Ever gone skiing or suddenly slipped on an icy path? The 'villain' behind these scenes is none other than friction, a force that plays hide and seek between two surfaces in contact.

Imagine a setup where

• A metal object sits on a wooden block.
• One end of the metal is connected to a spring balance (which measures force in Newtons). Let's call this a newton-meter.
• The other end of this metal links up to a winch (kinda like a crane).
• The entire wooden block sits on rollers, ready to slide.
• When everything is relaxed, the spring balance reads a big fat zero.

What happens when the game begins?

• When the winch starts its magic, it reels in the string attached to the wood.
• Initially, both metal and wood are like two best friends, not letting go of each other, hence no movement.
• But as the pull gets stronger, there comes a point where the metal is like, "Bye, Wood!" and they start to slide away from each other. Interestingly, less force is then needed to keep this slide show going.

A graph (Figure 26(b)) would show that

• Initially, as force increases, the wood doesn't move.
• Then, BOOM! At a certain force, it starts moving. But then the force required drops.
• Occasionally, the platform has moments of "stick-slip" where it pauses and then suddenly moves again.

Based on observations, we can describe two types of friction

• Static Friction: When there's no movement between surfaces (like the metal and wood initially).
• Dynamic Friction: The kind you experience when things are on the move (like the metal sliding over the wood).

Almost every surface pair will display these two friction types.