## Motion on Curves

The

net forceon a car traveling around a curve is thecentripetal force,F_{c}= m v^{2}/ r, directed toward the center of the curve.For a

level curve, the centripetal force will be supplied by the friction force between the tires and roadway.A

banked curvecan supply the centripetal force by the normal force and the weight without relying on friction.

Level Curves

For a

level curve, the centripetal force will be supplied by the friction force between the tires and roadway.

ExampleWhat must be the coefficient of friction between the tires and the level roadway to allow a car to make a curve of radius r = 350 m at a speed of 80 km/h?For a level curve, the force of friction is the

onlyhorizontal force on a car and provides the centripetal force. This can be seen from the free-body diagram:The net force must be horizontal -- pointing toward the center of the circle -- and only the friction force is available to provide it. The normal force and the weight simply cancel each other.

Calculations are easier if the speed is expressed in m/s so we shall convert that first.

km 1000 m h v = 80 -- [------] [------] h km 3600 s v = 22.2 m/s Now we can calculate the necessary centripetal force,

F _{c}= M v^{2}/ r(22.2 m/s) ^{2 }Fc = M ------------ 350 mF _{c}= 1.41 M (m/s^{2})F

_{N}= M g = (M) (9.8 m/s^{2})For this flat curve, the centripetal force is supplied by the friction force, F

_{f},F _{f }= F_{N}F

_{f }= M 9.8 m/s^{2}F

_{f }= F_{net}= F_{c}M 9.8 m/s

^{2}= 1.41 M (m/s^{2})= 0.141

Of course, any coefficient of friction

greaterthan 0.141 will keep the car from slipping; this is the minimum value for . Notice that the force of friction is perpendicular to the velocity. But it still is in the direction necessary to oppose the motion that would occur without friction. The car will slide away from the center (to the right in this diagram) if it encounters a patch of low-friction surface (like ice or oil); therefore the friction force is directed toward the center.Notice, too, that we are dealing with

static friction.The piece of the tire in contact with the road is momentarily at rest with respect to the road! The force of static friction is somewhat greater than the force provided by kinetic friction, if the tires start to skid.

Banked CurvesHow can a car ever travel at all on a slippery highway? Some curves are banked to compensate for slippery conditions like ice on a highway or oil on a racetrack. Below is a car making a banked turn. Without friction, the roadway still exerts a normal force

nperpendicular to its surface. And the downward force of the weightwis present. Those two forces addas vectorsto provide a resultant or net forceFwhich points toward the center of the circle; this is the centripetal force. Note that it points to the center of the circle; it is_{net}notparallel to the banked roadway.Resolve the forces into their components. Since we are interested in the force that points

toward the centerof the circle, we choose a coordinate axis that lies along that direction. There is no acceleration in the y-direction so the sum of the forces in the y-direction must be zero.Remember, the

net forcemust point toward thecenter of the circle. It doesnotpoint down, along the surface. Thenet forceon an object moving in a circle may also be called thecentripetalforce.F _{net,y}= n cos - w = 0n cos = w

n = w / cos

F

_{net,x}= n sinF

_{c}= m v^{2}/ rbut

F _{c }= F_{net,x}substituting, this provides

F _{c}= mv^{2}/ r = n sin = [w / cos ] sinF

_{c}= mv^{2}/ r = w [ sin / cos ]F

_{c}= mv^{2}/ r = w tanm v

^{2}/ r = m g tantan = v

^{2}/ g rThis gives the angle necessary for a banked curve that will allow a car to travel in a curve of radius r with constant speed v and

require no frictionforce. A banked curve is designed for one specific speed. If the banked curve is icy so there is no friction force at all then traveling at higher than design speed means the car will slide out, up, and over the edge and traveling at lower than design speed means the car will slide in, down, and off the bank.

Return to ToC, Ch 6, Application of Newton's Laws(c) Doug Davis, 2005; all rights reserved