Gravity in Motion

Section 6-1 Pages 150 - 156

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In the beginning….

According to the Greek philosopher Aristotle, an object’s mass affects how fast it will fall  He said that objects with larger masses will fall faster  Aristotle never actually tested his theory  Italian scientist Galileo argued that the mass of an object des not affect the time it takes to fall to the ground  He proved this by dropping two different massed canon balls from the Leaning Tower of Pisa and they landed at the same time

Gravity and Acceleration

 The reason the two cannon balls hit the ground at the same time is because acceleration due to objects gravity is the same for all  As we’ve talked about before, acceleration depends on mass force and  A heavier object experiences greater gravitational force but is harder to accelerate due to its increased mass  The extra mass of the heavier object balances the additional gravitational force

Acceleration due to gravity

 Remember that acceleration is the rate at which velocity changes over time  All objects accelerate toward Earth of 9.8 m/s 2 at a rate  This means that for every its second velocity an object falls toward the Earth, increases by 9.8 m/s

Calculating change in velocity

 Δv = g x t   Δ (delta) stands for change in Δv = change in velocity  g = acceleration due to Gravity (always 9.8 m/s 2 )  t = time the object takes to fall (in seconds)  Do Math focus on page 151 as a class

Air resistance

 Two pieces of paper, one flat and one crumpled  They will not fall at the same rate  This is because of air resistance  The force that opposes motion of objects in the air  Its pushes up against the object, going against the force of gravity  The amount of air resistance depends on the size, shape and speed of the object    The more surface area, the more air resistance The more mass, the less air resistance The faster the object moves, the more air resistance

We have reached Terminal Velocity

 Objects do not continue to accelerate (increase their velocity) forever until they hit the ground  This is because air resistance continues to increase as speed increases  Air resistance increases until it is equal to the force of gravity  Results in a net force of 0N  Is there any further change in motion (increase in speed) if net force is 0N?

 NO!!

 When net force is 0N, the object falls at a constant velocity which is called its terminal velocity  Terminal means at the end  Terminal velocity is a good thing  Think about hail storms  If large chunks of hail did not reach a terminal velocity (usually between 5 and 40 m/s), they could reach velocities of up to 350 m/s!

 Terminal velocity is reached at a faster pace when using a parachute – allows sky divers and airmen to reach the ground at a safer speed

Free Falling

 When you ride the Free Fall at Six Flags, you are not really in free fall  Free Fall occurs when gravity is the only force acting on an object  Therefore, the only place you can experience free fall is in space where there is no air  Why?

 If there is air, there is air resistance, which is a force

Weightless or not?

 Astronauts are said to be weightless when they are in space and that is why they float, but…  You can never truly be weightless  Why not?

 Everything creates a gravitational force so there will always be something around you that causes you to have weight  So why do astronauts float?

 They are in free fall as they orbit the Earth

Orbits – not the gum

 An object is orbiting when it is traveling around another object in space  It is caused when an object is moving forward, but it is also in free fall (being pulled towards the Earth or another object due to gravity)  The two forces combine to create a circular path of motion  Remember that any object traveling in a circular path is constantly changing direction  A force that causes circular motion is called a centripetal force

Aim High

 Projectile motion is the curved path an object takes when it is thrown near the Earth’s surface  Two parts to projectile motion  Horizontal motion  Ie: Caused by the force of your hand throwing the ball  Vertical motion  Caused by gravity  When these two parts combine, they cause a curved path of motion