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Free Body Diagrams and Newton’s Laws Free Body Diagram A diagram of all the external forces on an object. The object is drawn as is floating in space (“free”). •Only include forces on the diagram, not other vectors such as acceleration or velocity. •Only include the actual individual forces, not components or net forces. Example Ftable Free Body Diagram Fg Object or System of Objects System Problem #1 A 55 N box is at rest on a table. What is the force of the table on the box? Ftable F 0 F Ftable Fg Ftable Fg 0 Ftable Fg 55 N Fg Problem #2 A 20 kg box is at rest on a table. What is the force of the table on the box? Ftable F 0 F Ftable Fg Ftable Fg 0 Ftable Fg mg Fg mg 20 9.8 196 N Problem #3 Two 20 kg boxes are tied together with a rope. A force of 40 N is applied to the block on the right. What is the acceleration of the blocks? Treating the boxes as one system F ma F 40 40 20 20 a a 1.0 m 2 s Problem #4 Two 20 kg boxes are tied together with a rope. A force of 40 N is applied to the block on the right. What is the tension in the rope? Examine the forces on one box F ma F 40 T 40 T 20 1.0 T 20 N Problem #5 Two 20 kg boxes are tied together with a rope. A force of 60 N is applied to the block on the right at an angle of 30 degrees to the horizontal. What is the acceleration of the blocks? Fx ma Treating the boxes as one system Fx 60cos30 Fy 0 Fy Ftable 60sin 30 20 20 9.8 60cos30 40a 51.9 40a a 1.3 m 2 s Problem #6 The length of the chain between the car and the tree is 15.0 m and the perpendicular force causes the chain to deflect 0.5 m from the dotted line in the diagram. If the perpendicular force is 100 N what is the force T on the car if the car doesn’t move? Examine the point in the chain where the perpendicular force is applied 0.5 o sin 3.8 7.5 1 T T Fx T cos3.8 T cos3.8 0 Fy T sin 3.8 T sin 3.8 F 0 F┴ 2T sin3.8 100 0 100 T 754 N 2sin 3.8