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Forces and Motion 3.6
Some other situations
of falling objects from a feather to a hammer and in different gravitational
fields, thought experiments described & explained!
Doc Brown's Physics exam study revision notes
INDEX for physics notes on
acceleration of falling objects, experiments, friction, drag effects,
gravity, terminal velocity
3.6
Some other situations
of falling objects from a feather to a hammer and in different gravitational
fields, thought experiments described and explained!
The earliest theories on falling objects suggested that the
heavier the object, the faster it falls.
We now know that all objects fall at the same speed if only
the force of the local gravitational field is acting on the them.
Some 'actual' and 'thought' experiments.
You need to consider the W (weigh, depends on
local gravitational field strength), U (upthrust) and F (friction, drag) forces
described in the ball bearing - liquid filled tube experiment and the four
objects illustrated above.
(a) The hammer and the feather
experiment on the moon
When the first astronauts landed on the
moon they carried a beautifully simple experiment.
One astronaut held up a feather and a
hammer and let them both fall simultaneously.
Both objects (i) fell to the ground in
the same time experiencing the same gravitational acceleration and (ii) they
seemed to fall to the ground more slowly than if they had been dropped on
Earth.
(Weight = mass x gravity, W = mg)
(i) Both objects experience the same
gravitational acceleration of 1.62 m/s2, BUT the Moon has no atmosphere so
there is no air resistance. With no drag effect due to friction both
objects, whatever their mass or shape (surface area) are free to fall with
maximum acceleration due to the Moon's gravity.
(ii) Acceleration on Earth due to the
gravitational field is 9.80 m/s2. The Moon has a much smaller mass and
gravity is much weaker and only produces an acceleration of 1.62 m/s2.
Therefore objects on the Moon will fall much more slowly than on Earth
(ignoring air resistance).
On Earth: Hammer weight = 0.20 x 9.80 = 1.96 N
and feather weight = 0.002 x 9.80 = 0.0196 N
On the Moon: Hammer weight = 0.20 x 1.62 =
0.32 N and feather weight = 0.002 x 1.62 = 0.0032 N
(b) The hammer and the feather
experiment on Earth
If you drop the feather and hammer
simultaneously on Earth, the feather takes much longer to fall to the ground
because it has a relative greater surface area/mass ratio greatly increasing
drag effect.
If it wasn't for air resistance, no drag
effect can happen and all objects would fall with the same acceleration.
You can do a laboratory experiment with
two objects (one 'light' like a feather and one heavy like a lead weight) by
allowing them to fall vertically in a large tube from which the air has been
pumped out to create a vacuum.
It seems uncanny when they both hit the
bottom of the tube at the same time.
(c) Dropping two spheres of
equal surface area, but different mass
If you simultaneously dropped the dense
pool ball and light hollow plastic ball with the same radius (giving same
surface area) the pool ball will hit the ground before the lighter hollow
plastic ball.
This is because the greater weight of the pool ball will overcome
the drag effect of air resistance more than the less 'weighty' plastic ball and
the pool ball
will appear to accelerate at a greater rate.
The surface area is the same, and if both surfaces
are smooth, the friction effect should be the same.
On the Moon you would not notice any
difference, see (a) for the argument.
(d) The general behaviour of a
falling object on a planet or a moon
If you drop any object (safely!) from a
tall building on Earth, it will always achieve a terminal velocity due to the
friction effect of air resistance. The terminal velocity s
reached when the downward weight force equals the air resistance
friction force.
Remember the drag effect increases with
speed because more air particles are brushing and colliding against the
surface of the falling object in the same time interval.
The drag effect is most prominent if a parachute is
used.
If there is no atmosphere, there is continuous
acceleration to the surface of the planetary objet.
(5) Falling objects on other planets
Graph of distance-time graphs for falling objects:
Speed or
velocity = gradient on the graph i.e.
∆d/∆t
(1) A huge moon or planet with no atmosphere, no
terminal velocity observed, velocity increasing all the time (∆d/∆t
gradient gets steeper).
(2) A smaller moon or planet with no atmosphere, as for
(1) but smaller acceleration.
(3) A moon or plane with an atmosphere, terminal
velocity observed (∆d/∆t
gradient becomes constant).
(4) A smaller planet or moon with no atmosphere, acceleration
much smaller than the (1) or (2) due to the body having a much smaller
mass e.g. (3) and (4) could be a comparison of our Earth and our Moon.
Physics notes index on
acceleration of falling objects, experiments, friction, drag effects,
gravity, terminal velocity
Keywords, phrases and learning objectives for
objects falling in a gravitational field
Be able to describe behaviour of falling objects
e.g. feather or a hammer
in different gravitational fields e.g. those of our moon, different planets
and be able to make predictions from thought experiments -
theoretical investigations!
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Physics notes index on
acceleration of falling objects, experiments, friction, drag effects,
gravity, terminal velocity
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