Explanation

(c) The explanation are given below
(i)If a body is moved up in inclined plane, then the work done against friction force is zero as there is no friction. But a work has to be done against the gravity. So, this statement is incorrect.
(ii)If there were no friction, moving vehicles could not be stopped by locking the brakes. Vehicles are stopped by air friction only.
So , This Statement is correct.
(iii) In this situation the normal reaction is given by
[image:6_image76.jpeg]
$R = mg\cos\alpha\ \ \ \ \ \ \ldots\ldots.(i)$
If $\alpha$ increase then the value of cos $\alpha$ also decreases.
So, this Statement is incorrect.
(iv)When the duster is rubbing upward then an external force is applied and its value is
[image:6_image77.jpeg]
$F$’=0.5g+$\mu R$
$\therefore\ \ F^{''} = 0.5g + 0.5 \times 11$
Or $F^{''} = (0.5 \times 10 + 5.5)N$ (Here R=11 N)
Or $F^{''} = 10.5N$
Hence, work done in rubbing the duster through a distance of 10 cm.
$W = F^{''} \times d$
$\Rightarrow \ \ \ \ \ \ \ W = 10.5 \times \frac{10}{100}J$
Or $F^{''} = 10.5J$