Simple Nature - Light and Matter

n / If we imagine dividing a taut rope up into small segments, thenany segment has forces pulling outward on it at each end. If the ropeis of negligible mass, then all the forces equal +T or −T , where T , thetension, is a single number.If you look at a piece of string under a magnifying glass as youpull on the ends more and more strongly, you will see the fibersstraightening and becoming taut. Different parts of the string areapparently exerting forces on each other. For instance, if we think ofthe two halves of the string as two objects, then each half is exertinga force on the other half. If we imagine the string as consisting ofmany small parts, then each segment is transmitting a force to thenext segment, and if the string has very little mass, then all theforces are equal in magnitude. We refer to the magnitude of theforces as the tension in the string, T .The term “tension” refers only to internal forces within thestring. If the string makes forces on objects at its ends, then thoseforces are typically normal or frictional forces (example 29).Types of force made by ropes example 29⊲ Analyze the forces in figures o/1 and o/2.⊲ In all cases, a rope can only make “pulling” forces, i.e., forcesthat are parallel to its own length and that are toward itself, notaway from itself. You can’t push with a rope!In o/1, the rope passes through a type of hook, called a carabiner,used in rock climbing and mountaineering. Since the rope canonly pull along its own length, the direction of its force on thecarabiner must be down and to the right. This is perpendicular tothe surface of contact, so the force is a normal force.force acting on carabinerrope’s normal force on carabinerforce related to it by Newton’sthird lawcarabiner’s normal force onrope(There are presumably other forces acting on the carabiner fromother hardware above it.)In figure o/2, the rope can only exert a net force at its end thatis parallel to itself and in the pulling direction, so its force on thehand is down and to the left. This is parallel to the surface ofcontact, so it must be a frictional force. If the rope isn’t slippingthrough the hand, we have static friction. Friction can’t exist withoutnormal forces. These forces are perpendicular to the surfaceof contact. For simplicity, we show only two pairs of these normalo / Example 29. The forcesbetween the rope and otherobjects are normal and frictionalforces.Section 3.2 Force In One Dimension 159