When a Falling Object Accelerates Only Due to Gravity We Call This Free Fall Knowing What We Know Now Why does a brick with twice the amount of mass as another brick not fall twice as fast ID: 403694
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Slide1
Acceleration of a Falling ObjectSlide2
When a Falling Object Accelerates Only Due to Gravity, We Call This….
Free FallSlide3
Knowing What We Know Now
Why does a brick with twice the amount of mass as another brick not fall twice as fast?
mass
2 massSlide4
Well This is Confusing
The greater the mass of an object, the greater is the gravitational force of attraction between it and earth,
So why doesn’t the brick with double the mass move to earth twice as fast?Slide5
Remember Inertia!
Acceleration of an object depends not only on the force (in this case mass), but also on the object’s resistance to motion – inertia
Inertia is a
resistance
to acceleration Slide6
So twice the force exerted on twice the inertia produces the same acceleration as half the force exerted on half the inertia
mass
2 mass
F
--- = g
m
2F
--- = g
2mSlide7
So acceleration of a free falling object is independent of an object’s mass
A boulder 100 times more massive than a pebble falls with the same acceleration.
Although the force on the boulder is 100 time greater, its resistance to change (inertia) is 100 times that of the pebbleSlide8
Okay Take That All In….Slide9
When Acceleration is Less Than g
Nonfree
FallSlide10
Not in a Vacuum
In a vacuum, the net force is the weight because it is the only force.
In the presence of air resistance, however, the net force is less than the weight – it is the weight minus air drag (the force arising from air resistance) Slide11
The force of air drag of a falling object depends on 2 things:
The frontal area of the falling object
- the amount of air the object must plow through as it falls
The speed of the falling object
- the greater the speed, the greater the number of air molecules an object encounters per second, and the greater the force of molecular impactSlide12
A Falling Feather
Air drag greatly affects feathers as they fall
Because a feather has so much area for an object so light in weight, it doesn’t have to fall very fast before the upward acting air resistance cancels the downward –acting weight.
The net force on the feather is then 0 and acceleration terminates
When acceleration terminates we say the object has reached its
terminal speedSlide13
Skydiving
As a falling skydiver gains speed, air drag may finally build up until it equals the weight of the skydiver
If and when this happens, the net force becomes zero and the skydiver no longer accelerates – reaching
terminal velocity
For a skydiver this is about 200 km/h
A smaller terminal velocity may be obtained by spreading out like a flying squirrel
Slide14
Skydiving
The large frontal area provided by a parachute produces low terminal speeds for safe landings
**Read last paragraph of pg. 59Slide15
Problem
Nellie Newton skydives from a high flying helicopter. As she falls faster and faster through the air, does her acceleration increase, decrease, or remain the same?
a= mg-R/m
As R increases, a decreases
If she falls fast enough so that R=mg then a =0
she will have no acceleration and fall at a constant speedSlide16
Terminal Velocity
Terminal Velocity Video Slide17
Practice
Review questions pg. 61 #
27-35
Ranking pg.63 #4
Exercises pg. 64 #33, 36, 41-54