Ravi suggested his friend Karim the following ways to reduce or minimise friction. (i) Smoothening the surfaces in contact. (ii) applying lubricants between the surfaces (iii) streamlining the bodies of objects moving in fluids (iv) Applying coarse material between the surfaces is contact. Which of the methods suggest is correct? Options :
(i) and (iv)
only (iv)
(ii) and (iii)
(i), (ii) and (iii)

3rd option

Explanation:

According to modern theory of friction, friction cannot be reduced by making smooth. example two mirrors are smooth but if kept in contact there is very high friction. It is due to intermolecular force of attraction but not roughness.

But if you go towards old theory then last option is correct as friction is due to roughness

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find the sum of the two horizontal vectors and the sum of the two vertical vectors

isoceles right angled triangle