Falling Objects Ch 2 Sec 3

What is free fall? Free falling bodies undergo constant acceleration due to the influence of gravity only. All objects in free fall in the earth’s gravitational field have the same constant acceleration (g=-9.81 m/s2). *Why does this seem untrue?* Watch a video and then verify this for yourself!! http://www.youtube.com/watch?v=aCTHVhCQSQs http://www.youtube.com/watch?v=_Kv-U5tjNCY

Air Friction Friction- A force that resists the relative motion or tendency to such motion of two bodies or substances in contact To simplify life in this physics course, we will ignore friction most of the time by carefully choosing our materials. But in real life, friction cannot be ignored, especially for objects that have a large surface area and a low weight. Ex: a parachute, flat sheet of paper experience a lot of air friction Terminal velocity- maximum velocity a falling object will travel as limited by air resistance -shape, size and weight affect terminal velocity -aerodynamic objects have a higher terminal velocity

Did you know…? Free fall acceleration is denoted with the symbol “g” (gravity). We will study this later, but the value of g is relevant to the mass that is exerting the force. “gEarth” is approximately 9.81 m/s2 “gMars” is approx. is 3.73 m/s2 “gVenus” is approx. is 8.7 m/s2 “gJupiter” is approx. 24.79 m/s2 (wow!)

If something is falling, the downward direction is negative. When an object is thrown into the air, it has a positive velocity because the direction is positive, but a negative acceleration (this means that the object is slowing down). a = -g = -9.81 m/s2

Let UNITS be your guide!!! distance or displacement = meters (m) *note: Δx occurs in the horizontal direction Δy occurs in the vertical direction v, vi, vf = meters per second (m/s) a or g = meters per second per second (m/s2) Δt = seconds (s)

Falling Object Equations Falling Object Equations **These are the same equations for uniform acceleration. vf = vi + gt (Notice that “g” has replaced “a”.) y (height) = yi + vit + ½ gt2 Vf2 = vi2 + 2gΔy Note: yi and vi can also be shown as y0 or v0 If an object is falling, the displacement is negative. As it goes from high to low, the distance covered is in the negative direction… and that’s OKAY!!

Solving Problems In most free fall problems, you are solving for height or speed. Remember: Total distance traveled by the object is double the height of the path. The object takes the same amount of time going up as it takes coming down, so the total time is both of those added together. Velocity at the very top of the path is 0 m/s. H E I G T Path of an object thrown up and falling back down.

Try this one… A stone is dropped down a well and it takes 1.6 seconds to reach the bottom. How deep is the well? (You may assume the initial speed of the stone is zero.)

Answer: Δy = vit + ½ gt2 Δy = (0m/s)(1.6s) + (.5)(-9.81m/s/s)(1.6^2) Δy = -12.55 meters (the negative sign refers to the direction the distance occurs)