5/20/2014 13 Physics 4A Slide 49 1.Draw a sketch. 2. For one object, draw a free-body diagram, showing all the forces acting onthe object. Make the magnitudes and directions as accurate as you can. Label each force. If there are multiple objects, draw a separate diagram for each one. 3. Resolve vectors into components. 4. Apply Newton’s second law to each component. 5.Solve. Two boxes, A and B, are connected by a lightweight cord and are resting on a smooth table. The boxes have masses of 12.0 kg and 10.0 kg. A horizontal force of 40.0 N is applied to the 10.0-kg box. Find (a) the acceleration of each box, and (b) the tension in the cord connecting the boxes. (1.82 m/s2and 21.8 N) Physics 4A Slide 50 Example Strings Pulling the mystery box. Suppose a friend asks to examine the 10.0-kg box you were given previously, hoping to guess what is inside; and you respond, “Sure, pull the box over to you.” She then pulls the box by the attached cord along the smooth surface of the table. The magnitude of the force exerted by the person is FP= 40.0 N, and it is exerted at a 30.0°angle as shown. Calculate (a) the acceleration of the box, and (b) the magnitude of the upward force Fexerted by the table on the box. (3.46 m/s2and 78N) The figure shows two blocks with two forces acting on the pair. Is the net force on the larger block (A) less than 2 N, (B) equal to 2 N, or (C) greater than 2 N? The figure shows two blocks with two forces acting on the pair. Is the net force on the larger block (A) less than 2 N, (B) equal to 2 N, or (C) greater than 2 N? Solving Problems with Newton’s Laws: Free-Body Diagrams N Physics 4A Slide 51 Slide 4-16 Physics 4A Slide 52 Slide 4-17
5/20/2014 14 Physics 4A Slide 53 A Catalog of Forces: Weight Weight • The falling box is pulled toward the Earth by the long-range force of gravity. The gravitational pull on a object on or near the surface of the Earth is called weight , for which we use the symbol w . • Weight is the only long range force that we will consider in Physics 2A. The agent for the weight force is the entire Earth pulling on the object. Weight acts equally on objects at rest or in motion. The weight vector always points vertically downward, and it can be considered to act at the center of mass of the object. 2 9.81 m/s ; g w mg Physics 4A Slide 54 A Catalog of Forces: Normal Normal Force • A solid object resists the action of another force which compresses it with what we call the normal force . • The normal force always acts outward and perpendicular to the surface of the compressed object. The symbol for the normal force is n . • The normal force may be equal to, greater than, or less than the weight. Physics 4A Slide 55 A Catalog of Forces: Spring Spring Force • A stretched or compressed spring exerts one of the most common contact forces. • A spring can either push (when compressed) or pull (when stretched). • In either case, the tail of the vector force is attached to the contact point. • There is no special symbol for the spring force, but we can use sp F Physics 4A Slide 56 A Catalog of Forces: Tension Tension Force •
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