Fundamentals of Physics Extended (10th Edition)

Published by Wiley
ISBN 10: 1-11823-072-8
ISBN 13: 978-1-11823-072-5

Chapter 6 - Force and Motion-II - Problems: 71a

Answer

35.3 Newtons

Work Step by Step

To be on the verge of moving means that the applied force is equal to Ff,max. To find Ff,max or the maximum frictional force we need to find the normal force, N. To do this we balance the forces in the y direction and see that there is only the weight of the block downwards and the normal force upwards. The two must balance since the box is not moving in the y direction so N = mg = 8kg*9.8 = 78.4 Newtons. Then, this means that Ff, max = 78.4N * 0.450 (coefficient of friction given) = 35.3 Newtons. If the force is directed horizontally then it needs to exceed 35.3 Newtons.
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