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Physics 101: Application of Newton's Laws Review of the different types of forces discussed in Chapter 4: Gravitational, Normal, Frictional, Tension Forces Practice Examples Physics 101: Lecture 9, Pg 1 Solving Problems Identify forces acting on an object by drawing a free- body diagram. Do not include forces that the object exerts on its environment. This is the most important step! Select x and y axes and decompose all forces in the free-body diagram into x and y components. Select the axis so that as many forces as possible point along the x and y axis. x and y components Write Fnet=m a for each axis: Fnet,x=m a x and Fnet,y=m a y (equilibrium: ax=0, ay=0) Solve the two equations for the desired quantities ! Strong suggestion: » work problem algebraically » plug in numbers only at the end Physics 101: Lecture 9, Pg 2 Example: Free-Body Diagram, Fnet=0 You are driving a car up a hill with constant velocity. On a piece of paper, draw a Free Body Diagram (FBD) for the car. How many forces are acting on the car? 1 2 correct 3 4 5 FN V f W Physics 101: Lecture 9, Pg 3 Example: Free-Body Diagram, Fnet=ma You are driving a car up a hill with constant acceleration. How many forces are acting on the car? 1 2 correct 3 4 5 FN a f W Physics 101: Lecture 9, Pg 4 Example: Tension A lamp of mass 4 kg is stylishly hung from the ceiling by two wires making angles of 30 and 40 degrees. Find the tension in the wires. Physics 101: Lecture 9, Pg 5 Example: Tension Consider two blocks of mass m1 and m2, respectively, tied by a string (massless). Mass m1 sits on a horizontal frictionless table, and mass m2 hangs over a pulley. If the system is let go, compute the acceleration and the tension in the string. Physics 101: Lecture 9, Pg 6 Example: Normal Force A 80 kg man is standing in an elevator which is accelerating downwards at 4 m/s2. What is the force of the elevator floor on the man ? Physics 101: Lecture 9, Pg 7 Example: Normal and Kinetic Frictional Force A block of mass m is sliding down a frictionless inclined plane making an angle q with the horizontal. What is the acceleration of the block down the plane and what is the normal force exerted by the plane on the block ? Repeat the problem with friction. Physics 101: Lecture 9, Pg 8 Example: Static Frictional Force A boy is playing with a pull-toy of mass m on the floor of an elevator. Find the minimum horizontal force with which the boy must pull the string in order to set the toy into motion along the elevator floor. Assume that the elevator is accelerating upwards with acceleration a and the coefficient of static friction between the toy and the floor is ms. Physics 101: Lecture 9, Pg 9