GRAVITY

INTRODUCTION Friction explains why a book comes to a stop when it is pushed. But why does a book fall to the ground if you lift it and let it go?

INTRODUCTION Newton realized that a force acts to pull objects straight down toward the center of the Earth. He called this force gravity. Gravity is the force that pulls objects toward Earth.

INTRODUCTION So how does gravity affect a skydiver when they jump out of an airplane?

INTRODUCTION When the only force acting on an object is gravity, the object is said to be in free fall. An object in free fall accelerates as it falls. In free fall the force of gravity is an unbalanced force and unbalanced forces cause an object to accelerate.

INTRODUCTION Exactly how fast do objects fall? On Earth the acceleration due to the force of gravity is 9.8 m/s 2, this means that for every second the object falls its velocity increases by 9.8 m/s.

INTRODUCTION At the start, the skydiver’s velocity is 0 m/s. At the end of the first second, his velocity is 9.8 m/s. After two seconds his velocity is 19.6 m/s ( ). After three seconds his velocity is 29.4 m/s ( ). The velocity tends to increase as long as the object falls.

INTRODUCTION So why do some objects seem to fall faster than other objects, like a leaf compared to an acorn? According to Galileo all objects fall at the same speed, right?

INTRODUCTION While it is true that all objects dropped at the same time will fall at the same rate, on Earth they are falling through a fluid, air, that produces a force working against the fall called air resistance.

INTRODUCTION Air resistance is not the same for all objects. The greater the surface area of an object, the greater the air resistance. That is why a leaf falls more slowly than an acorn. In a vacuum, all objects fall at the same rate of acceleration.

INTRODUCTION Elephant and feather falling through the air on Earth. Elephant and feather falling through a vacuum.

INTRODUCTION Air resistance also increases with velocity. So as a falling object speeds up, the air resistance working against it increases also. Eventually the air resistance equals the force of gravity. Remember that when forces are balanced there is no acceleration.

INTRODUCTION So even though the object continues to fall, its velocity no longer increases. This velocity, the greatest velocity the object reaches, is called terminal velocity.

INTRODUCTION At terminal velocity, an object will be weightless because the forces acting on it are balanced.

INTRODUCTION The force of gravity on a person or object at the surface of a planet is known as weight. When you step on a bathroom scale you are determining the force with which Earth is pulling you. Do not confuse weight with mass.

INTRODUCTION Weight is a measure of the force of gravity on an object, and mass is a measure of the amount of matter in that object. Since weight is a force, you can use Newton’s second law to find weight. weight = mass x acceleration due to gravity.

INTRODUCTION Weight is in newtons, mass is in kilograms, and acceleration due to gravity is in m/s 2. So a 50 kilogram person would weigh 490 newtons. 50 kg x 9.8 m/s 2 = 490 newtons

INTRODUCTION Newton realized that the Earth was not the only object that exerts gravitational force. Gravity acts everywhere in the universe. What Newton discovered is now called the law of universal gravitation. Any two objects in the universe, without exception, are attracted to each other.

INTRODUCTION Gravitational force depends on the distance between two objects. The farther apart the objects are, the weaker the force, the closer together they are, the stronger the force. We say that the force of attraction between two objects varies with mass and distance.

EXAMPLE

OBJECTIVES Explain how mass differs from weight. State the universal law of gravitation. Describe the effects of gravity and air resistance on an object in free fall.

VOCABULARY Gravity – force of attraction between two bodies. Free fall - falling motion (acceleration) of a body due to gravity.

VOCABULARY Air resistance - force that opposes motion of an object through a fluid. Terminal velocity – rate at which a falling object cannot fall faster.

IN QUESTION How do the forces of gravity and air resistance act on a parachute so that the person using the parachute falls safely to the Earth? Gravity pulls the person and chute down while air resistance pushes up. The parachute when open gives a large enough surface area that it slows the fall of the person using the chute.

NOTES Gravity pulls to the center of an object. Objects fall because of unbalanced forces. Objects fall at 9.8 m/s 2 on Earth. Air resistance opposes gravity.

NOTES Greater surface area means more air resistance. When air resistance and gravity equal each other a falling object is at terminal velocity. At terminal velocity an object will be weightless.

NOTES Weight measures how hard gravity pulls on an object. Any object with mass has gravity. Universal law of gravitation states - Any two objects are always attracted to each other.

NOTES Force of gravity is affected by size and distance. Bigger objects = more gravity Closer distances = more gravitational attraction.

OUT QUESTION Why would a crumpled piece of paper hit the ground before a flat piece of paper if they have the same weight? There is less surface area on the crumpled paper so there is less air resistance and it falls faster.