Simple Nature - Light and Matter

The following is another example in which we can profit by checkingagainst our physical intuition for what should be happening.Rappelling example 28As shown in the figure below, Cindy is rappelling down a cliff. Her downward motion is at constant speed, andshe takes little hops off of the cliff, as shown by the dashed line. Analyze the forces in which she participatesat a moment when her feet are on the cliff and she is pushing off.force acting on Cindyforce related to it by Newton’s third lawplanet earth’s downward gravitational force Cindy’s upward gravitational force on earthon Cindyropes upward frictional force on Cindy (her Cindy’s downward frictional force on the ropehand)cliff’s rightward normal force on Cindy Cindy’s leftward normal force on the cliffThe two vertical forces cancel, which is what they should be doing if she is to go down at a constant rate. Theonly horizontal force on her is the cliff’s force, which is not canceled by any other force, and which thereforewill produce an acceleration of Cindy to the right. This makes sense, since she is hopping off. (This solutionis a little oversimplified, because the rope is slanting, so it also applies a small leftward force to Cindy. As sheflies out to the right, the slant of the rope will increase, pulling her back in more strongly.)I believe that constructing the type of table described in thissection is the best method for beginning students. Most textbooks,however, prescribe a pictorial way of showing all the forces acting onan object. Such a picture is called a free-body diagram. It shouldnot be a big problem if a future physics professor expects you tobe able to draw such diagrams, because the conceptual reasoningis the same. You simply draw a picture of the object, with arrowsrepresenting the forces that are acting on it. Arrows representingcontact forces are drawn from the point of contact, noncontact forcesfrom the center of mass. Free-body diagrams do not show the equaland opposite forces exerted by the object itself.Discussion QuestionsA When you fire a gun, the exploding gases push outward in all directions,causing the bullet to accelerate down the barrel. What Newton’sthird-lawpairs are involved? [Hint: Remember that the gases themselvesare an object.]B In the example of the barge going down the canal, I referred toa “floating” or “hydrostatic” force that keeps the boat from sinking. If youwere adding a new branch on the force-classification tree to represent thisforce, where would it go?Section 3.2 Force In One Dimension 157