Physical Science Gravity Objectives Explain that gravitational force

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. 8 m/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. 0 kg x 9. 8 m/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. 8 m/s 2 Venus, g= 8. 8 m/s 2 Mars, g= 3. 7 m/s 2 Neptune, g= 11. 8 m/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