Frictional force is a fundamental concept in physics. It's the force that opposes motion when two surfaces are in contact. Friction always acts parallel to the contact surface and in the opposite direction to motion or attempted motion. When you push a block across a table, the applied force tries to move it forward, while friction acts backward, resisting the motion.
There are two main types of friction. Static friction prevents an object from starting to move. It acts when you apply a force but the object remains stationary. Kinetic friction, also called sliding friction, opposes motion once the object is already moving. An important characteristic is that static friction is typically stronger than kinetic friction, which is why it's often harder to start moving an object than to keep it moving.
The magnitude of friction can be calculated using specific formulas. For static friction, the force is less than or equal to the coefficient of static friction times the normal force. For kinetic friction, the force equals the coefficient of kinetic friction times the normal force. The coefficient of friction depends on the materials in contact, while the normal force is typically equal to the weight of the object on a horizontal surface.
Several factors affect the amount of friction between surfaces. Surface roughness plays a major role - rougher surfaces have more microscopic contact points, creating greater friction. The normal force also matters - heavier objects press down harder, increasing friction. Finally, the materials themselves have different friction coefficients. For example, rubber on concrete has much higher friction than ice on ice.