An object is released from rest at the top of a long vertical tube filled with a thick viscous fluid at time t=0 seconds. Take x(0)=0 and v(0)=0, where x is the object’s displacement from the top of the tube measured in metres, and v=dtdx is its velocity in m s−1.
Initially, the resistance is modelled as proportional to the velocity, giving the differential equation
dt2d2x=9.81−0.9(dtdx)
where 9.81 is in m s−2 and the constant 0.9 has units s−1.
The maximum velocity approached by the object as it falls is known as the terminal velocity.
An experiment is performed in which the object is placed in the fluid on a number of occasions and its terminal velocity is recorded. It is found that the terminal velocity was consistently smaller than that predicted by the model used. It was suggested that the resistance to motion is actually proportional to the velocity squared and so the following model was set up:
dt2d2x=9.81−0.9(dtdx)2
In this second model, the constant 0.9 has units m−1.
At terminal velocity, the acceleration of an object is equal to zero.