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What’s the hurry?

What’s the hurry?. The Kinematics of Freefall. What happens as objects fall ??. Physicists DO NOT KNOW WHY objects fall! But, we can describe HOW they fall As they fall, THEY GO FASTER This means that they ACCELERATE ! They ACCELERATE at a CONSTANT RATE. “g” - The “Magic” Number.

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What’s the hurry?

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  1. What’s the hurry? The Kinematics of Freefall

  2. What happens as objects fall?? • Physicists DO NOT KNOW WHY objects fall! • But, we can describe HOW they fall • As they fall, THEY GO FASTER • This means that they ACCELERATE! • They ACCELERATE at a CONSTANT RATE

  3. “g” - The “Magic” Number • “Little g” is a ‘shorthand’for ACCELERATION DUE TO GRAVITY • All LARGE OBJECTS have a “little g” value! • Examples • “g” is -1.67 m/s2 on the Moon • “g” is -26 m/s2 on Jupiter • “g” is -9.81 m/s2 on Earth

  4. Uses for “g” • An object falls from rest. Calculate its velocity at the end of one second. Two seconds. Three seconds. t = 0 vi = 0 m/s t = 1 s v = -9.81 m/s v = -19.62 m/s t = 2 s t = 3 s v = -29.43 m/s

  5. Uses for “g” • An object falls from rest. Calculate how far it has fallen at the end of one second. Two seconds. Three seconds. t = 0 d = 0 m t = 1 s d = -4.905 m d = -19.62 m t = 2 s t = 3 s d = -44.145 m

  6. Throwing Downward a = -9.81 m/s2 vi = -10 m/s • An object is thrown downward from the top of a 175 meter building with an initial speed of 10 m/s. • What acceleration does it experience? • -9.81 m/s2 • What is the object’s initial velocity? • -10 m/s “a” and “vi” in SAME DIRECTION

  7. Throwing Downward (cont.) • What is the object’s velocity as it hits the ground? • vi = -10 m/s • d = -175 m • a = -9.81 m/s2 • vf = ? • How long does it take the object to hit the ground? • t = ?

  8. Initial Upward Velocity t = 2 s v = +30.38 m/s • A cannon fires a shot directly upward with an initial velocity of 50 m/s. • What acceleration does the cannonball experience? • -9.81 m/s2 • What is the cannonball’s initial velocity? • +50 m/s t = 1 s v = +40.19 m/s t = 0 s vi = +50 m/s How does the velocity of the cannonball change as time goes on?

  9. Initial Upward Velocity (cont.) 0 m/s (All objects momentarily STOP at the top of their flight) • What is the object’s velocity as it reaches the top of its flight? • How long does it take the cannonball to reach the top of its flight? • Calculate the maximum height of the cannonball.

  10. SUMMARY • What is the acceleration due to gravity on Earth? • What is the vi when object is dropped from rest? • How does vi change when the object is thrown downward? • How does vi change when the object is thrown upward? • What is the velocity of the object when it is at maximum height? • What is the acceleration of the object at maximum height?

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