**UFBA**

(Parte **1** de 3)

•8–1.Determine the minimum horizontal force P required to hold the crate from sliding down the plane.The crate has a mass of 50 kg and the coefficient of static friction

© 2010 Pearson Education,Inc.,Upper Saddle River,NJ.All rights reserved.This material is protected under all copyright laws as they currently exist.No portion of this material may be reproduced,in any form or by any means,without permission in writing from the publisher.

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8–2.Determine the minimum force Prequired to push the crate up the plane.The crate has a mass of 50 kg and the coefficient of static friction between the crate and the plane

© 2010 Pearson Education,Inc.,Upper Saddle River,NJ.All rights reserved.This material is protected under all copyright laws as they currently exist.No portion of this material may be reproduced,in any form or by any means,without permission in writing from the publisher.

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8–3.A horizontal force of is just sufficient to hold the crate from sliding down the plane,and a horizontal force of is required to just push the crate up the plane.Determine the coefficient of static friction between the plane and the crate,and find the mass of the crate.

© 2010 Pearson Education,Inc.,Upper Saddle River,NJ.All rights reserved.This material is protected under all copyright laws as they currently exist.No portion of this material may be reproduced,in any form or by any means,without permission in writing from the publisher.

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*8–4.If the coefficient of static friction at Ais and the collar at Bis smooth so it only exerts a horizontal force on the pipe,determine the minimum distance so that the bracket can support the cylinder of any mass without slipping.Neglect the mass of the bracket.

x 100 m

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•8–5.The 180-lb man climbs up the ladder and stops at the position shown after he senses that the ladder is on the verge of slipping.Determine the inclination of the ladder if the coefficient of static friction between the friction pad Aand the ground is .Assume the wall at Bis smooth.The center

B 10 ft

3 ft u

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8–6.The 180-lb man climbs up the ladder and stops at the position shown after he senses that the ladder is on the verge of slipping.Determine the coefficient of static friction between the friction pad at Aand ground if the inclination of the ladder is and the wall at Bis smooth.The center of gravity for the man is at G.Neglect the weight of the ladder. u = 60°

B 10 ft

3 ft u

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8–7.The uniform thin pole has a weight of 30 lb and a length of 26 ft.If it is placed against the smooth wall and on the rough floor in the position ,will it remain in this position when it is released? The coefficient of static

A d

B 26 ft

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*8–8.The uniform pole has a weight of 30 lb and a length of 26 ft.Determine the maximum distance dit can be placed from the smooth wall and not slip.The coefficient of static

A d

B 26 ft

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•8–9.If the coefficient of static friction at all contacting surfaces is ,determine the inclination at which the identical blocks,each of weight W,begin to slide. ums

B u

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8–10.The uniform 20-lb ladder rests on the rough floor for which the coefficient of static friction is and against the smooth wall at B.Determine the horizontal force Pthe man must exert on the ladder in order to cause it to move.

8 ft 5 ft

5 ft

6 ft P

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8–1.The uniform 20-lb ladder rests on the rough floor for which the coefficient of static friction is and against the smooth wall at B.Determine the horizontal force Pthe man must exert on the ladder in order to cause it to move.

8 ft 5 ft

5 ft

6 ft P

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*8–12.The coefficients of static and kinetic friction between the drum and brake bar are and , respectively.If and determine the horizontal and vertical components of reaction at the pin O. Neglect the weight and thickness of the brake.The drum has a mass of 25 kg.

B O125 m

700 m500 m

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•8–13.The coefficient of static friction between the drum and brake bar is .If the moment , determine the smallest force Pthat needs to be applied to the brake bar in order to prevent the drum from rotating. Also determine the corresponding horizontal and vertical components of reaction at pin O.Neglect the weight and thickness of the brake bar.The drum has a mass of 25 kg.

B O125 m

700 m500 m

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8–14.Determine the minimum coefficient of static friction between the uniform 50-kg spool and the wall so that the spool does not slip.

0.6 m 0.3 m

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8–15.The spool has a mass of 200 kg and rests against the wall and on the floor.If the coefficient of static friction at B is ,the coefficient of kinetic friction is ,and the wall is smooth,determine the friction force developed at Bwhen the vertical force applied to the cable is .P=800 N

0.1 m G A

0.4 m

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*8–16.The 80-lb boy stands on the beam and pulls on the cord with a force large enough to just cause him to slip.If the coefficient of static friction between his shoes and the beam is ,determine the reactions at Aand B. The beam is uniform and has a weight of 100 lb.Neglect the size of the pulleys and the thickness of the beam.

5 ft

4 ft 1 ft

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•8–17.The 80-lb boy stands on the beam and pulls with a force of 40 lb.If ,determine the frictional force between his shoes and the beam and the reactions at Aand B.The beam is uniform and has a weight of 100 lb.Neglect the size of the pulleys and the thickness of the beam.

5 ft

4 ft 1 ft

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8–18.The tongs are used to lift the 150-kg crate,whose center of mass is at G.Determine the least coefficient of static friction at the pivot blocks so that the crate can be lifted.

500 m A

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8–19.Two blocks Aand Bhave a weight of 10 lb and 6 lb, respectively.They are resting on the incline for which the coefficients of static friction are and . Determine the incline angle for which both blocks begin to slide.Also find the required stretch or compression in the connecting spring for this to occur.The spring has a stiffness of .k=2 lb>ft

A u

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*8–20.Two blocks Aand Bhave a weight of 10 lb and 6 lb, respectively.They are resting on the incline for which the coefficients of static friction are and . Determine the angle which will cause motion of one of the blocks.What is the friction force under each of the blocks when this occurs? The spring has a stiffness of and is originally unstretched.k=2 lb>ft

A u

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•8–21.Crates Aand Bweigh 200 lb and 150 lb, respectively.They are connected together with a cable and placed on the inclined plane.If the angle is gradually increased,determine when the crates begin to slide.The coefficients of static friction between the crates and the u u

8 Solutions 44918 1/27/09 1:52 PM Page 703

8–2.A man attempts to support a stack of books horizontally by applying a compressive force of to the ends of the stack with his hands.If each book has a mass of 0.95 kg,determine the greatest number of books that can be supported in the stack.The coefficient of static friction between the man’s hands and a book is

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8–23.The paper towel dispenser carries two rolls of paper. The one in use is called the stub roll Aand the other is the fresh roll B.They weigh 2 lb and 5 lb,respectively.If the coefficients of static friction at the points of contact Cand D are and ,determine the initial vertical force Pthat must be applied to the paper on the stub roll in order to pull down a sheet.The stub roll is pinned in the center,whereas the fresh roll is not.Neglect friction at the pin.

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*8–24.The drum has a weight of 100 lb and rests on the floor for which the coefficient of static friction is .If ft and ft,determine the smallest magnitude of the force Pthat will cause impending motion of the drum.

•8–25.The drum has a weight of 100 lb and rests on the floor for which the coefficient of static friction is .If ft and ft,determine the smallest magnitude of the force Pthat will cause impending motion of the drum.

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8–26.The refrigerator has a weight of 180 lb and rests on a tile floor for which .If the man pushes horizontally on the refrigerator in the direction shown, determine the smallest magnitude of horizontal force needed to move it.Also,if the man has a weight of 150 lb, determine the smallest coefficient of friction between his shoes and the floor so that he does not slip.

3 ft

3 ft

1.5 ft G

A 4 ft

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(Parte **1** de 3)