Physical Science 8th grade (1).pdf

Because this acceleration is used so frequently in

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Because this acceleration is used so frequently in physics, the letter g is used to represent its value. When you see the letter g in a physics question, you can substitute the value 9.8 m/s 2 . Gravitational force, mass, and distance Gravitational force exists between all objects that have mass. Newton’s law of universal gravitation says the strength of the force depends on the mass of the objects and the distance between them. The greater the masses and the smaller the distance, the greater the force. You do not notice gravity between ordinary objects because it takes a huge amount of mass to create enough force to notice. You feel the force of gravity between you and Earth because the planet’s mass is huge. Weight Your weight is the force of gravity between you and Earth. It depends on your mass, Earth’s mass, and your distance from Earth’s center. You calculated weight with the formula W=mg . The g in the weight formula is the same g that describes the acceleration due to gravity. The value of g depends on Earth’s mass and the distance between its center and surface. If you travel to a planet or moon with a different mass and/or radius, the value of g and your weight would change.
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302 U NIT 5 M OTION AND F ORCE Orbital motion Gravitational force and acceleration Newton’s second law can be used to explain the motion of planets, moons, and satellites in orbit. For example, the moon moves in a 384,000 km orbit around Earth at nearly constant speed (Figure 14.16). While the moon’s speed is constant, its direction is not. Therefore the moon is accelerating. The gravitational force between the Moon and Earth causes the acceleration. Why the moon doesn’t fall to Earth The moon orbits or moves around Earth just as Earth and the other planets orbit the sun. But why doesn’t the force of gravity just pull the moon into Earth? To answer that question, imagine kicking a ball off the ground at an angle. If you kick it at a slow speed, it curves and falls back to the ground. The faster you kick the ball, the farther it goes before hitting the ground. If you kick it fast enough, the curve of the ball’s path matches the curvature of Earth. The ball goes into orbit instead of falling back to Earth. The moon falls around Earth The same idea applies to the motion of the moon. The orbiting moon falls around Earth. But as it falls, Earth curves away beneath it. What do you think would happen if the gravitational force between the moon and Earth were to disappear? The moon’s inertia would cause it to move in a straight line at a constant speed. The moon would fly off into space! Satellites Many human-made satellites orbit Earth providing weather data, communications services and scientific research. These satellites orbit much closer than the moon and must be launched at speeds high enough to reach orbit and not fall back to Earth.
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