# Gravity and Free Fall.

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Gravity and Free Fall

Gravity Sir Isaac Newton ( ) developed a law of gravitation which said all objects in the universe attract each other through gravitational force Universal Gravitation Equation F = G m1m2 d2 Equation shows grav. Force ↑ as both masses ↑ Equation shows grav. Force ↓ as the distance b/w masses ↑

Free Fall Free fall- when gravity is the only force acting on an object (free fall acceleration is directed toward center of earth) The acceleration of gravity (g) for objects in free fall at the earth's surface is 9.8 m/s2. Galileo found that all things fall at the same rate in the absence of air resistance, regardless of their mass

Free Fall The rate of falling increases by 9.8 m/s every second.
Height = ½ gt2 For example: ½ (9.8 )12 = 4.9 m ½(9.8)22 = 19.6 m ½ (9.8)32 = 44.1 m ½ (9.8)42 = 78.4 m

Air Resistance In air… In a vacuum A stone falls faster than a feather
Air resistance affects stone less In a vacuum A stone and a feather will fall at the same speed.

Mass and Weight  different!
► Mass is an intrinsic property of an object. It is completely determined by the number and type of atoms that make up the object. It does not depend on the environment in which the object is located. ► But weight is different. Weight depends on both the object itself, and on some other object that exerts the gravitational force. In everyday life, the words “mass” and “weight” mean somewhat the same thing. But in physics they are different. For one thing, they have different units (kg and newtons, respectively). So what is the difference between mass and weight? {READ} The everyday meaning of words may be vague and fuzzy, but in science words must be defined strictly and precisely. So, for example, the mass of an object would be the same on the moon or the Earth; but the weight would be different.

What does weight depend on?
The weight of an object depends on the object’s mass. In fact, an object’s weight is directly proportional to the object’s mass. The weight of an object also depends on the object’s location. In fact, an object’s weight is directly proportional to its free fall acceleration, g at its current location.

W = m g Weight = the force of gravity
The weight of an object is, by definition, the strength of the force of gravity pulling the object downward. force of gravity W = m g newtons kg Another word for the force of gravity is the weight of the object on which gravity is acting. {READ THE SLIDE}

Weight of a 1 kg object Since W = mg, the weight of a 1 kg object is:
W = (9.8 m/s2)(1 kg) = 9.8 N on Earth W = (1.6 m/s2)(1 kg) = 1.6 N on the Moon

Mass vs. Weight Mass is not Weight
Weight is a force - an interaction between 2 objects involving a push or a pull. One of these objects is typically VERY big - the Earth or the Moon, for instance. Weight is NOT a property of an object

Weight In symbols: W = mg W g m

Gravity What is the force acting on the mass m due to the Earth’s gravity? F Solution If released, the acceleration of m would be … g = 9.81 m/s2 By Newton’s second law the force on m must be … The first example is the force of gravity. A force is a push or a pull. The Earth’s gravity pulls any mass toward the center of the Earth. {READ all} DEMO [5/6] Drop a ball. Note that F=ma {board} Or, equivalently, W=mg {board} because the force F is W (the weight) and the acceleration a is g. That is, the magnitude, or strength, is mg and the direction is downward. By the way, what is the reaction force?

2-5. Free Fall A ball thrown horizontally will fall at the same rate as a ball dropped directly.

2-5. Free Fall A ball thrown into the air will slow down, stop, and then begin to fall with the acceleration due to gravity. When it passes the thrower, it will be traveling at the same rate at which it was thrown.

Example. Playing catch with a softball
The trajectory has 3 parts. free fall the throw the catch What is the force acting on the ball, during each part of the trajectory?

Projectile Motion An object thrown upward at an angle to the ground follows a curved path called a parabola. combines vertical and horizontal motion Orbiting objects- forward motion combines with free fall and object follows a curved path

2-6. Air Resistance Free Fall
A person in free fall reaches a terminal velocity of around 54 m/s which means the person stops accelerating b/c: Force of air resistance = gravitational force Called terminal velocity (balanced forces) With a parachute, terminal velocity is only 6.3 m/s Allows a safe landing

Free Fall Review Dropping something from a resting position (gains speed as it falls so it accelerates) Gravity pulls objects down (air resistance can affect how fast) (no air resistance- in a vacuum) When gravity is the only thing that affects falling object → FREE FALL Acceleration due to gravity is 9.8 m/s2

Free Fall equations V = g x t D = 1 gt2 2