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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?