Free Fall

1
Free Fall

• Acceleration due to gravity

2
Free Fall and Galileo

• All objects at same location above earth fall
  vertically with same acceleration
• Ideal motion where air resistance is ignored
  free fall
• For small falls acceleration is constant
• Constant acceleration means we can use the big 5
  kinematics equations

3
Galileo

• Galileo famously rocked the scientific
  establishment by refuting Aristotles claim that
  bigger objects fall faster
• At the end of the Apollo 15 mission the
  astronauts tried it on the moon- without air
  resistance
• http//www.youtube.com/watch?v4mTsrRZEMwA

4
Lab

• Gravity- measuring g with a stopwatch
• Each group will have a different size object
• We will drop the object 10 times to average
• Solve for acceleration due to gravity known as g

5
Acceleration due to gravity

• g9.80m/s2
• We will use this value but g actually varies
  with
• Altitude- why?
• Latitude- why?

6
Problem solving

• Since a is constant, same kinematics equations
  apply
• Generally we use y for displacement since it is
  vertical

7
Example finding displacement

• A stone is dropped from the top of a tall
  building. After 3.00s of free fall, what is the
  displacement of the stone?
• We know

y a v v0 t
? -9.80m/s2 0m/s 3.00s
8
Example continued

• We dont know v so we can use
• yv0t1/2 at2
• y(0m/s)(3.00s) 1/2 (-9.80m/s2)(3.00s)2
• y-44.1m

9
Example- how high?

• A football game customarily begins with a coin
  toss to determine who kicks off. The referee
  tosses the coin up with an initial speed of
  5.00m/s. In the absence of air resistance, how
  high does the coin go above its point of release?
• Write what you know
• Whats missing? t
• So which equation can we use?

10
Acceleration vs. Velocity

• Think about flipping a coin-
• Where is displacement 0?
• Where is instantaneous velocity 0? Draw the
  velocity vector at different parts of coins
  flight.
• Is acceleration ever 0?

11
Symmetry in Free fall

• Remember the saying, what goes up must come
  down?
• Objects in free fall experience symmetrical
  motion
• At any displacement on the way up, objects speed
  will be the same on the way down
• Object loses 9.80 m/s2 on the way up and gains it
  on the way down
• Average speed will occur halfway up/down

12
Hmmsome questions

• If I throw a rock straight up, what will be the
  acceleration at the moment it reaches the top of
  its trajectory?
• 9.8m/s2- the instantaneous velocity will be 0 as
  it reverses direction but acceleration is always
  g!