Gravity And Upthrust : Two Opposite Forces of Pull and Push.

Gravity Page

GRAVITY:

If you lift a brick and let go of it. It falls to the floor.What causes this to happen? Bricks do not move sideways without being pushed or puled.The do not move upwards unless they are thrown or something is pulling them. For the brick to move downwards there must be some force pushing it or pulling it towards the ground.You can feel this force if you hold anything in your hand or try to lift anything off the floor It is called weight.

Gravitational force

GRAVITATIONAL FORCE:

The first person to discover why things have a weight and why they fall to the ground was Sir Isaac Newton.

Sir Isaac Newton.

Newton`s Apple

It is said that one day he was siting in his mother`s orchard when he saw an apple fall from a tree. This started him thinking why such a thing should happen. His  explanation was that the earth and the apple pull each other together. The earth attracts the apple and the apple attracts the earth.The earth is very large and is not affected by the pull of the apple.The apple, being much smaller, is pulled down towards the earth.This force attracting things through space is called gravity.It is this force that holds everything on the earth`s surface and causes them to have a weight.

Isaac Newton also realized that the force of gravity between things gets smaller as they get further apart.People high up in an Aeroplane do not weigh as much as they do on the ground. This is because they are further from the earth and so the pull of gravity on them is weaker. We do not usually notice this change in weight because it is so small.

Astronauts are eventually float in the zero gravity Cabin

Astronauts in a rocket notice that the pull of gravity begins to get weaker as they go further from the earth. They can eventually float around the cabin. They can also do this in orbit around the earth because the pull of earth is balanced by

the centrifugal force.

As a spacecraft goes further from the earth towards the moon it begins to be affected by the gravity of the moon. The Gravity on the surface of the moon is less than the gravity of earth. A women who weighs 60 kg on earth would weigh 10 kg on the moon.She would be throw things much further and jump much higher because the force pulling her down would be smaller.

The planets move almost in circular elliptical orbits based on the force of gravity. The sun's gravitational pull is the most powerful gravitational force in the solar system. The other heavenly bodies have a much smaller gravitational force on one another called perturbations.

 The weight of things also depends on how big they are and on what they are made of. What weighs more, a kilogram of lead or one of feathers? Many people without thinking say a kilogram of lead weighs more .Of course this is a trick question because they both weigh the same, but people think of feathers as being lighter than lead. If you hold a cricket ball in one hand and an apple in the other the cricket ball weighs more although they are almost the same size .This is because the stuff (or matter) of which cricket balls are made is more tightly packed than the matter of which apples are made .Scientists say that the cricket ball has a greater density than the apple.lead has a greater density than feathers.

The total amount of matter in anything is called the mass .The cricket ball has more mass than the apple although they look the same size. A kilogram of feathers has the same mass  as a kilogram of lead. However the size of the pile of feathers would be much greater than the size of the piece of lead.

Newton`s law of universal gravitation

According to the Newton`s law of universal gravitation, In universe any two objects attract each other in the direction of its connecting straight line. This attraction force(F) is directly proportional to the multiplication of those two object`s masses(M,m), and also inversely proportional to the square of those two objects distance(d).

So the final equation is:

           m M

F = G -------- 
             ^   

            d|2    

(F = force, m and M = masses of two objects, d = distance, G = universal gravitational constant).

                                                                                 Animation Showing Different decreasing g due to increasing distance from Earth.

 

The force of attraction of the  earth toward any object is known as object`s weight.

Therefore  W = m*g (W = weight, m = mass , g = acceleration due to gravity) 

Weight of Metal big Pot on Earth is 100 kg . Therefore man on the picture cannot pull the Metal big pot due to increased G on earth

Weight of Metal big Pot on 10000 km from earth is 14 kg . Therefore man on the picture can easily  pull the Metal big pot due to decreasing  G away from the earth.

the rate of change of g in an object is equal to its weight change. As we go from the surface of the earth the effect of gravity become weaker with the increasing distance and things weight less.

Weight of Metal big Pot on 15000 km from earth is 8 kg . Therefore man on the picture can more easily  pull the Metal big pot due to more decreasing  G away from the earth.

Weight of Metal big Pot on 20000 km from earth is only 6 kg . Therefore man on the picture can more easily  pull the Metal big pot due to most decreasing  G away from the earth.

FLOATING:

Physics of Floating page

If your throw a stone and cork into a pond.The stone sinks and cork floats.If you gently lower a piece of wood into water you will find that it feels lighter and lighter until finally it appear to have no weight at all. It is now being supported by the water instead of your hand.The support from the water is called an upthrust, because the water is pushing up against the bottom of the wood. When the upthrust is equal to the weight of the wood , it floats.

Stone, Metal Ball, Weight bar sink and wooden bar and bread Float.

There is also an upthrust against things that do not float.Lower a brick into water and it will seem to become less heavy.But an ordinary brick will not float.

                                                                             Animation showing Floating and Sinking of different  elements.

ARCHIMEDES PRINCIPLE:

weight of an object weather it fully or partially submerged into a liquid is apparently decrease.This decreased weight is equal to weight of a liquid which is displaced by the submerged object.

Picture of  Archimedes.

Archimedes. an ancient Greek scientists, was the first to study upthrusts.He found that the upthrust was equal to the weight of fluid displaced.This is known as Archimedes principle.It means that if an object pushes aside(displaces) one kilogram of water then there will be an upthrust equal to one kilogram acting on the object.

Suppose that a brick and a  block of wood of the same size are put into water that they  are both totally covered.Because they are the same size,they both displace the same weight of water.This means that the upthrust on each of them is the same . The upthrust on the wood is greater than its weight so it rises to the surface and floats.The brick sinks because the upthrust on it is smaller than its weight.

Weather a substance will float in water or not depends on its density. Density is the weight of the substance divided by its volume.If its weight is greater than the  weight of an equal volume of water it will sink. If its weight is less than the weight of the equal volume of water it will float.The brick has greater density than water and sinks.Wood floats because the density of the wood is less than that of water.

It is possible to float one liquid on another.Oil floats on water.This is why oil slicks from tankers float ashore and pollute beaches.It is difficult to put out a petrol fire with water, because the petrol floats on the top of the water and keeps burning.

Oil and petrol float because they are less dense than water.Mercury sinks because its density is 13 time greater than water.

A ship floats

A ship floats Because it has large volume of air in its hull. Giving it an over all density less than water.If a ship is badly holed by a rock or a torpedo.Water floods in .This increases its density and it may sink.

Aeroplane float in air

Things are also float in air .For example airship and balloons.This is because they are filled with helium which is less dense than air.

                                                                     Animation Showing Archimedes Principle experiment:

Archimedes Principle experiment:

Take a weight of metal object by a spring balance. Take a glass pot with a side tube opening. Then hang this metal object into the glass pot fully submerge into the glass pot`s water without touching the side of the glass pot and weight it now with a spring balance, weight of the metal object now apparently decrease. Some water is displace through side tube opening glass pot , conserve this displace water in a separate glass pot and weight it. The difference between the weight of metal object at initial stage (before the submerge) and  from submerge stage is call decrease weight which is equal to the weight of the water which displace from the glass pot.

Take a weight of metal object by a spring balance

Then hang this metal object into the glass pot fully submerge into the glass pot`s water without touching the side of the glass pot

Then hang this metal object into the glass pot fully submerge into the glass pot`s water without touching the side of the glass pot and weight it now with a spring balance, weight of the metal object now apparently decrease. Some water is displace through side tube opening glass pot , conserve this displace water in a separate glass pot