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Gravity
Gravity
The Earth is attracted to you.
You are attracted to the Earth
You fell for the Earth
Mr Cowley Lecture 1
Mr Cowley Lecture 1
Watch this short Lectures. This is: Mass, Gravity, Weight. Gravitational Potential Energy
Gravity x Mass = Weight
Gravity x Mass = Weight
The Relationship between Gravity, Mass and Weight
The Relationship between Gravity, Mass and Weight
You already know there is a relationship between these things.
Think about it - you are weightless on the moon. You are weightless in space. What is weight?
We stand on our bathroom scales to see our weight, but they lie, the bathroom scales lie, the Briscoes woman who sold you the scales....she lied to us!! They don't tell you your Weight! They tell you your MASS!!!!!!
What is the difference?
Mass is the stuff of an object. It's the quantity of matter within the object. If you go into space, you would be weightless, but you would not be massless. To be massless is not to exist.
If you add more matter to your body and eat more McDonalds, your mass increases.
If you reduce the amount of matter in your body by replacing McDonald's with a carrot, your mass will decrease.
Your mass is how many Kilograms of matter your body consists of.
Your mass will be the same if you teleported to any part of the Universe
However.
Your weight will change depending on where in the Universe you are.
On the moon, you weigh only 1/6th of what you weigh on Earth
On the international space station, you weigh nothing
Mr Cowley Long Lecture
Mr Cowley Long Lecture
Mr Cowley Lecture - Weight, Mass and Gravity
Mr Cowley Lecture - Weight, Mass and Gravity
The difference between the Moon, the International Space Station and Earth is GRAVITY.
Well what is gravity
Gravity is the force of attraction between objects. All objects!
You and that person over there are attracted to each other - even if you aren't - because of gravity
Gravity is a very weak force. (luckily)
If I put my chocolate cup, and my coffee cup 1 cm apart on a highly varnished desk and left them their for 10 years, then they will be....... still 1 cm apart.
It is a weak force. If you stood on the Mars Moon Deimos, which is twice the size of Mount Everest, then you did a running jump, you would launch yourself into space and never return.
The more massive the object, the more gravity it generates, but it needs to be really massive for it to be noticeable.
The earth is massive. It is large enough to have cleared it's orbit of all other rocky matter.
The earth is so massive, that it pulls you onto it. No matter how hard you jump, it will always pull you back.
The Earth's gravity will always pull you down. And the bigger you are the harder you'll fall.
But as the earth pulls you down, you push back - with an equal and opposite force. You know this innately, because if you stand for a long time, you will eventually feel tired and want to sit.
The interaction between the force the earth pulls on you and your mass is called weight
To calculate it take your mass and times it by the force of gravity which on earth is 9.81N/ kg.
This 9.81N/kg means that each kg is pulled towards the earth with a force of 9.81 Newtons
The more kilograms of matter an object has, the more downward force will be applied by gravity (and the more upward force, or Normal, will need to be generated to oppose it)
We can calculate that downward force, by taking the force exerted on each Kilogram and multiplying that by how many Kilograms there are.
The formula for this is: Weight = Mass x Gravitational Force
W = mg
G can either be:
9.81N/kg, which means that each kg is pulled towards the earth with a force of 9.81Newtons.
Or, because a 1Newton is equal to 1kg accelerated at a rate of 1m/s per second, you could cancel the kg and have:
9.81m/s/s
The value for Weight is Newtons, since it is the downward Force generated by the interaction between a mass and gravity
Gravitational Potential Energy
Gravitational Potential Energy
I climbed Mt Tongariro, I had to put alot of energy in to get to the summit.
The earth itself had to transform a lot of energy to create Mt Tongariro.
The Helicopter that takes Fabio Wibmer to the top of his Mountain Bike ride transformed alot of chemical potential energy into kinetic energy and then into gravitational potential energy.
To lift my dog up, I had to transform chemical potential energy into kinetic energy and this energy into gravitational potential energy.
But how much did I transform? How much energy does Zed have when he is 2 meters above the ground? How much Gravitational Potential Energy does he have?
Gravitational Potential Energy = mass * gravity * height
Ep = m g h
Let's take Zed and myself.
Zed's mass is 20kg, mine is 75kg. The table is 1 meter high
How much GPE do we both have while standing on the table?
Zed's GPE
GPE = m g h
GPE = 20kg x 9.81ms-2 x 1m
Zeds GPE = 196 Joules
Mr Cowley's GPE
GPE = m g h
GPE = 75kg x 9.81ms-2 x 1m
GPE = 736 Joules
Then, what is Zed's GPE when I lift him so that he is 3 meters above the ground?
Zed lifted by Mr Cowley
GPE = m g h
GPE = 20kg x 9.81ms-2 x 3m
Zeds GPE = 589 Joules
That's a lot of energy for Zeds bones and joints to transform into elastic energy (bend without snapping). No wonder he doesn't want me to drop him from way up there.
My dog, Zed, likes to jump from the picnic table
However, will there be a difference if I stand on the table, pick him up, and throw him up?
The higher you are when you jump, the greater the impact. You know this when you are up high and look over the edge. Part of your brain goes, "That would be bad."
So, can we quantify the energy of something up high?
Yes!
The higher something is, the more energy it has. It has more energy because it transformed more energy to get up there.
We can calculate that by taking 3 factors
Mass - how massive is the object (kg)
Gravity - with how gravity is on that planet (ms-2 )
Height - how far above the other surface is something (m)
As we can see from this, how much mass something has matters. If I was lifting my big dog Luka up, then, as he is more massive, he would have more gravitational potential energy
Gravity matters as well, if we are on the moon, then jumping from 2 or 3 meters would be fun as there would be little ground shock
On Earth gravity has a value of 9.81ms-2
However, you can round gravity to 10ms-2
Height relative to a surface matters. Notice that it is not height relative to the ground. If I lift up Zed and I drop him on the table, that would be very different to dropping him onto the ground. He his closer to the table, so the height relative to the table is less, thus he has less fall time, his velocity will be less and thus so too will be his impact. So, when calculating height it is relative to a surface. If you are in the Skytower and jump and land on the same floor, because you are still inside, then you wont suddenly splatter on the floor. Even though, relative to the earth outside, your GPE is huge.
Mass, Gravity and Height.... Times these together and you get the objects Gravitational Potential Energy
Gravitational Potential Energy = mass * gravity * height
Ep = m g h
Note, GPE is often written as Energy potential, or Ep = mgh
This is because it is a very common calculation, so common that by dropping the G everyone still knows that the calculation is for GPE. And if you forget that Ep = GPE, thats ok because the g is in the other side of the formula :-)
Manu Championship
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