GravitationGravitation

Cavendish ExperimentCavendish Experiment

What do you need to calculate the force of What do you need to calculate the force of attraction between two bodies?attraction between two bodies?

the masses of the two objectsthe masses of the two objects the distance between the two objectsthe distance between the two objects the gravitational constant.the gravitational constant.

Therefore, to be able to Therefore, to be able to prove the law of prove the law of gravitationgravitation you need to be able to calculate the you need to be able to calculate the gravitational constant (G).gravitational constant (G).

The ProblemThe Problem

The strength of attraction between two small The strength of attraction between two small masses will be extremely small! Therefore, hard masses will be extremely small! Therefore, hard to measure in a laboratory.to measure in a laboratory.

Despite the weakness of the attraction, Henry Despite the weakness of the attraction, Henry Cavendish was able to perform an experiment to Cavendish was able to perform an experiment to measure the force between two small objects measure the force between two small objects which led to the calculation of the which led to the calculation of the gravitational gravitational constant (G) . constant (G) .

Artist's conception Artist's conception of Cavendish of Cavendish conducting his conducting his experiment. experiment.

He performed the He performed the experiment inside a experiment inside a closed shed and closed shed and observed the result observed the result from outside from outside through a through a telescope. telescope.

The opening in the The opening in the wall was added by wall was added by the artist to show the artist to show the apparatus. the apparatus.

Torsion BalanceTorsion Balance For his experiment in For his experiment in

1798, Cavendish hung a 1798, Cavendish hung a dumbell from a fine string. dumbell from a fine string.

He then placed two large He then placed two large lead weights below the lead weights below the dumbell, and was able to dumbell, and was able to see a small twisting in the see a small twisting in the string.string.

From this small twist in the From this small twist in the string he was able to string he was able to measure the force measure the force between the objects.between the objects.

After measuring the force, After measuring the force, masses, and distance, the masses, and distance, the gravitational constant gravitational constant could be calculatedcould be calculated

Did Cavendish determine Did Cavendish determine GG??

In actuality, Cavendish's only goal was to measure the In actuality, Cavendish's only goal was to measure the density of the Earth; he called it density of the Earth; he called it 'weighing the world'.'weighing the world'.

The method Cavendish used to calculate the Earth's The method Cavendish used to calculate the Earth's density consists in measuring the force on a small ball density consists in measuring the force on a small ball caused by a large ball of known mass, and comparing it caused by a large ball of known mass, and comparing it with the force on the small ball caused by the Earth, so the with the force on the small ball caused by the Earth, so the Earth can be calculated to be Earth can be calculated to be NN times more massive than times more massive than the large ball without the need to obtain a numeric value the large ball without the need to obtain a numeric value for for GG..

The gravitational constant does not appear in Cavendish's The gravitational constant does not appear in Cavendish's paper, and there is no indication that he regarded it as a paper, and there is no indication that he regarded it as a goal of his experiment. goal of his experiment.

One of the first references to One of the first references to GG is in 1873, 75 years after is in 1873, 75 years after Cavendish's work.Cavendish's work.

What is the difference between What is the difference between mass and weight?mass and weight?

MassMass The amount of matter in The amount of matter in

an objectan object Mass is measured in kg Mass is measured in kg

or gor g Measured using a Measured using a

balancebalance

WeightWeight The force of gravity on an

object Weight is measured in Weight is measured in

Newtons (N)Newtons (N) Measured using a scaleMeasured using a scale What does weight depend What does weight depend

on?on? massmass gravitygravity What does gravity depend What does gravity depend

on?on? mass of planetmass of planet distance to planetdistance to planet gravitational constantgravitational constant

Gravitational ForceGravitational Force

Weight is another name for the Weight is another name for the gravitational force from the Earth.gravitational force from the Earth.

What can we use to measure the What can we use to measure the gravitational force acting on an object?gravitational force acting on an object?

A spring balance/ Newton meterA spring balance/ Newton meterPeople often use the word ‘weight’ when People often use the word ‘weight’ when

they really mean ‘mass’they really mean ‘mass’

SummarySummary

1) Mass is a measurement of the amount of 1) Mass is a measurement of the amount of matter something contains, while Weight is the matter something contains, while Weight is the measurement of the pull of gravity on an object.measurement of the pull of gravity on an object.

2) Mass is measured by using a 2) Mass is measured by using a balancebalance comparing a known amount of matter to an comparing a known amount of matter to an unknown amount of matter. Weight is measured unknown amount of matter. Weight is measured on a on a scalescale..

3) The Mass of an object doesn't change when 3) The Mass of an object doesn't change when an object's location changes. Weight, on the an object's location changes. Weight, on the other hand does change with locationother hand does change with location

Compare your weight

Interplanetary Can ExperimentInterplanetary Can Experiment

You have nine cans from the You have nine cans from the nine planetsnine planets and each has the same mass. and each has the same mass.

Find the weight of each can using a Find the weight of each can using a Newton meter and work out Newton meter and work out which can is which can is from which planet. from which planet.

Here is one to get you started - is the Here is one to get you started - is the symbol for Earthsymbol for Earth

The mass of each can =581g =0.581kgThe mass of each can =581g =0.581kgGood Luck!Good Luck!

What is gravity?What is gravity?

We don't really know. We don't really know. We can define what it is as a field of We can define what it is as a field of

influence, because we know how it influence, because we know how it operates in the Universe. And some operates in the Universe. And some scientists think that it is made up of scientists think that it is made up of particles called gravitons which travel at the particles called gravitons which travel at the speed of light. However, if we are to be speed of light. However, if we are to be honest, we do not know what gravity "is" in honest, we do not know what gravity "is" in any fundamental way - we only know how it any fundamental way - we only know how it behaves.behaves.

