Gravity The Universal Force
Universal Gravitation What is gravity? Gravity makes things fall Gravity makes bubbles rise Gravity made the earth round, and makes the stars shine, but WHAT IS GRAVITY???
Universal Gravitation Isaac Newton is credited with first extending the thought of gravity throughout the universe. Newton knew that according to Galileo, objects travel in a straight line at constant speed unless acted upon by an outside force.
Universal Gravitation He also knew that the moon followed a circular path, which must mean there was an outside force acting upon it. He related this force to the same force that makes an apple fall. He realized that the apple accelerates as it falls, meaning it is under a constant force.
Universal Gravitation Newton knew that an apple would fall about 5 m in the first second of free fall. It was known at the time that the moon is about 60 times farther from the Earth s center than the apple from the tree. So Newton reasoned that the moon wouldn t fall 5 m in it s first second.
Universal Gravitation He reasoned that it would fall m, or about 1.4 mm. 2 1/60 of 5 if this were correct, then the moon must also travel 1.4 mm tangent to it s orbit in order to stay in orbit around the Earth. It turns out that Newton was almost perfect in his crude calculations.
Universal Gravitation Newton did not publish his findings right away because of ridicule he had already received from other publishings. Later on, he paired up with astronomer Edmund Halley (Halley s comet) and published his findings. Before doing this, he invented a whole new branch of math to prove where the center of gravity of spheres is..he called this math calculus.
Universal Gravitation These findings helped to prove Copernicus right no longer was the Earth considered the center of the universe. Although Newton did not discover gravity, he did discover that it is universal and that ALL things with a mass attract one another.
Universal Gravitation He came up with his law of universal gravitation, which mathematically expresses gravity as:
Universal Gravitation Which means that the force between two objects is directly proportional to the size of the objects, and inversely proportional to the square of the distance between the two. This proportionality is made exact by the universal gravitational constant (G), which -11 2 2 has a value of 6.67x10 N*m /kg
Universal Gravitation Problem: What is the Earth s Mass? Earth s radius is 6 6.4x10 m. Hint: Use the force of gravity on an object with mass of 1 kg to make it easy. Cavendish was the first to calculate it to kg 24 6x10 At what distance would the Earth s gravity drop to zero?
Universal Gravitation So while in orbit, all planets will act upon each other, and cause the other planets to wobble in their trip around the sun. We call this wobbling perturbations. We hav eused perturbations to discover planets we couldn t even see Pluto and Uranus!
Gravitational Interactions The fact that gravity extends in all directions around an object, and affects objects in its vacinity, makes it a type of force field. We call this a gravitational field. The strength of the gravitational field is a property of the mass and size of the object
Gravitational Interactions What if Earth had the same mass, but a larger radius? What if the radius were double? What would the gravitional force be? What if you fell through a hole that had been bored all the way through the Earth?
Gravitational Interactions It turns out that you would at first 2 accelerate at 9.8 m/s, but would then begin to lose acceleration, to the point that at the Eart s center there would be no acceleration.if you could stop at the earth s center, you would in fact feel weightless. Why?
Gravitational Interactions Ocean tides are caused by the interaction between the Moon s gravity and our oceans. Ocean tides are also influenced by the sun s gravity, but to a much lesser extent. Why does the sun s gravity affect our travel through space more, but the moon s gravity affects our tides more?
Gravitational Interactions The sun pulls 180 times harder on the Earth than the moon does However, it is so far away from us, that it pulls just about as hard all the way around the earth.
Gravitational Interactions However, the Sun s attraction to the Earth can t be completely ignored. In fact, then the moon, sun, and Earth are aligned (full and new moons, and eclipses), then those forces add to one another, giving us extra big high tides, and extra small low tides (Called Spring Tides)
Gravitational Interactions When the two are at right angles to one another, we get extra low high tides, and extra high low tides (called Neap Tides). The Earth also causes tides on the moon but the moon is solid, so they are land tides. The atmosphere also exhibits tides, and the Earth itself undergoes land tides.
Gravitational Interactions But what about black holes? Black holes deal with an idea called escape speed the speed an object would have to reach initially such that once launched, it would never return to to that body (planet, sun, etc). For the Earth, this escape speed is 11.2 km/s The escape speed of the sun is 620 km/s
Gravitational Interactions But what happens to a bodies escape speed if it gets more massive? What if it s radius becomes smaller? The speed of light is 300,000 km/s. What if the escape speed of a very massive object were =/> 300,000 km/s? Not even light could escape!.black HOLE
Gravitational Interactions What would happen to the Earth if the sun became a black hole? (It can t by the way). Turns out nothing except we would get really cold but we would not get sucked into it.
Gravitational Interactions But, what IS gravity? To understand this, we have to understand a little something about Einstein and his Theory of relativity. We have to understand that we live in a 4 dimensional world!