GravityGravityWhat is it?What is it?

http://www.fi.edu/pieces/cych/apollo%2010/story/hoi/ball3.html

http://www.fi.edu/pieces/cych/apollo 10/story/hoi/ball.html

GravityGravity isis……

A natural A natural forceforce that that pulls all pulls all objects objects

toward the toward the center of center of the earth.the earth.

WHAT IS A FORCE?WHAT IS A FORCE?

A pull or a push on an

object

It is measured in Newtons (N)

or kg/ m/ s2.

Newton was a Smart GuyNewton was a Smart Guy

Sir Isaac Newton (1643-1727) an Sir Isaac Newton (1643-1727) an English scientist and mathematician English scientist and mathematician famous for his discovery of the law of famous for his discovery of the law of gravity also discovered the three gravity also discovered the three laws of motionlaws of motion

Today these laws are known as Today these laws are known as Newton’s Laws of MotionNewton’s Laws of Motion and and

describe the motion of all objects on describe the motion of all objects on the scale we experience in our the scale we experience in our

everyday lives.everyday lives.

11stst law: law: An object at rest tends to stay at An object at rest tends to stay at rest and an object in motion tends to stay rest and an object in motion tends to stay

in motion unless acted upon by an in motion unless acted upon by an unbalanced forceunbalanced force..

-Things will keep -Things will keep moving forevermoving forever unless it is stopped by a force.unless it is stopped by a force.

-Things will stay -Things will stay stationarystationary (in one (in one place) unless an place) unless an outside forceoutside force acts acts on it.on it.

If a golf ball was hit in space would it stop?If a golf ball was hit in space would it stop?If a golf ball was hit on Earth, would it to stop? What causes is to If a golf ball was hit on Earth, would it to stop? What causes is to stop?stop?

- - Earth’s Gravity causes things to stop. The opposing Earth’s Gravity causes things to stop. The opposing force that slows things down is force that slows things down is FrictionFriction

Some Examples from Real LifeSome Examples from Real Life

Two teams are playing tug of war. They are both Two teams are playing tug of war. They are both exerting exerting equal forceequal force on the rope in opposite on the rope in opposite directions. This directions. This balanced forcebalanced force results in no results in no change of motion.change of motion.

A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

Newton’s First Law is also Newton’s First Law is also called the called the Law of InertiaLaw of Inertia

-Inertia-Inertia: the tendency of an object to : the tendency of an object to resist changes in its state of motionresist changes in its state of motion

-All objects have inertia-All objects have inertia..

-The more -The more massmass an object has, the an object has, the more inertia it has (and the more inertia it has (and the harderharder it it is to change its motion).is to change its motion).

More Examples from Real LifeMore Examples from Real LifeA powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

22ndnd law: law: Force equals mass Force equals mass times acceleration.times acceleration.

GravityGravity is a is a forceforce.. AccelerationAcceleration is how quickly an is how quickly an

object is object is changing speedchanging speed Earth’s gravity has a constant Earth’s gravity has a constant

acceleration acceleration downwarddownward. All objects . All objects accelerate at accelerate at 1010 m/s m/s22. (9.8 m/s. (9.8 m/s2)2)

If we all jumped out of a plane we would fall at 10 If we all jumped out of a plane we would fall at 10 m/sm/s22. This goes for . This goes for all other objectsall other objects whether whether largelarge or or smallsmall..

Which do you think would fall first?Which do you think would fall first?

A A bouncybouncy ballball

Or a marble

-All things fall at the -All things fall at the same ratesame rate of of speedspeed, , because of Earth’s constant because of Earth’s constant downward acceleration. downward acceleration.

--ButBut things with more things with more mass mass (weight) will (weight) will fall harderfall harder and and hitshits the the earth with earth with more forcemore force!!

((GravityGravity has a has a stronger forcestronger force on things with on things with more massmore mass!)!)

It takes force to overcome gravity!It takes force to overcome gravity!

Throw a bean bag in the air and then…

Throw a basketball in the air.

Which one was harder to throw?

The basketball would be harder The basketball would be harder to throw. You had to use to throw. You had to use more more forceforce to throw the basketball to throw the basketball because it has because it has more massmore mass (weight)(weight)

What does F = ma say?What does F = ma say?F = ma basically means that the force of an

object comes from its mass and its acceleration.

-Something very small that’s changing speed very quickly, like a bullet, can still have a great force. -Something very small changing speed very slowly will have a very weak force.

-Something very massive that’s changing speed very slowly like a glacier, can still have great force.

