Friction k a is usually greater than the coefficient of static friction s b usually equals s c is usually smaller than s d equals the applied force that exceeds the maximum static force c 2 The purpos ID: 891176
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1 Friction 1. The coefficient of
Friction 1. The coefficient of kinetic friction μ k (a) is usually greater than the coefficient of static friction μ s (b) usually equals μ s (c) is usually smaller than μ s (d) equals the applied force that exceeds the maximum static force. (c) 2. The purpose of anti - lock brakes is to avoid the lock - up of the wheels to keep the car rolling rather than sliding. Why would rolling decrease the stopping distance? Kinetic friction (sliding) is less than static friction (rolling) 3. The picture left shows the front and rear wings of an Indy car. These wings generate down force , the vertical downward force by the air moving over the car. Why is such a down force desired? Will the additions of the wings slow down the car? An Indy car can create a down force e qual to twice its weight. Why not simply make the cars heavier? 4. (a) We commonly say that friction opposes motion. Yet, when we walk, the frictional force is in the direction of our motion.
2 Is there an inconsistency in terms of N
Is there an inconsistency in terms of Newtonâs second law ? Explain. (b) What effects would wind have on air resistance? (a) no, opposes slipping (b) increase or decrease depending on wind direction. 5. Why are racing tires wide and smooth whereas regular tires are narrower and have tread? Are there frictional and/or safety considerations? Treads displace water, wide and smooth tires increase friction. 6. In moving a 35.0 - kilogram desk from one side of a classroom to the other, a professor finds that a horizontal force of 275 N is necessary to set the desk in motion and a force of 195 N is necessary to keep it in motion with a constant speed. What are the coefficients of (a) static and (b) kinetic friction between the desk and the floor? (a) 0.802 (b) 0.569 7. A 40 - kilogram crate is at rest on a level surface . If the coefficient of static friction between the crate and the surface is 0.69, what horizontal force is required to get the crate moving? 2.7 X 10 2 N 8.
3 A wood block with a weight of 3.5 N is p
A wood block with a weight of 3.5 N is placed on a wood board, measuring 0.75 m X 0.75 m, that is elevated at one end to act as an inclined plane. What is the elevation of the higher end of the board when the block, once it is in motion, slides down with a constant velocity? a. .28 m 9. Suppose the slope conditions for a skier are such that he trave ls with a constant velocity. Could you find the coefficient off kinetic friction between the snowy surface and the skis? If so, describe how this would be done. a. Yes, tan x=μk 10. A 5.0 - kilogram wooden block is placed on an adjustable wooden inclined p lane. (a) what is the angle of incline above which the block will start to slide down the plane? (b) at what angle of incline will the block then slide down the plane with a constant speed? a. (a) 30° (b) 22° 11. A block that has a mass of 2.0 kg and is 10 cm wide on each side just slides down an inclined plane with a 30° angle of incline (as shown in the picture below). Anoth
4 er block of the same height and same mat
er block of the same height and same material has base dimensions of 20 cm X 10 cm and thus has a mass of 4.0 kg. (a) at what criti cal angle will the more massive block start to slide down the plane? (b) estimate the coefficient of static friction between the block and the plane. a. (a) 30° (b) 0.58 12. For the system in the picture below, if μ s =0.45 and μ k =0.35 between the blocks and the surface, what applied forces will (a) set the blocks in motion and (b) move the blocks with a constant velocity? a. (a) 26N (b) 21 N 13. In the picture below, m 1 = 10 kg and the coefficients of static and kinetic friction between m 1 and the ta ble are 0.60 and 0.40, respectively. (a) what mass of m 2 will set the system in motion? (b) with mass m 2 as in part (a), after the system moves, what is the acceleration? a. (a) 6.0 kg (b) 1.2 m/s 2 14. For the picture below, what is the minimum value o f the coefficient of static friction between the block (m 3 ) and the table t
5 hat would keep the system at rest if m 1
hat would keep the system at rest if m 1 =0.25 kg, m 2 =0.50 kg, and m 3 =0.75 kg? (Assume ideal conditions for the string and pulleys.) a. 0.33 15. If the coefficient of kinetic fri ction between the block and the table (see picture above) is 0.560, and m 1 = 0.150 kg and m 2 = 0.250 kg, (a) what should m 3 be if the system is to move with a constant speed? (b) if m 3 = 0.100 kg, what is the acceleration of the system? (assume ideal condi tions for the string and pulleys.) a. (a) 0.179 kg (b) 0.862 m/s 2 16. In the picture below, m 1 = 2.0 kg and the coefficients of static and kinetic friction between m 1 and the inclined plane are 0.30 and 0.20, respectively. (a) what is m 2 if both masses are to remain at rest? (b) what is m 2 if both masses are moving with constant velocity? Neglect all friction. a. (a) between 0.72 kg and 1.7 kg (b) between 0.88 kg and 1.5 kg 17. The coefficients of static and kinetic friction between a 50 - kilogram b ox and a horizon
6 tal surface are 0.60 and 0.40, respecti
tal surface are 0.60 and 0.40, respectively. (a) What is the acceleration of the object if a 250 - newton horizontal force is applied to the box? (b) What if the applied force is 350 N? (a) zero (b) 3.1 m/s 2 18 . A 1500 - kilogram automobile t ravels at a speed of 90 km/h along a straight concrete highway. Faced with an emergency situa tion, the driver jams on the br ak e s and the car skids to a stop. What will the stopping distance be for (a) dry pavement and (b) wet pavement? (a) 38 m (b) 53 m 19 . A hockey player hits a puck with her stick, giving the puck an initial speed of 5.0 m/s. If the puck slows uniformly and comes to rest in a distance of 20 m, what is the coefficient of kinetic friction between the ice and the puck? 0.064 20 . A packin g crate is placed on a 20 o inclined plane. If the coefficient of static friction between the crate and the plane is 0.65, will the crate slide down the plane? (Justify your answer.) Ѳ min = tan - 1 0.65 = 33 o , so it will not mo