# Strings or ropes are negligible interested only in

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of strings or ropes are negligible Interested only in the forces acting on the object Can neglect reaction forces Ropes : Ignore any frictional effects of the rope Ignore the mass of the rope The magnitude of the force exerted along the rope is called the tension The tension is the same at all points in the rope 4- 5 Applications of Newton’s Laws pg 100
Free-Body Diagram : A diagram of the forces acting on an object Must identify all the forces acting on the object of interest Choose an appropriate coordinate system If the free body diagram is incorrect , the solution will likely be incorrect Only forces acting directly on the object are included in the free body diagram Reaction forces act on other objects and so are not included The reaction forces do not directly influence the object’s motion 4- 5 Applications of Newton’s Laws pg 101
4- 5 Applications of Newton’s Laws Example : The magnitude of force T is the tension acting on the box and are the forces exerted by the earth and the ground g F n pg 100
Equilibrium : An object either at rest or moving with a constant velocity is said to be in equilibrium The net force acting on the object is zero (since the acceleration is zero) Easier to work with the equation in terms of its components: A zero net force does not mean the object is not moving , but that it is not accelerating 4- 5 Applications of Newton’s Laws 0 F 0 and 0 y x F F pg 101
A traffic light weighing 1.00 × 10 2 N hangs from a vertical cable tied to two other cables that are fastened to a support, as figure below. The upper cables make angles of 37.0 o and 53.0 o with the horizontal. Find the tension in each of the three cables . Example 4-6 pg 102
Find the tension in each of the three cables. Example 4-6 pg 102
Inclined Planes : Choose the coordinate system with x along the incline and y perpendicular to the incline Replace the force of gravity with its components 4- 5 Applications of Newton’s Laws pg 103
A sled is tied to a tree on a frictionless, snow- covered hill, as shown below. If the sled weighs 77.0 N , find the magnitude of the tension force exerted by the rope on the sled and that of the normal force exerted by the hill on the sled . Example 4-7 n T pg 103
Solution find the magnitude of the tension force exerted by the rope on the sled and that of the normal force exerted by the hill on the sled. T n pg 103