# Unit 2-4: Gravity. Free Fall The classic story of physics is the story of Sir Isaac Newton and the Apple. –As the story goes, Newton is working on his.

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Unit 2-4: Gravity

Free Fall The classic story of physics is the story of Sir Isaac Newton and the Apple. –As the story goes, Newton is working on his physics research while sitting underneath an apple tree (because it apparently was the one sunny warm day in England as compared to the cloudy wet days they normally have)

As Newton is working underneath the apple tree, an apple comes loose and falls towards the ground, striking Newton on the head. This makes Newton realize that a force pulled the apple towards the Earth, and that this force is also responsible for why the moon orbits the Earth. Free Fall

This incident inspires Newton to research gravity, the attractive force between all masses. Newton also realized that as objects fall, they must accelerate to reach the earth. –Think about it, the apple starts at rest, then begins moving as it falls to the Earth.

Free Fall For our purposes, we are going to imagine that there is no air resistance as objects fall –(it makes the math so much easier) An object that is falling with nothing else acting on it is in free fall. –Free falling objects are only affected by gravity.

Free Fall In free fall, we can measure the time that has passed since the beginning of the fall: –We call this elapsed time The more time passes, the faster the object will fall –Because the acceleration is acting on it for longer.

Free Fall Free Fall Speeds of Objects Dropped from Rest Elapsed TimeInstantaneous Velocity 0s0m/s 1s-10m/s 2s-20m/s 3s-30m/s 4s-40m/s What is the acceleration due to gravity? Accel.= change in Velocity Time Accel.= -10 m/s 2

Free Fall The acceleration due to gravity on Earth is -10m/s 2, customarily referred to as g. Algebraically, or formulaically, we can simply replace the a for acceleration in any 1-dimensional formula (I.e. any from the past three sections) with g for the acceleration due to gravity.

Free Fall We have looked at the example of an object dropping from rest, but what about an object thrown upwards initially? –It will move upwards for awhile, but slow down at a constant rate (g) until it stops. It only stops for a split-second. –Once it stops, it reverses direction, because g is still acting on it, then it drops back to Earth. As if it were dropped from rest!

Free Fall Describing how fast something falls is entirely different from describing how far it falls. We use our distance formulas to calculate how far something falls

Free Fall Formulas: (Note: x is still a displacement, merely a vertical displacement) X = 1/2gt 2 + v i t V f 2 = v i 2 + gt g = v f -v i t

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