Friction and drag

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Introduction

Friction is a force that opposes motion between surfaces in contact. It can be categorized as static friction, which prevents initial movement, or kinetic friction, which acts on moving objects. Additionally, drag forces act on objects moving through fluids like liquids and gases, also resisting motion.

Solid friction

Solid Friction:
- Solid friction is the force that acts between two surfaces that are in contact. It opposes motion, meaning that it acts in the direction opposite to the movement of an object.
- When you try to slide an object across a surface, friction between the two surfaces resists that movement.
- Friction may also cause heating due to the rubbing action between surfaces. For example, when you rub your hands together quickly, they become warm because friction converts kinetic energy into heat.
Types of Solid Friction:
- Static Friction: This is the friction that acts when an object is not moving. It must be overcome to start moving the object.
- Kinetic (or Sliding) Friction: This friction acts when an object is already in motion. It is usually slightly less than static friction.

Drag forces in fluids

Drag forces are a type of friction that acts on an object moving through a fluid (a liquid or gas). These forces oppose the object's motion and can have significant effects, such as slowing down the movement of vehicles or objects.

(a) Friction (Drag) in Liquids:
- When an object moves through a liquid (e.g., water), it experiences a drag force.
- This force is caused by the interaction between the object's surface and the liquid molecules, which resists the object's motion.
- The greater the surface area and speed of the object, the larger the drag force it experiences. Swimmers, for example, experience drag as they move through water, which is why their body position and streamlined shapes are important to reduce resistance.
(b) Friction (Drag) in Gases:
- When an object moves through a gas (e.g., air), it experiences air resistance (also known as drag).
- Air resistance depends on factors such as the speed of the object, its surface area, and the density of the air.
- For example, a parachute has a large surface area that increases air resistance, allowing it to slow down a skydiver safely.

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