Download Free Fall - Cobb Learning

Free Fall
• Consider an apple falling from a tree. We
know that it starts at rest and gains speed as it
falls, or accelerates.
• Gravity causes the apple to accelerate
downward and is said to be in free fall.
Free fall: when an object is only affected by
gravity
– SI unit: m/s2 ( for acceleration due to gravity)
– Ex: g = 10 m/s2 on Earth.
• The letter g represents the acceleration due to gravity.
– Equation:
v = gt
• v = velocity or speed (m/s)
• g = acceleration due to gravity (10 m/s2 on
Earth)
• t = elapsed time (s)
**Hint - as soon as you see any of the following
phrases in a word problem, write g = 10 m/s2
for a given: free fall, falling, dropped,
thrown**
Remember, the object’s velocity
increases by 10 m/s every second it falls
Time falling
0s
1s
2s
3s
4s
5s
6s
7s
8s
9s
velocity
Check Your Understanding
What would the speedometer reading on a
falling rock be 4.5 seconds after it drops from
rest?
v = ?
g = 10 m/s2
t = 4.5s
v = gt
v = (10 m/s2) (4.5s)
v = 45 m/s
How about 8 seconds?
 v=?
g = 10 m/s2
t = 8s
v = gt
v = (10 m/s2) (8s)
v = 80 m/s
How about 15 seconds?
 v=?
g = 10 m/s2
t = 15s
v = gt
v = (10 m/s2) (15s)
v = 150 m/s
• Now consider an object thrown straight up. It
will continue to move straight up, then it
comes back down.
• At the highest point, the object changes its
direction and the objects instantaneous speed
is 0 m/s.
• Whether the object is moving up or down, the
acceleration of the object is always 10 m/s2.
– It is just -10 m/s/s when the object is moving up
• Because gravity is slowing the object down instead of
speeding it up
• Because an object in free fall increases the
rate of distance covered every second, we
cannot use v =d/t to find distance.
– Equation: d = ½ gt2
• d = distance (m)
• g = acceleration due to gravity (10 m/s2 on
Earth)
• t = elapsed time (s)
Use the distance formula to find how
far the object will fall at each second
Time falling
0s
1s
2s
3s
4s
5s
6s
7s
8s
9s
Distance travelled
Check Your Understanding
What is the distance an object falls in one
second?
d = ?
g = 10 m/s2
t=1s
d = ½ gt2
d = ½ (10)(12)
d=5m
Warm up 9/17 (Write the questions and then answer them!)
1. What does it mean if an object is in
free fall?
2. How do we calculate speed of an
object if it is in free fall?
3. What does g represent? What’s is
it’s numerical value?
4. How do we calculate the distance
an object falls when it is in free fall?
The problem with air
• When you drop a crumpled piece of paper and a
flat sheet of paper they should fall and hit the
ground at the same time, but do they?
– The reason the crumpled piece of paper hits the
ground first is because it experiences less air
resistance
• A frictional force (means it goes against motion) caused by
air
• The greater the surface area of an object, the more air
resistance it feels
Next steps:
- You are going to work some free fall problems
on your own paper and put up the answer on
your white board for me to check
- Make sure you put the problem # with the answer
on the white board so I know which one to check
- At the end of class I will pass back last week’s
quiz
- You can complete quiz corrections and turn them in
by Wednesday for ½ credit back
- On a separate sheet of paper rework any questions
missed
- Be sure to include all work