Ans:
- When two solid bodies contact move relative to each other a friction force develops at the contact surface in the direction opposite to motion.
- The similar situation is also true when a fluid moves relative to the solid.(or vice-versa)
- For example an airplane moving relative to air will experience a resisting force from air.
- A swimmer experiences a higher resistive force in water compared to the resistive force exerted by air on a human body when a person walks on a street.