Free Fall

1
Drop it

• Free Fall
• Drop an object force of gravity acts
  continuously
• Initial velocity zero
• AS TIME GOES ON, VELOCITY INCREASES THEREFORE
• ACCELERATION.
• AT SURFACE OF EARTH ACCELERATION DUE TO GRAVITY
  IS 9.81 m/s2
• Well approximate to 10m/s2 most of the time

2
Drop it

• Free Fall
• Drop an object force of gravity acts
  continuously

Galileo Galilei born 1564 Argued in his time
that In a vacuum, where there is no air
resistance, all bodies fall at the same rate
regardless of mass/weight
3
Drop it

• Galileo rolled balls down inclined planes with
  gradually increased angles.
• Why not just drop from a building??
• Clocks in the 1500s???
• He slowed things down with a shallow inclined,
  then extrapolated to 90o
• The acceleration was always proportional

4
Drop it

• We will ignore air friction

• Lets look at 10 m/s per second squared

5
Drop it

• Drop object -- at what speed will the object be
  every second of travel?
• elapsed time speed
• 0 0
• 1 10
• 2 20
• 3 30
• etc

6
Drop it

• Rate of change of speed
• 10 m/s every second
• Not only falling, but on the way up, also.
• Throw an object up, it will decelerate at the
  rate of 10 m/s every second.

7
Drop it

• Speed at any time
• vat
• get this from a ?v / ?t
• Since acceleration due to gravity is constant
  free fall speed
• vgt, where g is acceleration due to gravity

8
Drop it

• Every second of travel, how far does the object
  fall?
• Between 0 and 1 second what is average speed?
  Starts 0, ends in 10 m/s. So
• 0 plus 10 divided by 2 5m/s
• How far? V x/t x vt (5 m/s)(1 s) 5 m
  
• Between 1 and 2 seconds of fall, what is average
  speed?
• 10m/s plus 20m/s divided by 2 15 m/s
• X vt (15 m/s)(1 sec) 15 m plus 5 m at 1
  sec
• 20 meters

9
Drop it

• Elapsed Time Distances Fallen
• o o
• 1 5
• 2 20
• 3 45
• 4 80
• etc.

10
Drop it

• Kinematic

Which comes from
11
Drop it
12
Work it out . . . . .
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14
Terminal Velocity

• Terminal Speed increases as the objects weight
  increases, and
• Decreases as its cross-sectional area increases.
• Galileo wrote
• A cannon ball weighing one or two hundred pounds,
  or even more, will not reach the ground by as
  much as a span ahead of a musket ball weighing
  only half a pound, provided both are dropped from
  a height of 200 cubits.

15
Terminal Velocity

• Terminal speed of a child is less than that of an
  adult.
• Small animals and insects surviving falls that
  are immense in proportion to their size
• How can we survive a hail storm?
• . . . . . Air friction saves us.
• Hailstones hit earth at 25 30 km/hr
• If air friction ignored, what would speed be if
  hail left a cumulonimbus cloud at 9.144 x 103 m?
  --

424 m/s .. Or 947 mph!
16
Record Free Fall
Lieutenant I. Chisov of former Soviet Union

• Ilyushin 4 January 1942

• Attacked by 12 German Messerschmitts

• Plane damaged beyond survival, so he bailed out
  at 21,980 ft
• Rather than be a sitting duck . . . . .free fell
• Wanted to pull rip cord at 1000 ft, but
• Passed out and hit edge of steep ravine at 120
  mph with 3 ft of snow slid along bruised and
  sore
• SURVIVED WITH JUST A FEW BONE INJURIES

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