Answer 3:
Deceleration is just a way of saying
acceleration in the direction opposite the
direction of motion, so making a heavy object
decelerate faster means making it accelerate
backward faster. The acceleration a of
an object of mass m is related to the net
force acting on the object F, as given by
a rearrangement of
Newton's Second Law:
to a = F/m.
Note that this is the net force, meaning the
sum of all of the forces, those acting in the
same direction as the motion of the object and
those acting opposite the direction of
motion (see not at bottom). To decelerate, meaning
a < 0, the net
force must be backward, F < 0 (using the
convention that F > 0 acts in the direction of
motion).
So, to make an object decelerate at all
the force acting backward on it must be
greater than whatever forces are acting to drive
it forward. Since a heavy object has a
greater mass than a feather (m_heavy >
m_feather), making the heavy object decelerate
faster means the force on the heavy object must be
more negative than that on the feather.
Assume the heavy object and the feather are simply
falling through the air. In this case the only
downward force is that produced by
gravity,
so positive a comes from gravity. However,
air produces
aerodynamic drag force on objects moving
through it. Drag opposes motion through air, so
here this force acts in the upward
direction, so decreasing a comes from drag.
Because they are acting in opposite directions,
adding these forces reduces the value of a.
Thus, to achieve a < 0
requires increasing the drag force enough that it
is larger than the force of gravity.
The drag equation reveals that increasing
area is one way to increase drag force. This is
exactly the working principle of parachutes.
Other factors are the shape and surface
roughness of the object. A heavy object that
catches more air will have a greater drag force
than one that cuts through the air.
So, to sum all of this up, to make a heavy
object decelerate faster than a feather requires
increasing the drag force on the heavy object by
enough to overcome the accelerating force of
gravity. One easy way to do this is to add a
parachute to the heavy object. Note that this only
covered passively falling objects; any upward
force on the falling heavy object will decrease
its acceleration. Some methods which seem
outside of the spirit of the question would be
attaching solid rocket boosters, hot air
balloons, or propellers to push/pull the heavy
object upward against gravity.
[As a bit of background beyond 5th grade, realize
that force, acceleration, and velocity are
vector quantities, meaning they have both a
size and a direction. When
adding (or
subtracting, multiplying, etc.) vector quantities,
the direction must also be considered. For this
question it is sufficient to know that vectors
with opposite directions have opposite signs and
adding them produces a vector with a smaller size,
like adding positive and negative numbers, with
the direction of the vector depending on the
sign.]
