CBSE Class 11-science Answered

Frictional force comes due to the contact of the surface of the body to the surface of place where it is kept. Now when these two surfaces come in contact, at molecular level there are interactions between molecules of body A with molecules of body B. They exert attractive force and when you increase the normal reaction, you simply help these attractive forces become more strong, hence more force will be required to oppose this force , hence more frictional force is needed in that case. When normal reaction increases, layers of body A comes close to B and helps in better interaction and holding and hence needs more force to break this interaction.