The force of friction between two solid surfaces in contact will increase under which of the following conditions?

Introduction / Context:
Friction is a contact force that resists the relative motion or tendency of motion between two surfaces in contact. Understanding what increases or decreases friction is essential for engineering, everyday life, and exam questions. This question asks you to identify the factor that will increase the force of friction between two surfaces.

Given Data / Assumptions:

• Two solid surfaces are in contact with each other.
• The nature of the surfaces and the normal force can be altered.
• We are asked to determine which specific change increases friction.

Concept / Approach:
For dry friction between solid surfaces, the maximum static friction and kinetic friction are approximately given by F = mu * N, where mu is the coefficient of friction and N is the normal reaction between the surfaces. If the coefficient of friction remains unchanged, increasing the normal reaction N will increase the frictional force. Lubrication generally reduces friction by introducing a low-shear layer. Removing irregularities can also reduce friction by making surfaces smoother. Creating an air gap removes contact and therefore reduces friction to almost zero.

Step-by-Step Solution:
Step 1: Recall the relation F = mu * N, where F is friction, mu is the coefficient of friction, and N is the normal force. Step 2: Recognise that increasing N while keeping mu constant will increase F. Step 3: Pressing two surfaces harder together increases the normal reaction between them. Step 4: Therefore, the frictional force increases when the surfaces are pressed harder, making option about increased pressure the correct choice.

Verification / Alternative check:
Think about trying to push a heavy box on the floor. If someone sits on the box, increasing its weight, the normal force between the box and the floor increases. As a result, it becomes harder to push, indicating that the frictional force has increased. This everyday example supports the theoretical formula and confirms the correct option.

Why Other Options Are Wrong:

• A layer of lubricant is placed between the two surfaces: Lubricants reduce friction by allowing surfaces to slide more easily over a thin fluid film.
• An air gap is created between the two surfaces: If the surfaces are separated, direct contact and thus solid friction disappear, greatly reducing friction.
• Irregularities on both surfaces are completely removed: Making surfaces smoother generally reduces friction because there are fewer microscopic interlocks.

Common Pitfalls:
Some students think that rougher surfaces always mean more friction and that pressing harder is less important. However, the frictional force depends on both the normal force and the coefficient of friction. For a given pair of surfaces, increasing the normal force is a direct and simple way to increase friction. Another mistake is assuming lubrication can increase friction, when its main purpose is in fact to reduce wear and friction.

Final Answer:
The frictional force will increase when the two surfaces are pressed harder against each other.