Forces & Motion

What is a Force? Force: push or pull Unit: Newton (N) Kg x m/s2 Vector: has both magnitude & direction

Combining Forces Use vector addition Net Force: the total force acting on an object Balanced Forces: when the net force is zero No motion occurs Unbalanced Forces: when the net force is not zero Motion occurs (acceleration or change in direction)

Balanced Forces:

Unbalanced Forces:

Friction Friction: a force that opposes (acts opposite to) motion Four types: Static Sliding Rolling Fluid

Types of Friction Static: friction force on objects that are not moving (greatest friction force) Sliding: friction force on an object as it slides over another Rolling: friction force on an object as it rolls over another (ex. Ball bearings) Fluid: friction force on an object in a fluid (liquid or gas) (ex. Air resistance)

Gravity Gravity: downward force pulling objects toward the center of Earth Acceleration on Earth due to gravity = 9.8 m/s2 Terminal velocity: constant velocity that a falling object reaches when the force of gravity is equal to the air resistance

Projectile Motion An object thrown forward will follow a curved path due to gravity The combination of an initial forward velocity and the downward vertical force of gravity causes the object to follow a curved path

Projectile Motion

Scientists who contributed to the ideas about Force & Motion Aristotle: incorrectly proposed that force is required to keep an object moving at a constant speed Galileo: studied constant acceleration due to gravity Newton: defined force & mass; introduced 3 laws of motion

Newton’s First Law of Motion Law of Inertia An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force If the net force = 0, then the object’s state motion will remain unchanged

Newton’s Second Law of Motion The force acting on an object is equal to the mass of the object times the object’s acceleration Force = mass x acceleration (or F = m x a) Units: N = kg x m/s2

Weight & Mass Weight: the force of gravity acting on an object W = m x g same equation as F = m x a Changes depending on the gravitational force Mass: a measure of inertia on an object (how much matter is within an object) Constant no matter what the gravity

Newton’s Third Law of Motion For every action there is an equal and opposite reaction Forces occur in pairs Not all forces create motion

Momentum The product of an objects mass times its velocity An object with large momentum is hard to stop Momentum = mass x velocity M = m x v Units kg*m/s = kg x m/s

Momentum

Law of Conservation of Momentum In a closed system, the loss of momentum of one object equals the gain in momentum of another object Momentum is conserved

Universal Forces Electromagnetic Forces: forces associated with charged particles (electric & magnetic) Nuclear Forces: forces that occur within the nucleus of an atom Gravitational Forces: an attractive force between any two masses

Newton’s Law of Universal Gravitation Every object in the universe attracts every other object Closer distance = greater force Larger mass = greater force