ScientificScientific Revolution Revolution

Modern work on gravitational theory began with the work Modern work on gravitational theory began with the work of of Galileo Galileo GalileiGalilei in the late 16th century in the late 16th century

In his famous experiment dropping balls from the In his famous experiment dropping balls from the Tower of PisaTower of Pisa, and later with careful measurements of , and later with careful measurements of balls rolling down balls rolling down inclinesinclines, Galileo showed that , Galileo showed that gravitation accelerates all objects at the same rate. gravitation accelerates all objects at the same rate.

This was a major departure from Aristotle's belief that This was a major departure from Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.) objects may fall more slowly in an atmosphere.)

Galileo's work set the stage for the formulation of Galileo's work set the stage for the formulation of Newton's theory of gravity.Newton's theory of gravity.

Is there a link?Is there a link?

On Earth, the acceleration of free fall is On Earth, the acceleration of free fall is 10m/s10m/s22

On the Earth, there is a gravitational force On the Earth, there is a gravitational force of 10 newtons on every kilogramof 10 newtons on every kilogram

These two facts are connectedThese two facts are connected

Here is what we do know...Here is what we do know... Gravity is a force of attraction that Gravity is a force of attraction that

exists between any two masses. Sir exists between any two masses. Sir Isaac Newton (1642 -- 1727) realized Isaac Newton (1642 -- 1727) realized that the force called "gravity" must that the force called "gravity" must make an apple fall from a tree. make an apple fall from a tree.

Newton's "law" of gravity is a Newton's "law" of gravity is a mathematical description of the way mathematical description of the way bodies are observed to attract one bodies are observed to attract one another, based on many scientific another, based on many scientific experiments and observations. experiments and observations.

The effect of gravity extends from each The effect of gravity extends from each object out into space in all directions, object out into space in all directions, and for an infinite distance. However, and for an infinite distance. However, the strength of the gravitational force the strength of the gravitational force reduces quickly with distance. (Earth reduces quickly with distance. (Earth and Sun) (Tides and moon)and Sun) (Tides and moon)

Actually….Actually….EinsteinEinstein later came along and later came along and

redefined gravity, so there are redefined gravity, so there are now two models -- Newtonian now two models -- Newtonian and Einsteinian. Einsteinian and Einsteinian. Einsteinian gravitational theory has gravitational theory has features that allow it to predict features that allow it to predict the motion of the motion of lightlight around very around very massive objects and several massive objects and several other interesting phenomena other interesting phenomena

Stay tuned!!!Stay tuned!!!

This was an enormous thing Newton did - This was an enormous thing Newton did - to invent a new kind of math to build a to invent a new kind of math to build a model that described in the same formula model that described in the same formula the observed motion of both falling objects the observed motion of both falling objects on Earth and the planets in the heavens.on Earth and the planets in the heavens.

BUT unfortunately, BUT unfortunately, Newtonian gravityNewtonian gravity falls apartfalls apart when we try to combine it with when we try to combine it with what we've learned about what we've learned about Special Special RelativityRelativity. .

Space time continuumSpace time continuum In the early twentieth century, Albert Einstein developed In the early twentieth century, Albert Einstein developed

his theory of general relativity in which he described his theory of general relativity in which he described gravity as a deformation in space, caused by the gravity as a deformation in space, caused by the presence of massive objects, similar to the way a heavy presence of massive objects, similar to the way a heavy ball would warp a sheet of rubber. ball would warp a sheet of rubber.

This deformation 'told' smaller things how to move This deformation 'told' smaller things how to move through space, so they either went into orbit or fell onto through space, so they either went into orbit or fell onto the larger celestial object. the larger celestial object.

This was a very different way to visualise space. In the This was a very different way to visualise space. In the past, it had been thought space was filled with a fluid past, it had been thought space was filled with a fluid known as ether. When no one could prove the existence known as ether. When no one could prove the existence of the ether, people began to think of space as simply of the ether, people began to think of space as simply empty. So, Einstein's idea that space was like a fabric empty. So, Einstein's idea that space was like a fabric stretched across the Universe was revolutionary. He stretched across the Universe was revolutionary. He called it the 'space-time continuum'. called it the 'space-time continuum'.

General relativity made a number of surprising General relativity made a number of surprising predictions that, over the subsequent decades, have predictions that, over the subsequent decades, have been observed to be true. been observed to be true.

Among them was that light passing by a Among them was that light passing by a massive object would be deflected from its massive object would be deflected from its original path and that light escaping from a original path and that light escaping from a gravitational field would lose energy. gravitational field would lose energy.

(In fact, satellite-based navigation systems (In fact, satellite-based navigation systems such as GPS have to take this second such as GPS have to take this second effect into account, in order to pinpoint effect into account, in order to pinpoint precisely the location of their users.) precisely the location of their users.)

The presence of mass or The presence of mass or concentrated energy causes a concentrated energy causes a local curvature in the space-time local curvature in the space-time continuum. This curvature is continuum. This curvature is such that the inertial paths of such that the inertial paths of bodies are no longer straight bodies are no longer straight lines but some form of curved lines but some form of curved (orbital) path, and this (orbital) path, and this acceleration is what is called acceleration is what is called gravitation gravitation

Balling ball and Lycra Balling ball and Lycra Demonstration Demonstration

Matter tells space how to bend. Matter tells space how to bend. Space tells matter how to moveSpace tells matter how to move

Only by testing the predictions of general Only by testing the predictions of general relativity, to the limits possible in space, relativity, to the limits possible in space, will scientists be able to gain clues about will scientists be able to gain clues about what the next breakthrough in our what the next breakthrough in our understanding of gravity might be.understanding of gravity might be.