Question

A boy prevent fall of his book on the ground by pressing it against a vertical wall. If weight of his book is 10 kg and coefficient of static friction is 0.2 . Find the minimum force needed by him in his attempt. (given=10m/s2)​

Answer 1

Answer:

Let F be the force applied by the finger on the block. N is the normal reaction of the wall on the block. Mg is weight of block acting vertically downwards. f is the force of friction between the wall and the block which acts upwards as shown in the figure.

The block will not fall, if

f=Mg

μN=Mg (∵f=μN)

μF=Mg(∵N−F)=F=

μ

Mg

Answer 2

Given: Weight of a book is 10 kg and coefficient of static friction is 0.2.

To find: The minimum force needed by him.

Solution:

• Friction is the force that acts between the book and the vertical wall that resists the falling of it.
• According to the formula of friction,

       $f = mg$

• Here, f is the frictional force, m is the mass of the book and g is the acceleration due to gravity.

• So,

       $f = 10kg * 10ms^{-2}$

       $f = 100 kgms^{-2}$ or $f = 100N$

• Frictional force is also given by the formula,

       $f = \mu F$

• Here, μ is the coeffient of static friction and F is the minimum force required to prevent the book from falling.

• So,

        $f = 0.2 * F$

• On equating both the formulas of friction, we get,

        $100N = 0.2 * F$

        $F = 500 N$

Therefore, the minimum force needed by him in his attempt is 500N.