PHYSICS and stars Some forces pull objects together Other forces push objects apart Forces also affect people The Four Fundamental Forces There are four basic or fundamental forces in nature They are the gravitational force the electromagnetic force the strong nuclear force and the weak nuclear force These forces pass through us and exist within us They hold everything together All forces in nature are related to one of the four fundamental forces Each of these forces serves a different purpose Gravitational Force The gravitational force is the force of attraction or pulling together It is powerful enough to hold the Earth in its orbit around the sun Still it is the weakest of the four fundamental forces The gravitational force of the Earth is often called gravity Gravity keeps things on the surface of the Earth from flying off into space It keeps the ground on the Earth and your feet on the ground Everything that has mass has gravity In deep space the force of gravity is very weak This is because objects with mass are so far apart Gravitational force keeps the planets in orbit around the sun Gravity pulls this diver into the water M O T I O N A N D F O R C E 7
8 PHYSICS M O T I O N A N D F O R C E Electromagnetic Force The electromagnetic force is either an attraction pulling toward or a repulsion pushing away The electromagnetic force occurs in many forms Except for gravity most of the forces in nature are caused by electromagnetic forces Strong Nuclear Force The strong nuclear force is the strongest of the fundamental forces It only works within atoms The strong nuclear force keeps the nucleus center of an atom from coming apart It is like a glue holding the subatomic particles or smaller parts of the atom together The strong nuclear force gets weaker the further the particles are from the center of the atom Weak Nuclear Force The weak nuclear force causes the nuclei centers of some atoms to break apart These atoms are radioactive or unstable They give off tiny particles over time These are called beta particles The weak nuclear force causes the subatomic structure or smaller parts of some atoms to change This process is called radioactive decay The weak nuclear force does not happen in all nuclei Velocity and Acceleration The speed of an object is called velocity The velocity of an object is how fast it is moving at a point in time Velocity is measured in distance traveled per unit of time For example cars may drive 62 miles per hour 100 kilometers per hour Most people Small pieces of iron are attracted to the magnet electromagnetic force i-lek-tro-mag-NET-ik forss a combination of electrical and magnetic forces that attract push towards or repel push away escape velocity ess-KAPE vuh-LOSS-uh-tee how fast something needs to travel to leave the Earth’s gravity gravity GRAV-uh-tee a force that pulls things toward Earth nucleus NOO-klee-uhss center section of an atom made of protons and neutrons radioactive decay ray-dee-oh-AK-tiv di-KAY when the center of an atom breaks apart subatomic particles suhb-a-TOM-ik PART tuh-kuhls the smaller parts of an atom including the protons neutrons and electrons
PHYSICS can walk at a speed of 2 miles per hour 3 kilometers per hour and run at a speed of 17 miles per hour 27 kilometers per hour They can only run this fast for a short time Scientists often measure velocity in kilometers per second km s In outer space velocity is measured Escape Velocity in a specific way It is measured by how fast something is moving away from or toward the Earth An object must travel a certain speed to escape the gravitational force pull of a planet or a moon This is called the escape velocity This speed depends on the mass of the planet or moon It also 25,000 miles per hour about 40,000 kilometers per hour Acceleration Acceleration is a change in velocity over a certain time In physics anything that is speeding up is accelerating The speed of a car changes when it starts to move The car is accelerating When it slows down it is decelerating also called negative acceleration A dropped object accelerates as it falls This acceleration is caused by gravity Gravity keeps things on the surface of the Earth from flying off into space The acceleration caused by the gravity of the Earth is sometimes called g At the surface of the Earth the acceleration caused by gravity is equal to 1g The Space Shuttle launches into the night sky depends on the distance of the object from the center of the planet or moon The escape velocity from the surface of the Earth is about 6 6 miles 11 kilometers per second This is almost SheiKra a roller coaster in Florida pulls 4g M O T I O N A N D F O R C E 9
10 PHYSICS M O T I O N A N D F O R C ENewton’s Three Laws of Motion All objects follow certain rules These rules are called laws because they apply to everything everywhere Isaac Newton discovered the three laws of motion These laws help scientists understand how objects move First Law of Motion Newton’s first law of motion is sometimes called the Law of Inertia It says that an object that is not moving will stay still It will stay still unless something pushes or pulls it It also says that an object that is in motion will keep moving The object will travel in a straight line at a constant speed Its movement will only change if it is affected by a force On the Earth gravity is a big force that affects objects Without gravity all objects would stay still or keep traveling in a straight line The moon would fly off its orbit if the Earth’s gravity did not pull on it It is easier to see the law of inertia in outer space A tool released by an astronaut floats away until it bumps into something Second Law of Motion The second law of motion says that acceleration is caused by a force acting on an object The acceleration increased speed of an object depends on the amount of the force acting on it It also depends on the mass of the object Light objects like pens and pencils are easy to pick up They require very little effort or force to lift Heavy objects like a refrigerator require a lot of force to move Newton’s first law states that an object will stay still unless something pushes or pulls it The second law of motion says that acceleration is caused by a force acting on an object PROJECTS Volume 106 1
