Question

An eraser weighing $2\,N$ is pressed against the blackboard with a force of $5N$ The coefficient of friction is $0.4$ How much force parallel to the blackboard is required to slide the eraser upwards?
A. $2N$
B. $2.8N$
C. $4N$
D. $4.8N$

Answer 1

In order to solve this question, we will first find the normal frictional force due to force acting on the eraser horizontally while it's pressed and then we will find the minimum force required to move the eraser upwards by balancing each of the forces.

Complete step by step answer:
According to the question, we have given that force applied horizontally on eraser is $5N$ and weight of eraser downwards is $2N$ and Let N be the normal reaction due to friction in downward direction as shown in figure and let F be the force required applied parallel to blackboard to move the eraser upward as shown in figure below,

Now as we have given that coefficient due to friction is $\mu = 0.4$ and horizontal force applied is $5\,N$ so normal force downward N is calculated as
$N = 5\mu$
$\Rightarrow N = 5 \times 0.4$
$\Rightarrow N = 2\,N$
Now, net force acting downward on the eraser is sum of weight which is $2\,N$ and normal reaction downward is $N = 2Newton$ so to balance, the force we required the force $F$ is given as,
$F = 2 + 2$
$\therefore F = 4\,N$
So, we need the $4N$ of force to move the eraser upward applied parallel to the blackboard.

Hence, the correct option is C.

Note: It should be remembered that, coefficient of friction is the resistance force acting between the surface of two bodies which opposes the relative motion between two bodies and Weight is also the force acting on a body due to the acceleration produced by acceleration due to gravity.