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Gravity

Gravity. Force at Impact. A falling ball on a paper surface can break through. The ball is exerting a force A falling ball on a hard surface rebounds. The surface is exerting a force There are forces both ways at contact. force of the ball on the surface. force of the surface on the ball.

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Gravity

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  1. Gravity

  2. Force at Impact • A falling ball on a paper surface can break through. • The ball is exerting a force • A falling ball on a hard surface rebounds. • The surface is exerting a force • There are forces both ways at contact. force of the ball on the surface force of the surface on the ball

  3. Force is Not Motion • The ball breaking paper doesn’t stop. • The force needed to break through is small. • But that small force is also exerted on the ball. • On a rebound the ground doesn’t move. • The force needed to break through is large • A force acted in both ways force of the ball on the surface force of the surface on the ball

  4. Third Law: Law of Reaction • For every action there is an equal and opposite reaction. • Forces between two objects act in pairs. • F12 = -F21 (Newton’s third law) reaction force acts on the rocket rocket hot gas is forced out rocket and gas are both affected

  5. Equal and Opposite • Newton’s law of reaction also applies to the force of gravity. • The Earth pulls the Moon • The Moon pulls the Earth • Newton used this to describe a Law of Gravity. FEM Moon FME Earth

  6. Newton realized that all objects obey that Law. Other planets Apples People The gravitational force is universal. The gravitational constant is G = 6.67 x 10-11 Nm2/kg2. What is the gravitational force between two students sitting in adjacent seats? Assume the students have a mass of 70 kg each. Assume that they are separated by 1m. F = (6.67 x 10-11 Nm2/kg2) x (70 kg)2 / (1 m)2 F = 3.3 x 10-7 N. Universal Gravity

  7. The force of gravity on a mass is its weight. The force of gravity compared to the mass is the field strength. Consider the force on an unknown mass m. Use Earth’s mass M and Earth’s radius R Calculate the field strength. g = F/m = GM/R2 M = 5.97 x 1024 kg R = 6.37 x 106 m g = 9.81 N/kg = 9.81 m/s2 This field strength g only applies at Earth’s surface. Approximately, g is equal to 9.8 N/kg or 9.8 m/s2. Surface Gravity

  8. Variations • Gravity varies over the surface of the Earth. • The height of the surface varies – so the radius does, too • The material under the surface is not uniform • The earth isn’t exactly round • The tides affect the earth as well as the oceans • One unit of gravitational acceleration used on the Earth is the Galileo. • 1 Gal = 1 cm/s2 = 0.01 m/s2 = 0.01 N/kg. • g = 981 Gal

  9. Effect of Height • High areas have a greater distance from the center of the Earth. • An increase of 1 km should decrease g by 300 mGal. • Type of rock affects g. USGS

  10. Latitude Changes • As the Earth spins the equator slightly bulges. • The radius is about 22 km bigger compared to the pole. • Expect a few Gal difference from equator to pole. • Equator: 9.780 m/s2 • North Pole: 9.832 m/s2 Colorado School of Mines

  11. Earth’s Rotation • The earth is made of layers of different types of rock. • These rocks can move due to daily tides. • Much less than ocean tides • Period is 12 hours like the ocean • The force of gravity from the Moon is countering some of the force of the Earth. Colorado School of Mines next

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