Using Force = mass x acceleration Using Force = mass x acceleration

Calculate which would land with more Calculate which would land with more force:force: Object 1 has a mass of 20 grams = .02 kgObject 1 has a mass of 20 grams = .02 kg Object 2 has a mass of 100 grams = .1 kgObject 2 has a mass of 100 grams = .1 kg Acceleration is 10 m/sAcceleration is 10 m/s22..

Force of Object 1?Force of Object 1? Force of Object 2?Force of Object 2?

33rdrd Law: Law: For every action there is For every action there is an equal and opposite reaction.an equal and opposite reaction.~ For every force acting on an object,

there is an equal force acting in the opposite direction.

-Right now, gravity is pulling you down in your seat, but your seat is pushing up against you with equal force. This is why you are not moving.

-There is a balanced force acting on you– gravity pulling down, your seat

pushing up.

Think about it . . .Think about it . . .What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it,

the more force the rock exerts back on your toe (and the more your toe

hurts).

Satellites are objects that move Satellites are objects that move fast fast enoughenough that they that they overcomeovercome earth’s earth’s

gravitational pull, but still stay within the gravitational pull, but still stay within the earth’s orbit.earth’s orbit.

(As you know Satelittes move around our earth. But (As you know Satelittes move around our earth. But did you know that is a problem?)did you know that is a problem?)

““There are currently over eight There are currently over eight thousand satellites and other thousand satellites and other large objects in orbit around the large objects in orbit around the Earth, and there are countless Earth, and there are countless smaller pieces of debris smaller pieces of debris generated by spacecraft generated by spacecraft explosions and by collisions explosions and by collisions between satellites. Until between satellites. Until recently, it has been standard recently, it has been standard practice to put a satellite into practice to put a satellite into orbit and leave it there. orbit and leave it there. However, the number of However, the number of satellites has grown quickly, and satellites has grown quickly, and as a result, the amount of orbital as a result, the amount of orbital debris is growing rapidly. debris is growing rapidly. Because this debris is travelling Because this debris is travelling at orbital speeds (7-8 km/s!), it at orbital speeds (7-8 km/s!), it poses a significant threat to the poses a significant threat to the space shuttle, the International space shuttle, the International Space Station, and the many Space Station, and the many satellites in Earth orbit.”satellites in Earth orbit.”

http://images.google.com/imgres?imgurl=http://http://images.google.com/imgres?imgurl=http://www.tethers.com/WebImages/www.tethers.com/WebImages/OrbitalDebris.jpg&imgrefurl=http://www.tethers.com/OrbitalDebris.jpg&imgrefurl=http://www.tethers.com/OrbitalDebris.html&usg=__JnSEf589SrzzPevOvU5XTyPALss=&h=2OrbitalDebris.html&usg=__JnSEf589SrzzPevOvU5XTyPALss=&h=250&w=258&sz=46&hl=en&start=18&tbnid=rrwVQ4lVZalWAM:&t50&w=258&sz=46&hl=en&start=18&tbnid=rrwVQ4lVZalWAM:&tbnh=109&tbnw=112&prev=/images%3Fq%3Dearth%2B%252Bbnh=109&tbnw=112&prev=/images%3Fq%3Dearth%2B%252B%2Bsatellites%26gbv%3D2%26hl%3Den%26safe%3Dactive%2Bsatellites%26gbv%3D2%26hl%3Den%26safe%3Dactive

When do Satellites become subject to earth’s When do Satellites become subject to earth’s gravity?gravity?-Objects must move -Objects must move

very quicklyvery quickly to to orbit orbit the earth.the earth.

- If an object does not - If an object does not meet the required meet the required speed it would be speed it would be slowly draggedslowly dragged into into the Earth’s the Earth’s atmosphere. atmosphere.

-Objects-Objects fartherfarther from from earthearth experience experience less less gravitational pullgravitational pull..-The closer to earth’s surface the more

gravitational pull is felt.

-The Moon -The Moon orbitsorbits the Earth and is the Earth and is therefore a therefore a SatelliteSatellite..

Despite that the moon is much Despite that the moon is much fartherfarther it it is is still attractedstill attracted to the Earth’s to the Earth’s gravitationalgravitational pull, pull,

-but it moves so quickly it -but it moves so quickly it revolvesrevolves the the planet at a fixed distance.planet at a fixed distance.

-The Moon is -The Moon is also also 1/6.251/6.25 the the size of the size of the earth and earth and therefore has therefore has 1/6.251/6.25 of the of the gravitational gravitational pull.pull.

-Earth’s gravity -Earth’s gravity is is 9.8 9.8 m/sm/s22..

-Moon’s gravity -Moon’s gravity is is 1.5681.568 m/s2. m/s2.