Key Concepts• What factors affect the

gravitational force between two objects?

• Why do objects accelerate during freefall?

GravityMassWeightFree fall Air resistanceTerminal velocityprojectile

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

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/s2

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/s2

Goalfinder

http://www.aps.org/publications/apsnews/200510/zerogravity.cfm

http://www.williamsclass.com/EighthScienceWork/ImagesEighth/gravityEarthPull.gif

http://learn.uci.edu/media/OC08/11004/OC0811004_L6GravityForce.jpg

http://www.melancholyrhino.com/images/gravity.jpg http://imagine.gsfc.nasa.gov/YBA/M31-velocity/

images/twomass.gif http://www.daviddarling.info/encyclopedia/W/

weight.html http://www.bkpc.co.uk/freefall.jpg http://leedsmathgeeks.com/wp-content/uploads/

2009/02/terminal_velocity.gif http://www.mredwards.net/images/

ProjectileMotion.GIF