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Gravity and Free fall

An object is in free fall when it accelerates due to the force of gravity and no other forces are acting on it. Gravity and Free fall. For Example…. If you drop a rock off a cliff, the rock is in free fall , because only gravity is force .

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Gravity and Free fall

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  1. An object is in free fall when it accelerates due to the force of gravity and no other forces are acting on it. Gravity and Free fall

  2. For Example… • If you drop a rock off a cliff, the rock is in free fall, because only gravity is force. • However, if you throw a ball up toward the sky, it is also in free fall. Only gravity is force, when it is falling. • Why do birds, helicopters, and planes not fall from the sky?

  3. EARTH: gravity always makes things accelerate the same amount. We call this variable “g” for gravity. g = 9.8 m/s2

  4. You can find the velocityof an object in free fall at any time during the fall with the equation: Vf = gt + vi The Final Velocity is equal to the gravity multiplied (times) time in seconds in addition to (plus) the initial velocity.

  5. Example • A rock falls off a cliff and splashes into a river 5 seconds later. How fast is it going after 2.5 seconds? Vf = g = t = Vi =

  6. …after 2.5 seconds? 24.5 m/s

  7. Example • A rock falls off a cliff and splashes into a river 5 seconds later. How fast is it going when when it hits the water? Vf = g = t = Vi =

  8. …when it hit the water? 49 m/s

  9. Remember this clue: • When a question says an object “falls off a cliff,” or “falls off a desk,” or “you drop an object,” that is “code” for free fall, where gravity is the force

  10. Upward Launch 9.8 m/s -9.8 m/s -19.6 m/s 19.6 m/s

  11. If you throw a ball upward, the ball will: • slow down as it moves up, • stop for an instant, and then • speed up as it falls back down. 9.8 m/s -9.8 m/s -19.6 m/s 19.6 m/s

  12. Upward launches • As it moves upward, it slows down by 9.8 m/s every second until it stops. 9.8 m/s -9.8 m/s -19.6 m/s 19.6 m/s

  13. Upward launches • The ball then reverses direction and starts falling downward. 9.8 m/s -9.8 m/s -19.6 m/s 19.6 m/s

  14. Upward launches • As it falls downward, the ball gets faster by 9.8 m/severy second. 9.8 m/s -9.8 m/s -19.6 m/s 19.6 m/s

  15. In free fall, there is constant acceleration. • This means the object speeds up or slows down by the same amount every second.

  16. Changes in velocity • In free fall, is there also constant velocity? • No – because the velocity is constantly changing.

  17. Changes in velocity • In free fall, we can find average velocity. • initial velocity (vi ) • final velocity (vf)

  18. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity?

  19. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity? Vavg Vf Vi

  20. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity? Vavg --- trying to find Vf = Vf=gt+Vi Vi

  21. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity? Vavg --- trying to find Vf = Vf=gt+Vi Vf=(9.8m/s2) (5s)+Vi Vi = 0 m/s

  22. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity? Vavg --- trying to find Vf = Vf=gt+Vi Vf=(9.8m/s2) (5s)+Vi Vf=49 m/s + 0

  23. A rock fell off a cliff and splashed into a river 5 seconds later. What was the rock’s average velocity? Vavg = (0m/s + 49m/s)/2 = 24.5 m/s

  24. Calculating distance • You can also figure out how much distance an object fell during free fall using the equation:

  25. A skydiver falls for 6s before opening her parachute. Calculate the distance she has fallen in 6 s. Vf =gt+Vi Vf =(9.8m/s2) (6s)+Vi

  26. A skydiver falls for 6s before opening her parachute. Calculate the distance she has fallen in 6 s. Vf =gt+Vi Vf =58.8 m/s + 0 m/s

  27. A skydiver falls for 6s before opening her parachute. Calculate the distance she has fallen in 6 s. Vf =58.8 m/s Vavg =(0+58.8) m/s 2 Vavg = 29.4m/s

  28. A skydiver falls for 6s before opening her parachute. Calculate the distance she has fallen in 6 s. Vavg = 29.4m/s D = 29.4 m/s x (6s) D= 176.4 m

  29. Weight vs. Mass

  30. Weight vs. Mass • Say you become an astronaut. Will you WEIGH more, less, or the same on the moon? • Less • Why? • Because the moon has less gravity than Earth.

  31. Gravity and Weight • The force of gravity on an object is called weight(Fg). • A force is measured in units - Newtons • The equation for weight is: • Fg = mg

  32. Weight depends on mass and gravity MASS stays the same on the moon, but WEIGHT changes on the moon.

  33. Weight and mass Legend says that about 1587, Galileo dropped two balls from the Leaning Tower of Pisa to see which would fall faster. Suppose the balls had masses of 1.0 kg and 10 kg. Use the equation for weight to calculate the force of gravity on each ball. b. Use your answers from part a and Newton’s second law to calculate each ball’s acceleration.

  34. Weight and mass • Looking for:… the force due to gravity (Fw) and the acceleration for each ball • Given:… one ball’s mass = 1.0 kg. • Relationships: Use: Fw = mg and a = F ÷ m • Solution:For the 1.0 kg ball: • a) Fw= (1.0 kg)(9.8 m/s2) = 9.8 N • b) a = (9.8 N) ÷ (1.0 kg) = 9.8 m/s2

  35. Weight and mass • Looking for:… the force due to gravity (Fw) and the acceleration for each ball • Given:… other ball’s mass = 10 kg. • Relationships: Use: Fw = mg and a = F ÷ m • Solution:For the 1.0 kg ball: • a) Fw= (10 kg)(9.8 m/s2) = 98 N • b) a = (98 N) ÷ (10 kg) = 9.8 m/s2

  36. Weight and mass • Both balls have the same acceleration, so they would fall at the same time. • Gravity is the same force on both, but the weight is different. • What could make them fall different? Think about a feather.

  37. Air resistance • When something falls through air, the air exerts a force called.... • air resistance, • It acts opposite to the direction of the object’s motion.

  38. What direction is the skydiver’s motion? What direction is air resistance? Draw the Forces.

  39. The object falls faster. • Eventually, air resistance is equal to force of gravity, and it can’t go any faster. • This fastest speed is called terminal velocity.

  40. NASA has been conducting parabolic flights since the 1950s to train astronauts. Scientists and college students have also gone on parabolic flights to perform a wide variety of chemistry, biology, and physics experiments. ZERO-G flights contain three types of parabolas: Martian gravity (1/3 Earth gravity), Lunar gravity (1/6 Earth gravity), and zero gravity. Parabolic Flights http://www.vegas.com/attractions/off-the-strip/zero-g/

  41. Review Questions • A falling object under the influence of only gravity is in _____________________________ • Acceleration due to gravity (g) on Earth = __________________ • What is the weight of a 5 kg backpack on Earth? ____________ • What is the weight of the same backpack on the moon? (g = 1.6 m/s2) ______________

  42. 5. A rock falls off the edge of a cliff. It lands 7 seconds later. a. What is the rock’s velocity after 1 s? b. What is the rock’s speed the instant before it lands? c. What is the rock’s average velocity for the whole drop? d. What distance does the rock fall in 7 s?

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