99. ssm A student presses a book between his hands, as the drawing indicates. The forces that he exerts on the front and back covers of the book are perpendicular to the book and are horizontal. The book weighs 31 N. The coefficient of static friction between his hands and the book is 0.40. To keep the book from falling, what is the magnitude of the minimum pressing force that each hand must exert?

Free Body Diagram

Sum forces

Since book is not moving horizontally, acceleration is zero.

The forces the hands are pressing on must be equal if the book is not moving horizontally,

They cancel and leave

Which means the Normal forces left and right must be the same, so let’s call them N

Now sum forces vertically

Again book is not moving yet vertically. For it to just start to slide, static friction must be a max so.

Normal force is the opposition force to F so F = N

Solving for F

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This page last updated on January 11, 2020