School Physics notes: Friction, drag effects and terminal velocity
3. Acceleration, deceleration, friction, drag effects and terminal velocity
Doc Brown's school physics revision notes: GCSE physics, IGCSE physics, O level physics, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of physics
This page will help answer questions such as ...
What causes the drag effect in fluids? What is air resistance? What has it got to do with friction? What do we mean by 'terminal velocity? Describe an experiment to illustrate terminal velocity? How and why does a parachute work?
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(a) Friction and motion (solid surface contact and objects moving through a fluid)
You first appreciate friction when two solid surfaces rub together. When you pull a heavy object across the floor you experience the resistive force between the two surfaces. This resistive force is called friction and is caused by the atoms of the two surfaces bumping in to each other from opposite directions. The moment you stop applying a force, the dragged object ceases moving immediately.
The force of friction always acts in the opposite direction to an object's movement and it can involve ANY type of surface contact effect.
If an object is moving at a steady speed the thrust or driving force (engine or gravity etc.) is being balanced by the opposing force of friction.
If a moving object has no thrust force acting on it, then it will always slow down and come to a halt e.g. on a level road, if you take your foot of the accelerator, the car will eventually come to a halt as the resistive forces act against the car's forward motion. The total friction effects will bring the car to a halt.
(b) How does speed affect the drag force and how can we reduce it?
In these next examples you are reducing the loss from a kinetic energy store to the surrounding air/water thermal energy store - the destination of most wasted or dissipated energy.
The faster an object moves through a fluid the greater the rate of particle collisions between the object's surface and the fluid (e.g. air or water.
To reduce the drag effect its not always easy to reduce the surface area, hence reduce friction, but you can design the shape of an object to allow the fluid to flow more easily across the surface.
There are times when we wish to increase the drag effect - see parachuting further down the page.
(c) Terminal velocity (balancing forces - rolling down hill and objects falling downwards under gravity in fluids)
(c)(i) Terminal velocity and a trolley - running board experiment
(c)(ii) The terminal velocity of a small sphere falling down through a liquid
You can demonstrate the effect of resistive forces in a fluid using the experiment illustrated in the diagram above.
Theory: When an object falls through a fluid there are three forces to take into consideration.
(d) The complex behaviour of a falling parachutist
This is an interesting case because it involves an acceleration, a deceleration and two terminal velocities!
1. 2. 3.
The three possible 'force' situations as the parachuting person is descending after jumping out of an aeroplane.
Note the relative size and direction of the arrows.
1. When drag force F1 is less than the weight force F2, the parachutist is accelerating.
2. When drag force F1 equals the weight force F2, the parachutist is descending at a steady speed - a terminal velocity.
3. When drag force F1 is more than the weight force F2, the parachutist will decelerate (to another terminal velocity).
AND, remember the drag force on an object due to friction (air resistance), increases with speed i.e. as more air brushes over the surface of the object in the same time.
speed/velocity-time graph for a parachute descent
weight due to gravity W↓ and the drag effect due to air resistance D↑
(I'm ignoring the minor effect of upthrust U↑ that applies to any fluid - gas or liquid)
force vectors operating in the jump
(1) The parachutist jumps out of the aircraft and immediately accelerates due to gravity (weight force W↓), but the parachute is not opened yet, so the parachutist is in free fall!
Between (1) to (3) acceleration is taking place, but although W↓ is constant, the drag force D↑ due to air friction (air resistance) is increasing, so the acceleration is decreasing (gradient decreasing).
At (3) the terminal velocity is first reached when the drag force D↑ = weight W of the parachutist.
Between (3) to (5) the descent continues at the (1st) constant terminal velocity because the resultant force is zero.
At (5) the parachutist opens the parachute to increase air resistance. The wide area brushing through the air produces a massive air resistance (friction) drag effect so D↑ massively increases, and is far greater than W, so rapid deceleration immediately takes place.
Between (5) to (7) the deceleration continues and as the parachutist decreases in speed, so the drag forces decreases too.
At (7) the drag force once again equals the weight force and the parachutist attains a 2nd, somewhat slower, terminal velocity.
Between (7) and (8) the parachutist descends far more slowly with constant speed to land safely at ~15 mph, again the resultant force is zero.
Terminal velocity experiments
You do simple experiments with toy parachutes attached to a weight to determine their terminal velocities.
(e) Other falling object situations described and explained!
Some 'actual' and 'thought' experiments.
You need to consider the W (weigh, depends on local gravitational field strength), U (upthrust) and F (friction, drag) forces described in the ball bearing - liquid filled tube experiment and the four objects illustrated above.
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