Chapter 12. Fraction

1.Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play
(a)only if the objects are solid.
(b)only if one of the two objects is liquid.
(c)only if one of the two objects is gaseous.
(d)irrespective of whether the objects are solid, liquid or gaseous.

2. Photo

Fig. 12.1
In Fig.12.1, a boy is shown pushing the box from right to left. The force of friction will act on the box
(a)from right to left (¥)    (b)from left to right (¦)    (c)vertically downwards (œ)  (d)vertically upwards (›)

3.To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
(a)stone     
(b)plastic block  (c)wooden block  (d)glass block