Physics 20 Unit A Acceleration Due

to Gravity“Hey Kids! Make sure to write

down the notes from this PowerPoint! The parts in Newton’s Apple Red are particularly important!”

Gravity What is it?

• Gravity is a Force…you don’t know what a “force” is yet, but you’ll learn.

Who “invented” gravity?

Who “discovered” gravity?

Who was the first to “explain” gravity?

Mr. Langdale

No one “invented” Gravity You can’t “invent” gravity. Gravity’s

been around since the beginning of the universe, it has always been present and it always will be.

An invention is typically a human-produced technology…not a force of nature.

No one “Discovered” Gravity The effects of gravity have been

observed by even our earliest human ancestors.

It wasn’t as if it wasn’t around until one day someone discovered it and bang everything fell to the ground.

Who was the first to Explain Gravity?

Early Greek Natural Philosopher Aristotle (384-322 B.C.E [before common era]) made a detailed study of many topics including gravity.

I probably didn’t look like this…

Natural Place Aristotle theorized that objects such as

rocks and people fell to the ground because that was their ‘natural place’. Objects such as birds or paper airplanes acted against gravity because the sky was their ‘natural place’.

Sounds wacky, but this idea was powerful enough to stick around for nearly 2000 years!

The World According to Aristotle…

More Aristotelian Ideas Aristotle used thought

experiments just like Galileo, Einstein, Schrödinger and ourselves!

Aristotle didn’t actually perform these experiments, he just thought them out in his head.

One experiment involved dropping two objects, a feather and a rock, through a column of air.

Aristotle believed that heavier objects fell more quickly than lighter objects and visa versa.

He could prove this by dropping a rock and a feather at the same time…

Dropping an object through Air

Minor Problem Today, we know that while this is

what we would see if we dropped a feather and a rock at the same time, it is only because of the air resistance acting on the feather that slows it down…but hey, it was, like, 3000 years ago, give the guy a break! Mythbusters wasn’t around yet…

Sir Isaac Newton Born in 1642 at

Woolsthorpe, to a family of farmers.

A terrible farmer, so he went to school (Cambridge) to become a Priest, instead studied Math.

Went home from 1665-1667 b/c of Black Plague

Fruitful Years During this break, Newton developed a

mathematical language called Calculus, a powerful tool for analyzing scientific phenomenon (we still learn about it today years later).

Became a renown scientist and mathematician after publishing “Principia Mathematica” in 1686, one of the most influential books on physics of all times. He then became the head of Mathematics at Cainbridge University (a spot now held by Steven Hawkings).

The Apple Popular thought is that Newton

“discovered” gravity when an apple dropped from a tree and hit him on the head.

More likely, this was a thought experiment, or perhaps Newton saw a falling apple and began to think about the mathematics which governed falling objects.

He concluded that gravity not only made objects fall to the ground, but it also kept the planets in their orbits.

Gravity!

Galileo, Galileo, Galileo Figaro It wasn’t until

famed astronomer Galileo Galilei (1564-1642) hit the scene that notions about gravity changed.

But Galileo did more than just study the effects of gravity…

Inventions Galileo became

renown for “inventing” many devices we still use today, including the telescope (which he really just marketed from an idea by James Gregory), eye glasses and the Galilean Thermometer.

Sky watcher at night He also made many important

astronomical observations, including reasoning that said the Earth moved around the sun, and was the first to observe the moons of Jupiter, the rings of Saturn, craters on the moon and sun spots.

His countless astronomical observations were made through home-made telescopes (the first of their kind) and recorded by hand!

Study of Gravity He performed

some experiments using incline planes and rolling objects, to “dilute the force of gravity so I can study it”.

Diluting Gravity

Fg

Fg

Finally, some numbers From these experiments, Galileo

concluded that all objects, regardless of their mass, fall at the same acceleration. (Newton had came to the same conclusion.)

This is called the acceleration due to gravity, g.

g = 9.81 m/s2 I done good.

g Found on your formula sheet. An exact number*, so it does not count

towards sig digs. Is actually only 9.81 m/s2 at sea level and

will change slightly at different elevations and on different planets (more on that later).

Like all acceleration, represents a vector. Is sometimes written as negative because

the direction of the acceleration is always downwards.

http://www.hq.nasa.gov/alsj/a15/a15v.1672206.mov

Of course, we ignore air resistance for these calculations, and you should also know that 9.81 is an average value only used for objects close to the earth (it actually changes the farther you get away from the centre of the earth…but we’re learn more about that later.)

Question Time!!!Ex) An apple is dropped from a height

of 1.25 m. a) How long did it take the apple to hit

the ground?

b) With what velocity was the ball moving just before it hit the ground?

The same apple is now thrown up into the air with the from a height of 1.25m at a velocity of 2.5m/s.

How High does it go? How fast is it travelling when it hits

the ground? ****hard one – how long does it take?