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NIU PHYS 210 - Free Fall

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Free FallUp and DownFalling on the EarthSurface GravitySlide 5Initial VelocityExceptions to Free FallFree FallFree FallUp and DownUp and DownTwo balls are thrown at the same initial speed, one Two balls are thrown at the same initial speed, one upward and one downward with negligible air upward and one downward with negligible air resistance. When the balls strike the ground the ball resistance. When the balls strike the ground the ball thrown upward has a speed equal to 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 E) one quarter the speed of the ball thrown downward.downward.Falling on the EarthFalling on the EarthObjects fall to the earth.Objects fall to the earth.Items that start by going up, stop and then fall again, Items that start by going up, stop and then fall again, reversing direction.reversing direction.Items that fall from greater heights end up with more Items that fall from greater heights end up with more speed.speed.The velocity changes, there must be an acceleration.The velocity changes, there must be an acceleration.Surface GravitySurface GravityAt the surface of the Earth, gravity creates a nearly At the surface of the Earth, gravity creates a nearly constant acceleration.constant acceleration.This gravitational acceleration is called This gravitational acceleration is called gg..•gg = 9.8 m/s = 9.8 m/s22 and is directed downward. and is directed downward.Note that Note that gg is an acceleration. is an acceleration.•Don’t be fooled by phrases like g-force.Don’t be fooled by phrases like g-force.•gg can be used as a unit of acceleration. can be used as a unit of acceleration.Up and DownUp and DownThe constant acceleration The constant acceleration equations apply for equations apply for aa = - = -gg..The position is now The position is now yy, in the , in the vertical direction.vertical direction.00221ytvgty    000202ygvvvgvvy vtyty0v0Initial upwards velocityNegative slope = -gVelocity here is –v0Initial VelocityInitial VelocityA person throws a ball up in A person throws a ball up in the air releasing the ball at the air releasing the ball at 1.5 m. If it lands on the 1.5 m. If it lands on the ground after 2.0 s, find the ground after 2.0 s, find the initial velocity.initial velocity.Simplify the time equation, Simplify the time equation, with with •yy00 = 1.5 m = 1.5 m•yy = 0 m = 0 m•tt = 2.0 s. = 2.0 s.tvygt0022100221ytvgty tygtv0021vv00 = 9.1 m/s = 9.1 m/sExceptions to Free FallExceptions to Free FallOn Earth’s surface downward acceleration is On Earth’s surface downward acceleration is constant.constant.Why do we observe that different objects accelerate Why do we observe that different objects accelerate differently?differently?•Friction reduces the total acceleration.Friction reduces the total acceleration.•We think about velocity instead of acceleration.We think about velocity instead of


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NIU PHYS 210 - Free Fall

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