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Homework Problems Follow these general steps
to solve the problems below. 1. A block moving on a horizontal, friction-free surface at 6 m/s now encounters a rougher surface where u =.3 How far does the block slide before stopping? How long does it take it to stop? 2. We desire to drag a crate across the floor by applying a horizontal force to it. The crate has a mass of 100 kg and the coefficient of friction between crate and floor is .25. a) What should be the value of this applied force if the object is to move at a constant speed? b) What should be the value of this applied force if the object is to move with an acceleration of 2 m/s^2 3. A block (m = 4 kg) rests on a horizontal table where u = .2. A force FAPPLIED is made to act on the block. For each situation described below, draw a picture of the block and draw and label the forces acting on the block Determine the the frictional force acting on the block if: a) FA = 0; FA = 4 N; FA = 8 N 4. Yet another block (mass = 7 kg ) is dragged across floor. This time we notice that the force required to move the block at some constant speed is 40 N. Determine a) the weight of the block; b) the value of the normal force; c) the coefficient of friction for this arrangement. 5. A 70 kg fireperson (politically correct, n’est-ce pas?) slides down a firepole with an acceleration of 1 m/s2 downward. What downward force does the fireperson exert on the pole? 6. A 40 kg crate sits in the back of a pickup truck where u = .3 For now the truck is idle and at rest..If the truck were to accelerate uniformly, what maximum acceleration can the truck have without the crate moving? 7. I see kids in the cafeteria playing table shuffleboard where the object of the game is to slide a triangular wad of paper across the table so that it just extends over the edge without falling off. (Clearly, some brains are more easily sated than others.) If the table is 1.6 m long and u = .4, what should be the initial speed of the paper wad if it is score a point? 8. A block slides to the right at 4 m/s across a horizontal surface and encounters an inclined plane sloping up at an angle of 25 degrees. Neglect friction on all surfaces. a) What acceleration does the block experience on the plane? b) How far up the plane does the block move before stopping? c) How long does it take to get there? Now the block slides down the plane. d) What is its acceleration? e) How fast is it moving at the bottom? f) How long does it take to get there? 9. Repeat #8 with the coefficient of friction between the block and the incline u = .2 (Plan some time for this.) 10. A block (m = 30 kg) rests momentarily on an inclined plane where u = .4 and the angle of elevation of the plane is 37 degrees. We desire to lower the block down the plane at a constant speed. What force should be applied to the block to accomplish this? Does this force point up or down the plane? 11. A block (m = 3 kg) slides to the right at 4 m/s across a horizontal surface and encounters an inclined plane sloping up at an angle of 25 degrees. The block slides up the plane, stops, and does not slide down again. What must be the minimum coefficient of friction for the block-plane interface? 12. A block (mass = 5 kg) is attached to a string. Find the tension on the string if a) v = 0, the block is at rest; b) the block is moving upward with constant speed of 4 m/s; c) the block is moving downward with constant speed of 4 m/s; d) the block is accelerating upward at 3 m/s/s; e) the block is accelerating downward at 3 m/s/s. BEFORE ATTEMPTING THE NEXT FEW PROBLEMS, ASK AND ANSWER THE QUESTION "WHAT DOES A BATHROOM SCALE REALLY MEASURE. THE CORRECT ANSWER IS NOT "WEIGHT". 13. A 100 kg man is standing on a bathroom scale in an elevator (it can happen, you know). Describe the motion of the elevator if the scale reads a) 1200 N b) 800 N. 14. An 800N (Is this his mass or his weight? How do you know for sure?) man stands on a bathroom scale in an elevator. What does the scale read if the elevator is a) at rest?; b) moving upward at a constant speed of 2 m/s?; c) moving upward with an acceleration of +2 m/s^2?; d) moving upward with an accceleration of -2 m/s^2? 15. Cables are made to support block whose mass = 6 kg. Cable 1 is attached to the left and makes an angle of 53 degrees with the horizontal. Cable 2 is attached to the right and makes an angle of 40 degrees with the horizontal direction. Find the tension on each cable. 16 Repeat # 15 except that the angles are 5 degrees and 6 degrees, respectively. What are the tensions now? What implecations does this problem have for tight rope walkers?
This page last edited 01/23/09 |
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