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Unformatted text preview: Chapter 6 solved problems 24. Picture the Problem : The spring stretches while the backpack remains at rest. Strategy: There are two forces acting on the backpack, the spring force to the left and the friction force to the right. The magnitudes of these two forces must be equal for the backpack to remain at rest. Use Newtons Second Law to relate the magnitudes of the forces and Hookes Law (equation 6-4) to find the spring force and hence the friction force. Solution: 1. (a) Write Newtons Second Law in the horizontal direction: x s x F f F ma =- = = 2. Use Hookes Law to find the spring force and solve for the friction force. Note that the spring stretches in the negative x direction. ( 29 ( 29 150 N/m 0.0200 m 3.0 N s f F kx = = - = -- = 3. (b) No, the answer to (a) doesnt depend on the mass of the backpack, it only depends upon the spring force and the fact that the backpack remains at rest. Insight: The static friction force changes with the applied force, always matching its magnitude until the maximum friction force is reached. If the applied force is increased even more, the backpack will slide (see the next problem). 35. Picture the Problem : The physical situation is depicted in the figure at right. The tension in the string between the two blocks is T 1 and the tension in the string that is tied to the wall is T 2 . A free-body diagram for the upper block 2 is also shown at right.. Strategy: Set the x-axis parallel to and pointing up the incline, and the y-axis perpendicular to the incline. Write Newtons Second Law in the x direction for each block and use the resulting equations to find T 1 and T 2 . The acceleration is zero everywhere in the system. Solution: 1. (a) Write Newtons Second Law for block 1 and solve for T 1 : ( 29 ( 29 1 1 block 1 1 1 2 1 sin sin 1.0 kg 9.81 m/s sin 31 5.1 N x F m g T T m g T = - + = = = = 2. (b) Write Newtons Second Law for block 2 and solve for T 2 : ( 29 ( 29 1 2 2 block 2 2 2 1 2 sin sin 5.1 N 2.0 kg 9.81 m/s sin 31...
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- Spring '11