Laws of Motion 2 Question 3
3. A block of mass $m$ is at rest under the action of force $F$ against a wall as shown in figure. Which of the following statement is incorrect?
$(2005,2 M)$
(a) $f=m g$ (where $f$ is the frictional force)
(b) $F=N$ (where $N$ is the normal force)
(c) $F$ will not produce torque
(d) $N$ will not produce torque
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Answer:
Correct Answer: 3. (c)
Solution:
- This is the equilibrium of coplanar forces. Hence,
$$ \begin{array}{rlrl} & & \Sigma F _x & =0 \\ \therefore & F & =N \\ & \therefore & \Sigma F _y & =0, f=m g \\ & \Sigma & \Sigma \tau _c & =0 \\ & \text { Since, } & \tau _N+\tau _f & =0 \\ & \therefore & \tau _f & \neq 0 \\ & & \tau _N & \neq 0 \end{array} $$
