# Free Fall. Up and Down  Two balls are thrown at the same initial speed, one upward and one downward with negligible air resistance. When the balls strike.

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Free Fall

Up and Down  Two balls are thrown at the same initial speed, one upward and one downward with negligible air resistance. When the balls strike the ground the ball thrown upward has a speed equal to A) four times the speed of the ball thrown downward. A) four times the speed of the ball thrown downward. B) two times the speed of the ball thrown downward. B) two times the speed of the ball thrown downward. C) the same speed of the ball thrown downward. C) the same speed of the ball thrown downward. D) one half the speed of the ball thrown downward. D) one half the speed of the ball thrown downward. E) one quarter the speed of the ball thrown downward. E) one quarter the speed of the ball thrown downward.

Falling on the Earth  Objects fall to the earth.  Items that start by going up, stop and then fall again, reversing direction.  Items that fall from greater heights end up with more speed.  The velocity changes, there must be an acceleration.

Surface Gravity  At the surface of the Earth, gravity creates a nearly constant acceleration.  This gravitational acceleration is called g. g = 9.8 m/s 2 and is directed downward.g = 9.8 m/s 2 and is directed downward.  Note that g is an acceleration. Don’t be fooled by phrases like g-force.Don’t be fooled by phrases like g-force. g can be used as a unit of acceleration.g can be used as a unit of acceleration.

Up and Down  The constant acceleration equations apply for a = -g.  The position is now y, in the vertical direction. v t y t y0y0 v0v0 Initial upwards velocity Negative slope = - g Velocity here is –v 0

Initial Velocity  A person throws a ball up in the air releasing the ball at 1.5 m. If it lands on the ground after 2.0 s, find the initial velocity.  Simplify the time equation, with y 0 = 1.5 my 0 = 1.5 m y = 0 my = 0 m t = 2.0 s.t = 2.0 s. v 0 = 9.1 m/s

Exceptions to Free Fall  On Earth’s surface downward acceleration is constant.  Why do we observe that different objects accelerate differently? Friction reduces the total acceleration.Friction reduces the total acceleration. We think about velocity instead of acceleration.We think about velocity instead of acceleration. next

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