Gravity in MotionSection 6-1

Pages 150 - 156

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 gravity is the same for all objects As we’ve talked about before,

acceleration depends on mass and force

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 at a rate of 9.8 m/s2

This means that for every second an object falls toward the Earth, its velocity 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/s2) 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