Free Fall

1
Free Fall

• Physics
• Mrs. Coyle

Stuntmans Free Fall Great Adventure 2004
2
Questions

• Do heavier objects fall faster than lighter ones
  when starting from the same position?
• Does air resistance matter?
• If the free fall motion has a constant
  acceleration, what is this acceleration and how
  was it found?
• How do we solve problems involving free fall?

3
Apollo 15 -Astronaut David Scott on the Moon
(1971)

• Hammer and Feather on the Moon
• http//www.archive.org/details/NIX-LV-1998-00046

4
Galileo (1564 1642) and the leaning tower of
Pisa.
5
Does Air Resistance Matter?
6
Air Resistance

• The force of friction or drag acting on an object
  in a direction opposing its motion as it moves
  through air.

7
Hammer Feather in the presence of air
8
Hammer Feather in the absence of air
9

• If the free fall motion has a constant
  acceleration, what is this acceleration and how
  was it found?

10
Galileos Ball and Channel Experiment
http//www.ifa.hawaii.edu/barnes/ast110_06/rots/p
ftim19_01.png
11
Galileos Ball and Channel Experiment

• He varied the starting position of the ball along
  the channel.
• He measured the times for the ball to travel the
  various lengths.
• He raised the channel until it was steep enough
  to simulate free fall.

12
Galileos Finding

• We compared the time for the whole length with
  that for the half, or with that for two-thirds,
  or three-fourths, or indeed for any fraction in
  such experiments, repeated a full hundred times,
  we always found that the spaces traversed were to
  each other as the squares of the times, and this
  was true for all inclinations of the plane, i.e.,
  of the channel, along which we rolled the ball.
  Galileo Two New Sciences
• http//galileoandeinstein.physics.virginia.edu/lec
  tures/gal_accn96.htm

13
How Far?
14
To Find Distance from t and g

• dvit ½ g t2

15
How Fast?
16
Acceleration due to Gravity, g

• g-9.8m/s2 (we often use -10m/s2)
• g v f v i
• t
• When vi0 vfgt
• Note the down direction is usually assumed
  negative.

17
Equations of Motion for Uniform Accelerated Motion

• vf vi gt
• vavg ½ (vf vi)
• d ½ (vf vi)t
• d vit ½ gt2
• vf2 vi2 2gd
• d is the displacement (or ?d)
• Assume that ti0

18
Example 1 Free Fall

• A ball is dropped from rest from the top of
  a building. Find
• The instantaneous velocity of the ball after 6
  sec.
• How the ball fell.
• The average velocity up to that point.
• Answers -60m/s, 180m, -30m/s

19
Example 2 Free Fall on the Moon

• A hammer is dropped on the moon. It reaches the
  ground 1s later. If the distance it fell was
  0.83m
• Calculate the acceleration due to gravity on the
  surface of the moon.
• Calculate the velocity with which the hammer
  reached the ground and compare to the velocity it
  would have, if it was dropped on the earths
  surface.
• Answer-1.66m/s2, -1.66m/s, -9.8m/s