Velocity-time graph
Skydiver’s velocity-time graph
If a velocity-time graph of the motion shows a rising or falling line, this means that the velocity is changing. Therefore the forces acting must be unbalanced, and the skydiver is accelerating or decelerating.
Newton’s Second Law
A rising line means the skydiver is accelerating. Weight force is greater than drag, and the resultant force causes the acceleration. If the line is horizontal, then the speed is constant. This is the terminal speed. The forces acting are balanced at terminal speed. The drag is equal to the weight, so there is no resultant force to cause acceleration.
How is air resistance or drag created, and why does drag increase with speed?
Moving through the air causes collisions with air molecules. There is friction between the moving body and the particles, which causes some of the kinetic energy of the moving body to be converted into heat. Additionally, the air molecules bounce off the object gaining kinetic energy.
The faster an object moves, the greater the number of collisions with air molecules. This increases friction, and the air molecules bouncing off gain even more kinetic energy.
The first diagram tells you what is happening during a sky dive, and the second diagram shows you how to work out the values for terminal speed.
Image caption, Sky dive
This diagram tells you what is happening during a sky dive
1 of 2
The area under the graph represents the distance travelled. This will be the height of the plane.
Question
A coin, a feather, and a brick are dropped from a bridge into a river. In which order do they hit the water? How would the order change if the experiment was repeated in a vacuum?
In open air the coin would hit the water first, then the brick would follow and then the feather would hit it last. The coin is more streamlined than the brick and the feather, so there is less air resistance on the coin. The feather is light and has a relatively large surface area, and therefore reaches terminal velocity quickly.
Without air resistance, ie in a vacuum, all three objects would hit the water at the same time. They all accelerate at the same rate, and this does not depend on their mass or size.
Science presenter Jon Chase describes Aristotle’s and Galileo’s theories about falling bodies
Horizontal motion in air
Terminal velocity also applies horizontal motion when:
- a car accelerates forward when the thrust is greater than drag
- a car gets faster and the drag increases, reducing the resultant force
- the drag becomes equal to the thrust, there is no resultant force
- the forces are balanced, and the car travels at terminal speed
