Mechanics - Falling Bodies

Question 1

Free-fall

Answer 1

The only force acting on a body is its weight (ΣF=W∴ma=mg∴a=g).

Question 2

Apparent weightlessness

Answer 2

Weight is experienced as a support force. A body in free-fall has zero support force and therefore ‘feels’ weightless (e.g. in a falling lift, in an orbiting satellite).

Question 3

Bodies falling

Answer 3

in a fluid (such as air) do not experience free-fall because they are subject to drag.

Question 4

Drag is …

Answer 4

proportional to the square of velocity and therefore increases as the body accelerates downwards (provided fluid viscosity and the body’s drag coefficient remain constant).

Question 5

Drag decreases

Answer 5

the external resultant force on the body, which is the vector sum of the (constant downwards) weight and (increasing upwards) drag.

Question 6

The acceleration of the falling body…

Answer 6

The acceleration of the falling body therefore reduces (according to Newton’s Second Law) until weight and drag are equal in magnitude, the resultant force and acceleration are zero, and the body reaches terminal speed (obeying Newton’s First Law).

Question 7

what happens to bodies subject to drag

Answer 7

Bodies subject to drag therefore experience a lower average external resultant force, and hence a lower average acceleration, than bodies in free-fall.
a = g - (d/m)