# Key Concepts What factors affect the gravitational force between two objects? Why do objects accelerate during freefall?

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Key Concepts What factors affect the gravitational force between two objects? Why do objects accelerate during freefall?

 Gravity  Mass  Weight  Free fall  Air resistance  Terminal velocity  projectile

 Sir Isaac Newton concluded that a force acts to pull objects straight down toward the center of the Earth  Gravity is a force that pulls objects toward each other

 The force of gravity acts between all objects in the universe  Any 2 objects in the universe attract each other

 Two factors affect the gravitational attraction between objects: mass and distance  Mass - a measure of the amount of matter in an object (atoms)  SI unit of mass is kilogram - 1 kilogram is the mass of about 400 pennies = 2.2 pounds

 Mass – more mass an object has, the greater its gravitational force  The sun’s mass is so great that it exerts a large gravitational force on the planets – One reason why they stay in orbit  Distance – the farther apart two objects are, the less gravitational force between them

 Mass is the measure of the amount of matter in an object  Weight is a measure of the gravitational force exerted on an object  Weight varies with the strength of the gravitational force, but mass does not  Mass weight gizmo Mass weight gizmo

 Free fall –occurs when gravity is the only force acting on an object  An object in free fall is accelerating because the force of gravity is an unbalanced force

 Near the surface of the Earth the acceleration due to gravity is 9.8m/s 2  For every second an object is falling its speed increases by 9.8 m/s  All objects in free fall accelerate at the same rate

 Objects falling through air experience a type of fluid friction called air resistance  Friction is a force in the opposite direction of motion so air resistance is an upward force  Falling objects with greater surface area experience more air resistance  In a vacuum there is no air, all objects fall at the same rate of acceleration

 Air resistance increases with velocity  Eventually the falling object will fall fast enough that the upward force of air resistance will equal the downward force of gravity  At this point, the forces are balanced and the objects stops accelerating  The object continues to fall at constant speed  This is called terminal velocity – when the force of air resistance = weight of the object

 Projectile - An object that is thrown  Will an object that is thrown horizontally land on the ground at the same time as an object that is dropped?

 The force of gravity still acts on the object in the same way. It falls downward at the same rate of acceleration as a ball that has been dropped

 If you throw an object at an upward angle, the force of gravity will reduce its vertical velocity until it stops.  Then the force of gravity will pull it back to the ground  When it falls it will accelerate at 9.8m/s 2  Goalfinder Goalfinder

 http://www.aps.org/publications/apsnews/200510/zerogravity.cfm http://www.aps.org/publications/apsnews/200510/zerogravity.cfm  http://www.williamsclass.com/EighthScienceWork/ImagesEighth/ gravityEarthPull.gif http://www.williamsclass.com/EighthScienceWork/ImagesEighth/ gravityEarthPull.gif  http://learn.uci.edu/media/OC08/11004/OC0811004_L6GravityFor ce.jpg http://learn.uci.edu/media/OC08/11004/OC0811004_L6GravityFor ce.jpg  http://www.melancholyrhino.com/images/gravity.jpg http://www.melancholyrhino.com/images/gravity.jpg  http://imagine.gsfc.nasa.gov/YBA/M31- velocity/images/twomass.gif http://imagine.gsfc.nasa.gov/YBA/M31- velocity/images/twomass.gif  http://www.daviddarling.info/encyclopedia/W/weight.html http://www.daviddarling.info/encyclopedia/W/weight.html  http://www.bkpc.co.uk/freefall.jpg http://www.bkpc.co.uk/freefall.jpg  http://leedsmathgeeks.com/wp- content/uploads/2009/02/terminal_velocity.gif http://leedsmathgeeks.com/wp- content/uploads/2009/02/terminal_velocity.gif  http://www.mredwards.net/images/ProjectileMotion.GIF http://www.mredwards.net/images/ProjectileMotion.GIF

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