Don’t Let It Get You Down! Gravity Don’t Let It Get You Down!

Would you be surprised if you let go of a pen you were holding and it did not fall? You are so used to objects falling that you may not have thought about why they fall.

It all starts with an apple… One beautiful spring day in 1655, a man named Isaac Newton was sitting under an apple tree in his garden, enjoying a glass of tea. Suddenly, one of the apples fell and crashed on his head. Disclaimer: This last part has been fictionalized, most believe that the apple did not actually hit Newton on the head, but rather fell nearby and caught his attention.

A story about an apple That got Newton thinking (once the bump had gone down of course). “Why did the apple fall towards the Earth?” “Why did it not “shoot upwards'' when it came away from the branch?” And, “why did I plant that stupid apple tree anyway?!!''

A story about an apple Newton knew that unbalanced forces are necessary to move or change the motion of objects. So, he came up with the idea that the Earth must attract the apple towards it with some “unseen force''. He named this force gravity.

Why does the moon not fall straight down onto the earth?

The Truth About Gravity Gravity is a phenomenon The phenomenon results in a force which can accelerate objects with mass “g” represents the acceleration due to the force caused by the phenomenon of gravity

Gravity creates an attractive force between all objects that have mass. The atoms of one object pull on the atoms of another object. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work.

The Law of Universal Gravitation Now that we know what gravity is, let’s go back to Newton. He generalized his observation in something called the Law of Universal Gravitation. This law states: All objects in the universe attract each other through gravitational force.

The Law of Universal Gravitation The size of the gravitational force depends on two things: Mass of the objects (If mass increases, force also increases) Distance between the objects (If distance increases, force decreases)

The Importance of Being Gravity Gravity has basic properties that set it apart from the other forces: (1) it is long-ranged and thus can act over cosmological distances; (2) it always supplies an attractive force between any two pieces of matter in the Universe. Thus, although extremely weak, it always wins over cosmological distances and is the most important force for the understanding of the large scale structure and evolution of the Universe.

The law of universal gravitation The formula for this law is: F = G x m1 x m2 r2 F = force G = gravitational constant = 6.673 x 10-11 Nm2/kg2 - always an uppercase G, do not confuse with g, which is for gravity M = objects’ mass R = distance between objects

Newton’s Law of Universal Gravitation

Henry Cavendish’s experiment determined the proportionality constant in 1798 G= 6.67 x 10-11 N m2 / kg2

The Difference between Mass & Weight That’s heavy man. Heavy…

Mass vs. Weight Mass is sometimes confused with weight. Mass- is the amount of matter in an object Weight- is the force of gravity on an object The greater the mass the greater the force (weight) Weightlessness – free from the effects of gravity

Mass This does not change… ever! Your mass stays the same regardless of location in the universe. Mass is measured in units of kg Ex: On Earth Your mass is 65 kg on the Moon is 65 kg on Jupiter is 65 kg in space is 65 kg mass stays the same 65 kg Remember, mass is measured with a balance, where the mass of one object is compared to another object.

What is weight? W = m x g Weight is a measure of the gravitational force exerted on an object. Most of the time, when we’re talking about weight, we’re referring to the Earth’s gravitational force on an object. Since gravity is a force and weight is a measure of gravity, weight is expressed in newtons (N). On Earth, a 100 gram object would weigh 1 N.

Gravity and Weight If you look on the label of a bag of flour, it lists the “weight” in two units: 5 pounds in the English system and 2.3 kilograms in the metric system. As long as we are on Earth, where g = 9.8 N/kg a 2.3‑kilogram object will weigh 5 pounds. But on the moon, g = 1.6 N/kg, so a 2.3 kilogram object will weigh only 0.8 pounds.

Your weight on other worlds http:/www.exploratorium.edu/ronh/weight/

Isaac Newton Born 1642, the year Galileo died Loner, tinkerer, paranoid 1665-1666 Plague was very good for him Suffered mental breakdown 1675 Math, Chemistry, Theology, Parliament Died 1727 Has his picture on the British pound note He put the physics and mathematics to Kepler’s Laws!

Universal Law of Gravitation All objects in the Universe attract each other with a force that varies directly as the product of their masses and inversely as the square of their separation from each other. F = G m m r 1 2 gravity 2

Einstein’s View of Gravity Gravity is due to the curvature of spacetime. Spacetime is curved by mass.

Questions If the planets are orbiting the sun , what force is keeping them in orbit? What force keeps the moon in its orbit? Could the force of gravity be universal?

Change of Gravitational Force with Distance Law of universal gravitation is known as an inverse square law.

Problem 1 Two spheres of mass 35kg are 60m apart. What force does one exert on the other? If the mass of on is tripled and the radius is quadrupled how does the force change?

Problem 2 Two spheres of equal mass have a force of gravity of 7x10-9 exerted on each other. If the distance between them is 7m, find the mass.

Problem 3 Find the value of the gravitational acceleration g. The mass of the earth is 6.0 x 1024kg.