||
 * [[image:http://www.regentsprep.org/Regents/physics/phys06/bcentrif/wheelfas.gif width="320" height="240" align="center" caption="Not enough Fc (37k)"]] || In this animation, the tire is spinning faster which means a larger centripetal force is required to keep the mud in the circular path of the tire. The adhesive forces of the mud to the tire are not large enough to meet the requirement. The mud begins to move into a larger circular path but as soon as it is not touching the tread then there is no force (other than gravity) and so the mud continues with the velocity it had at the instant it was no longer touching the tread. (It went off on a tangent). It followed Newton's first law! ||

©2000 Science Joy Wagon

**The Force of Friction**

Friction is a force that is created whenever two surfaces move or try to move across each other. &amp;lt;embed SRC="friction.gif" WIDTH="580" HEIGHT="179"&amp;gt; In this simulation you see a block sitting on a level table. You can place an applied force on the object by pressing the "More Force" button. Each time you press the button the applied force will increase. As you use this simulation there are several things you should notice. The force of friction depends upon both surfaces in contact and the [|normal force]. A mathematical relationship can be created here. || || In this first example, a block of wood is shown sliding across a wooden table. (notice the cause of this sliding is not shown) Notice that the force of kinetic friction (fk) is equal to 40% of the normal force (FN). Another way of writing this relationship would be || The coefficients for static and kinetic friction are listed in some reference tables. The coefficient of static friction is usually a little bit higher than coefficient of kinetic friction for the same two surfaces. When coefficients are listed they must be given for one surface on another surface (ie wood-on-asphalt). The higher the coefficient, the greater the force of friction. The table below lists the coefficients for a few common surfaces used in physics. They are arranged from "sticky" to slippery. || surface-on-surface ||
 * Friction always opposes the motion or attempted motion of one surface across another surface.
 * Friction is dependant on the texture of both surfaces.
 * Friction is also dependant on the amount of contact force pushing the two surfaces together (normal force).
 * On a level surface, the [|normal force] (FN) is always equal and opposite to the weight (only on a level surface).
 * The force of static friction ( f s ) cancels out the applied force right up to and including when static friction reaches its maximum ( f smax).
 * For applied forces greater than the maximum force of static friction the block starts to slip and then the value for friction becomes kinetic friction ( f k ) and the box is then under a net force so it accelerates to the right.
 * || Now as we compare the first simulation to the next, we find that if the weight of the block is doubled, the normal force doubles, and the force of friction becomes doubled. Once again we find that the force of kinetic friction (fk) is equal to 40% of the normal force (FN). Another way of writing this relationship would be [[image:http://www.regentsprep.org/regents/physics/phys01/friction/ratio_k.gif width="71" height="53" align="absMiddle"]] Since this value is true for any weight of wood on wood, we say this value represents the coefficient of friction. (It's just the percentage of the normal force that can be friction.)The formula for the coefficient of kinetic friction is [[image:http://www.regentsprep.org/regents/physics/phys01/friction/coeff_k.gif width="82" height="62" align="absMiddle"]] ||
 * [[image:http://www.regentsprep.org/regents/physics/phys01/friction/kfricbig.gif width="350" height="100" caption="kfricbig.gif (10578 bytes)"]] ||^  ||
 * ^  || As we compare the simulation of wood on wood to wood on asphalt, we find that the amount of friction on the block increased for for the same amount of weight. Notice that the force of kinetic friction (fk) is equal to 60% of the normal force (FN) or we could say [[image:http://www.regentsprep.org/regents/physics/phys01/friction/coef_rub.gif width="76" height="31" align="absMiddle"]] ||
 * [[image:http://www.regentsprep.org/regents/physics/phys01/friction/kfricrub.gif width="350" height="100" caption="kfricrub.gif (10556 bytes)"]] ||^  ||
 * ^  || Another coefficient can be used to describe the relationship between the maximum force of static friction and the normal force. It's called the coefficient of static friction and its formula looks like [[image:http://www.regentsprep.org/regents/physics/phys01/friction/coef_s.gif width="108" height="78" align="absMiddle"]] The maximum force of static friction is used because static friction has a whole range from zero newtons up to the maximum force of static friction. ||
 * [[image:http://www.regentsprep.org/regents/physics/phys01/friction/mu_k.gif width="48" height="53" align="center"]] ||  ||
 * hook velcro-on-fuzzy velcro || >6.0 || >5.9 || [[image:http://www.regentsprep.org/regents/physics/phys01/friction/stickery.gif width="71" height="186" align="center" caption="stickery.gif (1676 bytes)"]] ||
 * avg tire-on-dry pavement || 0.9 || 0.8 ||^  ||
 * grooved tire-on-wet pavement || 0.8 || 0.7 ||^  ||
 * glass-on-glass || 0.9 || 0.4 ||^  ||
 * metal-on-metal (dry) || 0.6 || 0.4 ||^  ||
 * smooth tire-on-wet pavement || 0.5 || 0.4 ||^  ||
 * metal-on-metal (lubricated) || 0.1 || 0.05 ||^  ||
 * steel-on-ice || 0.1 || 0.05 ||^  ||
 * steel-on-Teflon || 0.05 || 0.05 ||^  ||
 * You should keep in mind that it isn't possible to give accurate values for the coefficient of frictions due to changing surface smoothness. For example, not all pieces of metal have the same surface smoothness. Some that are highly polished may be more slippery than others that are pitted or scratched. These values are just meant to give you the approximate values. ||

©1998 Science Joy Wagon

=Citations= Super Vision Beetles (Zane Allen, Morgan Super Vision Beetles (Zane Allen, Morgan