Propose another experiment that could be used to demonstrate that friction is a contact force.

Friction is defined as " the resisting force acts against motion of the body". It is a contact force and it is quiet an important in our daily life.

We can walk or run comfortably because of friction. How does it help us walk? let's think on it.

1. When we walk forward, we push our foot on ground in backward direction. In return the ground exert a force in forward direction which moves us forward. The force of the ground exerts is known as force of friction.

2. Another example,  try rolling a ball on an horizontal solid surface, what do you notice? You notice,  that the ball will slowdown and eventually comes to rest. This means, the surface applies a force in opposite direction of motion of ball, known as friction.

3. Consider boat moving in the water, in order to move the boat in water, the person in the boat must continuously row it.  If he stop rowing it, will the boat move forward? No,  just like the rolling ball stops at the solid surface, the boat will also stop eventually in water.  It stops because of force of friction between water and surface of boat which is at contact of water.

So, from all three examples what can you conclude about friction? Is it a contact force?or not?  Yes,  absolutely it is a contact force. Because in all three examples we noticed that the surface of the object contact with another surface.