Force Unit Part 2: Gravity

Objectives  Explain the law of universal gravitation and how applies to objects on Earth both still and falling and in space.  Explain how free-fall acceleration and terminal velocity are connected.  Describe the difference between mass and weight and their connection to gravity.  Explain how projectile motion works and how we use it to our advantage in sports and in space

Questions  Have you ever seen a videotape of the first astronauts on the moon?  Why did the astronauts-- who were wearing heavy spacesuits—bounce so easily on the moon?

Law of Universal Gravitation  Established by Sir Isaac Newton  It states that all objects in the universe are attracted to each other through gravitational force  Mathematically it is defined as: F = G m 1 m 2 d 2 d 2 G = _______________________ d = _________________________________

Fundamentals of Gravity  All matter is affected by gravity  Gravitational force increases as mass _________  Gravitational force decreases as distance ____________

Free Fall Acceleration  Happens when ________ is the only force acting on an object  The free-fall acceleration of an object is directed toward the center of Earth  It is abbreviated as g and equals ___________  Near the Earth’s surface it is constant when neglecting air resistance

Weight  It is the amount of gravitational force acting on the ________ of an object  It is equal to mass times free-fall acceleration  SI unit is the _____________  Weight also influences shape

The Velocity Connection  Velocity is a constant when air resistance balances weight  During free-fall, an object stops accelerating and reaches its maximum velocity, which we call _______________

Free Fall and Motion  An object is only in free fall if gravity is the only force acting on the object  Air resistance is a force on falling objects  Free fall can only happen where there is no ___, such as in a vacuum or space  Orbiting objects, such as moons and planets, are in free fall

Questions  Do sky-drivers experience free fall acceleration? Why or why not?  Are you weightless in space? Explain.  Why do astronauts “float” in space?

Projectile Motion  Projectile motion is a curved path that an object follows when thrown, launched or otherwise projected near the surface of the Earth  The orbit of a space shuttle around Earth is an example of projectile motion

Projectile Motion Continued  Has two components: _____________________________  These two components are independent of each other and thus have no effect on one another  The two components together form the curved path of motion for the object

Questions  Because objects in projectile motion accelerate downward, where should I aim when throwing a football at my teammate?  If I want to hit a bulls-eye, where should I aim the arrow? Why?

True or False Quiz 1. Disregarding air resistance, free-fall acceleration near Earth’s surface depends on mass. 2. The mass of an astronaut on the moon is the same as it is on Earth. 3. Terminal velocity is reached when the forces on a skydiver become balanced.