Capture 7.PNG

# Friction Suppose you push a skateboard with your hand. According to Newton's first law, if the net force on a moving object is zero, it will continue moving in a straight line at a constant speed. Does the skateboard maintain constant speed after you release it? You know the answer. The skatebo...

# Friction Suppose you push a skateboard with your hand. According to Newton's first law, if the net force on a moving object is zero, it will continue moving in a straight line at a constant speed. Does the skateboard maintain constant speed after you release it? You know the answer. The skateboard slows down and stops. According to Newton's second law, if the skateboard is accelerating (slowing down), there must be a net force acting on it. This force, which slows the skateboard and causes it to stop, is called friction. Friction opposes the sliding motion of two surfaces in contact. The magnitude of friction depends on two factors: 1. The type of surfaces (the materials) 2. The force pressing the surfaces together We express the type of surface with a friction coefficient, denoted by the letter *μ*. Every surface has its own friction coefficient, which depends on the material it is made of. The friction force (F) is related to the normal force (W) by the equation: F = μ * W The larger the friction coefficient, the greater the friction force, making it harder for objects to slide. Even smooth surfaces have imperfections at a microscopic level, which contribute to friction. Heavier objects experience a larger friction force. There are two types of friction: * **Static friction:** This force resists the initiation of motion. In the example, if your push on a box is insufficient to overcome static friction, the box will not move. * **Kinetic (sliding) friction:** This force resists the motion of two surfaces sliding past each other. To keep an object moving, you must continuously apply a force to overcome kinetic friction. Based on the fact that it is harder to start an object moving than to keep it moving, static friction is typically greater than kinetic friction. Friction is useful in many ways: * We are able to walk because of friction. * We are able to hold objects in our hand because of friction.