The Idea
Shortly after jumping from an airplane, skydivers reach a steady velocity instead of constantly accelerating as do other more streamlined objects subjected to Earth's gravity. When this happens, the skydiver falls at a terminal velocity that is nearly constant. This is fortunate because, once the parachute opens, it is much easier to slow the skydiver's fall. Had the skydiver been a rock in a vacuum without the benefit of air resistance, it would reach a much higher velocity.
What You Need
- coffee filter
- meterstick
- stopwatch
- coin, book, or another compact object
- optional: motion sensor
Method
- Drop a coffee filter from a measured distance.
- Compare the time it takes to fall with a coin or a book.
- Compare the distance versus the time graph generated by a motion sensor for each of the two objects.
Expected Results
Not only will the coffee filter take longer to descend, but more significantly, it falls at a steady velocity. The coin, typical of other objects in free-fall, accelerates as it falls and will have an ever-increasing velocity. The following shows distance versus time and velocity versus time graphs for these two objects. Notice the falling book continues to accelerate as it falls. This is indicated by the curved shape of the position-time graph and the positive slope for the velocity-time graph.
The coffee filter falls with constant velocity. The position-time graph is a straight line and the velocity-time graph is essentially a horizontal line, indicating a constant velocity during the descent.
Why It Works
When a falling object encounters significant air resistance, the faster the object falls, the greater the force opposing its descent. So, the more gravity tries to pull the object down, the more determined the air resistance is to oppose gravity. As a result, equilibrium is established with the object falling at a constant terminal velocity.
Other Things to Try
Compare the descent of bottle rockets (described in Project 27) with and without parachutes.
The Point
Free-fall is different than an object subjected to drag forces.
An object in free-fall accelerates with a constant rate of 9.8 m/s2. An object subject to a drag force does not accelerate, but reaches a steady constant velocity, called the terminal velocity.
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