3.2.3 Drag and Terminal Velocity

Question 1

Define drag.

Answer 1

The frictional force experienced by an object travelling through a fluid (a liquid or a gas, eg: air).

Question 2

In what direction does drag occur?

Answer 2

In the opposite direction to the motion of the object.

Question 3

Why must friction occur on seemingly smooth metals?

Answer 3

They seem smooth, on a MACROSCOPIC LEVEL; but at a MICROSCOPIC LEVEL, they are full of indentations, ridges and troughs.

Question 4

Name three equations which are utilized in drag equations, and state what is meant by each unknown.

Answer 4

1.F(small)d= 1/2pC(small)dAv^2
where:
F(small)d is the force of drag
A is the cross sectional area of the moving object (m^2)
v is the velocity of the moving object (ms^-1)
p is the density of the fluid (kgm^-3)
Cd is the coefficient of drag, which has no units and is different for each shape - this can only be determined by experimental means.
2. Fd (little d, this means ‘the force of drag) = k(a constant) v^2; and so it can be said that the force of drag is proportional to the square of velocity of the object.
3. F=ma, drag is a force - this is Newton’s second law.

Question 5

What is terminal velocity?

Answer 5

1. An object falls and accelerates;
2. The gravitational force is greater than the resistive forces;
3. The resistive forces catch up; and drag becomes equal to the weight of the falling object;
4. The forces are balances as the object has reached terminal velocity.
5. The resultant force on the object is zero - and it continues to fall at a constant velocity as the overall force on the object is zero;

Question 6

What are the forces acting on a bubble in a cup, and what is special about this?

Answer 6

1. Upthrust, w and drag - the bubble accelerated upwards - W DOES NOT CHANGE, and the bubble decelerates upwards at -g
2. Special facets: drag acts UPWARDS.

Question 7

Why are there two rounds of terminal velocity in a parachutists fall? What would this look like on what graph?

Answer 7

1. There are two terminal velocities as the parachute opens the resultant force acts upwards, and so the parachutist’s speed is decreased. This creates the second terminal velocity. The first is created when he falls from the plane and the forces balance.
2. On V-t graph, this would look like two straight horizontal lines proceeded by each other and connected by a curve.

Question 8

Describe the terminal velocity of a skydiver.

Answer 8

1. He jumps, the only force is his weight;
2. He accelerates and air resistance (FRICTIONAL FORCE) balances with weight.
3. On a V-t graph, this would be sen in a reduction of the gradient;
4. The parachute opens, which is followed by a rapid deceleration.