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GRAVITY Gravity is a force. It is a force of attraction between any two objects GRAVITY Gravity works close to the Earth. Objects and the Earth are attracted toward each other. Since the Earth cannot move toward the object because it is so big, the object moves toward the Earth. Gravity works out in space. Our moon is in the gravitational pull of the Earth. Because the moon is smaller than the Earth, it is pulled toward the Earth. The Earth is in the gravitational pull of the sun. Because the Earth is smaller than the sun, it is pulled toward the sun. The farther away two objects are, the less gravitational pull the objects will have. So, for instance, Pluto will have less gravitational pull than Earth, because it is farther away from the sun. GRAVITY Gravity makes things fall down because it is drawn toward the center of the Earth. When you drop an object it starts falling slowly, but then falls faster. It accelerates. This is called acceleration due to gravity (g). All objects on Earth have the same acceleration due to gravity. On Earth g= 9.8m/s2 Mass does not affect the acceleration of falling objects, IF gravity is the ONLY force acting on that object. Acceleration Due to Gravity The rate of falling increases by 9.8 m/s every second. On a side note: Remember that kinetic energy is energy of motion. We learned that an object has the greatest kinetic energy right before it hits the ground. Does it now make sense why? As an object falls it accelerates, so it has to be moving as fast as it can right before it hits the ground. Faster motion equals higher kinetic energy! I said earlier that mass does not affect the speed of falling objects, assuming there is only gravity acting on it. Is there ever a situation, here on Earth, in which gravity will be the only force acting on the object? Are there any other forces acting on a falling object? GRAVITY When something is falling gravity pulls it down. Air friction But another force, AIR friction, pushes it up. Remember that friction acts in the direction OPPOSITE the motion of an object Gravity GRAVITY When something starts to fall, gravity makes the object accelerate. At first they the object isn’t going very fast, so air friction is small. Air Friction Think about when you put your hand out the window on a car drive. There is more drag when you are flying down the highway then when you are driving 25mph down Main street! Gravity GRAVITY As the object falls faster, the force of friction gets bigger, so the acceleration of the objects begins decrease. The object doesn’t slow down, it is just not accelerating as much anymore. Air Friction Gravity GRAVITY After a while , the air friction force becomes the same size as the force of gravity. The object stops accelerating. It is still moving, but not accelerating anymore) It is moving at a constant velocity It is now moving at its terminal velocity Air Friction Gravity Terminal Velocity Terminal Velocity is the maximum speed an object can go. If a person jumps out of a plane, he will fall until he reaches his top speed. Once the force of gravity is equal to the force of air resistance, he cannot accelerate anymore. He could fall forever and not reach a higher speed than his terminal velocity. GRAVITY Air Friction Gravity Air Friction Gravity velocity time Air Friction Gravity What Affects Terminal Velocity? Mass can affect terminal velocity. The more mass an object has, the higher its terminal velocity will be. More massive objects have a greater gravitational pull toward the earth. A more massive object will also take longer to reach its terminal velocity. This is due to the object needing more time to reach a higher terminal velocity than a smaller object. Surface area also affects terminal velocity. If an object falls with a large surface area, it will experience a higher air resistance and will have a smaller terminal velocity. GRAVITY What happens when a parachute opens? The surface area gets bigger. The friction force gets bigger because of air resistance. The parachutist slows down because the force of air friction is larger than the force of gravity. GRAVITY Air Friction Gravity GRAVITY Air Friction Air Friction Gravity Air Friction Gravity velocity time Air Friction Gravity Gravity GRAVITY The falling object has slowed down. It now has a lower terminal velocity. So, Why does rain fall faster than snow? So what would it look like if there was no air resistance? On the moon, there is no air resistance to slow a falling object. Because of this, objects fall at the same speed. http://www.youtube.com/watch?v=L55jImE hNZ0 Without air resistance With air resistance Gravity is different on different planets!!! The more mass a planet has, the more gravity it has. Planets which have more mass than Earth would have more gravity than Earth. A person would weigh more on these planets than they do on Earth. How much would you weigh? Location Moon Mercury Venus Mars Jupiter Saturn Uranus Neptune Weight on Earth Gravity X X X X X X X X 0.17 0.38 0.86 0.38 2.87 1.32 0.93 1.23 Calculated Weight = = = = = = = = You just calculated your weight on different planets. Question…. Are mass and weight the same thing? NO!!!!!!! Mass is the amount of matter in an object. It is how much of something you have. Weight is the force of gravity acting on an object. Your weight is a measurement of how much force, the force of gravity, you are applying to a scale. If you hopped from planet to planet, your weight would change because of change in the gravity of each of the planets, but your mass would still stay the same. There would be the same amount of you here and on the moon….unless you cut off your arm on the space ride to the moon. Make sure to keep you arms and legs inside the space ship at all times. Safety first!