HU_Recitation # 7_Solution Sheet

HU_Recitation # 7_Solution Sheet - HOWARD UNIVERSITY PHYS...

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HOWARD UNIVERSITY PHYS 001-RECITATON ASSIGNMENT # 7 Part I. Work and Energy Practice Problems 1) Find the work done by a force of 300.0 N exerted on an object to lift it vertically to a distance of 240.0 cm above the ground. Answer: The work done on the object is W= F cos θ s=(300.0 N) (cos 0 ° ) (2.4 m)= 7.2 x 10 2 J. 2) An object of mass 450 g moves along a horizontal surface with constant velocity due to a force that pulls the object (traction force). Find the work done by the net force and the force of friction over a distance of 2 m if the coefficient of kinetic friction is 0.6. Answer: The work done by the net force is W=F net cos θ s= (0 N) cos 0 ° (2m)= 0 J. The work done by the force of friction is W=F frict cos θ s= μ k F N cos θ s=(0.6)(0.450 kg)(9.8 m/s 2 ) cos 180 ° (2m)= -5.292 J. 3) In practice problem 3, find the work done by the weight, and the work done by the force of friction. Answer: The work done by the weight is W=F g cos θ s= (0.450 kg)(9.8 m/s 2 )cos 90 ° (2 m)= 0 J. The work done by the force of friction is W=F frict cos θ s= μ k F N cos θ s=(0.6)(0.450 kg)(9.8 m/s 2 ) cos 180 ° (2m)= -5.292 J. 4) At a construction site, a plank of mass 425 kg is lifted to a height of 11.2m above the ground by a constant force in 1.00 minute. The plank is lifted at a steady pace. Find the power needed to produce this displacement. 1 | P a g e
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Answer: The power needed to produce this displacement is W s J s m s m kg F t W P g 5 . 777 60 46648 60 ) 2 . 11 ( 0 cos ) / 8 . 9 )( 425 ( cos 2 = = ° = = = t s θ . 5)
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HU_Recitation # 7_Solution Sheet - HOWARD UNIVERSITY PHYS...

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