Understanding Gravity: The Acceleration of Objects Toward Earth

good morning today today today today today we are going to continue with our investigation of

kinematics and specifically we're going to look at the earth

and gravity so there we are in our table of contents we're going to look at gravity

so let's commence operations for those of you following along in your notes please turn to this page now

the earth and any object that has mass attracts other objects that have mass this attraction is referred to as the

force of gravity we'll discuss forces in the next unit for now we're just going to investigate

the impact the earth's gravitational attraction has on the movement

of objects in the absence of air resistance all objects regardless of mass

accelerate toward the earth with an acceleration due to the earth's gravitational attraction that is this

value here negative 9.8 meters per second squared u stands for up which is the same as 9.8

meters per second squared and a d stands for down i really want to unpack this statement

because it's really an amazing statement that people take for granted so what does this mean no air resistance

no drag all objects regardless of mass accelerate towards the earth at the same

rate 9.8 meters per second per second so imagine we had a one kilogram object

and a five kilogram object and you could do this experiment for yourself just take two objects

of different mass but with enough weight and imagine we drop these objects from the same height

well according to this statement all objects regardless of mass accelerate toward the earth at the same

rate if we drop these two objects from the same height even though they're different mass

if there's no drag or no air resistance and for a heavy enough object from dropped from a low enough height

that's the case and they'll hit the earth at the same time

now in this picture you can see feathers and a bowling ball and these feathers and bowling ball

were dropped from the same height clearly the bowling ball has much more mass

except they were dropped in an area which did not have any air in it all the air was removed so there was no

air resistance and there was no drag and indeed when you can have this situation

a lighter object and a heavier object will accelerate at the same rate 9.8 meters

per second per second you can find this video by googling the following brian cox visits

the world's biggest vacuum and you'll find the video it's a beautiful video showing the effect of removing

air on falling objects if you can remove all the air so there's no more drag and no more air

resistance then feathers and a bowling ball when dropped from the same height

will hit the ground at the same time i suggest you check that out so let's now look at what this value

means this is something that students get confused about the question always comes up well when

do i use negative 9.8 meters per second squared up or when do i use 9.8 meters per second squared down

well first off i just want you to recognize that these numbers can be written in this

manner there's many different ways of writing the same number

and they all mean the same thing 9.8 meters per second per second it's the same as 9.8 meters per second

square down and it's the same thing as 9.8 meters seconds to the negative 2 power

in addition these values mean the same thing but they're just used in different circumstances in

different situations so let's explore the two different situations

so imagine you have this character standing on top of a very famous tower and the character decides to drop a ball

so first off you have to ask yourself when you're dropping an object what is its initial speed

well the initial speed is zero now if you really understand what 9.8 meters per second per second

means remember it's an acceleration and we discussed this in a previous video if you really understand

what 9.8 meters per second per second means then one second later you know what the

speed is one second later the speed has to be 9.8 meters per second what does this

mean it means for an object that's falling down

every one second its speed will increase by 9.8 meters per second so in a situation

where an object is falling we would use this value for the acceleration due to gravity

the downwards value two seconds later what would the speed be if you just

understand the unit really in physics try to focus on what the unit means

it makes physics that much easier you don't need any more equations if you understand the unit

every one second the object speed increases by 9.8 well if it was previously 9.8 and it

increases by another 9.8 the new speed will be 19.6 meters per second

so for objects moving downwards we use the positive value 9.8 meters per second

per second because gravity causes objects to speed up now the one thing i didn't mention is

we're assuming there is no drag and no air resistance if there is drag and there's air

resistance then it's a different story altogether now let's look at the opposite situation

imagine we have a person who wants to throw an object upwards well in this situation

when you throw a ball upwards we know it's not going to speed up we know it's going to slow down

so we use negative 9.8 meters per second per second up to describe its motion

so i just picked a random number let's say that we throw the ball upwards with a speed

of 30 meters per second and we're gonna throw it straight up and now we're gonna ask the same question as

we did before one second later what would the speed be again if you go back to the previous

video and recall what the negative means this means that every one second the object speed will decrease by 9.8

meters per second so if it starts off with an initial speed of 30 meters per second

and every one second it slows down by 9.8 well one second later the speed is going

to be 20.2 30 subtract 9.8 and another second later

two seconds later the speed will be 10.4 20.2 subtract 9.8 and another second later

well you can figure that out it's almost going to come to a stop this value for the acceleration to the

due to the earth's gravitational traction is only valid near the surface of the earth

so what does that mean well if we have an object near the surface of the earth

its acceleration due to gravity will be 9.8 however if we move this object now

at a distance of 6400 kilometers which is about the radius of the earth notice the value of acceleration due to

gravity drops significantly it's now only 2.4 it's not zero though there's a

misconception that in outer space there is no gravity that's not true there is still a significant amount of

gravity it would still fall towards the earth if we release this object it would still

fall towards the earth but it would be accelerating at a lower value of only 2.4 meters per

second squared compared to 9.8 when we're close to the surface of the earth

so keep that in mind that this 9.8 value only is valid near the surface of the earth

now for the international space station it orbits at around 400 kilometers above the earth

and the value there is still significant as well it's not 9.8 but it's 8.7 meters per second per second it's

still significant the acceleration to the gravity and again

sometimes you'll see videos of the astronauts on board the international space station and they'll look like

they're floating you'll think oh there's no gravity out in space that's not true if you take a grade 12 course in physics

you understand why the astronauts look like they're floating the earth's acceleration due to gravity

at 400 kilometers above the earth is still significant at 8.7 meters per second

per second for objects that experience air resistance the acceleration of the

object while falling will be lower than g will be lower than 9.8

so keep that in mind in everyday life for example you see a leaf falling from a tree

it's experiencing drag and clearly it's not accelerating at 9.8 meters per second

per second on the moon objects also experience gravitational attraction due to the moon

however the acceleration due to the moon's gravitational attraction is much smaller

than that of the earth and again if you're going to take a course in physics in the future a

grade 12 course or a senior course in physics you'll understand why the acceleration

due to the moon's gravitational attraction is smaller than that of the earth

i'll give you a hint it has to do with the size of the moon and its mass all right hope you enjoyed the lesson

have a great day