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Physical Science Gravity

Objectives Explain that gravitational force becomes stronger as the masses increase and rapidly become weaker as the distance between the masses increases, F=G(m 1 m 2 /d 2 ) Evaluate the concept that free-fall acceleration near Earth’s surface is independent of the mass of the falling object Demonstrate mathematically how free-fall acceleration relates to weight Describe orbital motion as a combination of two motions

The law of universal gravitation: –All objects in the universe attract each other through gravitational force (Sir Isaac Newton 1642-1727) –Universal Gravitation Equation F=G(m 1 m 2 /d 2 )

Gravitational force increases as one or both masses increases Gravitational distance decreases as the distance between the masses increases G is a constant –If the distance between two objects is doubled, the gravitational force between them decreases to ¼ the original value –If distance is tripled, gravitational force decreases by 1/9 the original value

Free Fall Free fall- the motion of body when only the force of gravity is acting on the body Free fall results from gravity= g~ 9.8m/s 2 Free fall acceleration of an object is directed toward the center of Earth

In a vacuum (an absence of air resistance) a feather and an apple accelerate at the same rate regardless of mass. Why? Newton’s second law of motion (acceleration depends on both force and mass)

Weight- the force on an object due to gravity Weight is equal to mass times free-fall acceleration Weight=mass x free-fall acceleration w= mg

Weight is a force, the SI unit of weight is the newton, N. w=mg Ex) 1.0kg x 9.8m/s 2 = 9.8 N

*Recall the difference between mass and weight Mass is the measure of the amount of matter in an object (this does not change) Weight changes as gravitational force changes

Space Planets in our solar system have different diameters and different masses. Therefore each planet has a unique value for g. Earth, g= 9.8m/s 2 Venus, g= 8.8m/s 2 Mars, g= 3.7m/s 2 Neptune, g= 11.8m/s 2

Velocity and air resistance A falling object stops accelerating when the force of air resistance = the gravitational force on an object (the weight) Terminal velocity- the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity

Free Fall An object is in free fall only if gravity is pulling it down and no other forces are acting on it Free fall can only occur in a vacuum- no air and no matter, or in space Orbiting objects are in free fall

Orbiting Two motions combine to cause orbiting Orbiting- when an object is traveling in a circular or nearly circular path around another object Linear motion forward + downward pull of gravity = curve of Earths surface

Projectile motion Two motions combine to form projectile motion Projectile motion- the curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth; the motion of objects that are moving in 2-D under the influence of gravity

Projectile motion Horizontal velocity + vertical downward pull of gravity= curved path The two velocities are independent of each other

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