Transcript Falling Objects

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
**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
H
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)