The gravitational acceleration decreases with the square of the distance from the centre of the earth. In the atmosphere, the motion of a falling object is opposed by the air resistance, or drag. The drag equation tells us that drag (D) is equal to a drag coefficient (Cd) times one half the air density (r) times the velocity (V) squared times a reference area (A) on which the drag coefficient is based.
The motion of a falling object can be described by Newton's second law of motion, Force (F) = mass (m) times acceleration (a). Which can be written as (a = F / m). The external force is equal to the difference between the weight and the drag forces (F = W - D). The acceleration of the object then becomes a = (W - D) / m . The drag force depends on the square of the velocity. So as the body accelerates its velocity (and the drag) will increase. It will reach a point where the drag is exactly equal to the weight. When drag is equal to weight, there is no net external force on the object, and the acceleration will become equal to zero. The object will then fall at a constant velocity. The constant velocity is
called the terminal velocity.
The following information was taken from a sky diving website and I have summarised the information give of the conditions they think best for skydiving. I think that it demonstrates how small changes could change the speed of the fall.
Drag Co-efficient The best way to describe this is the "thickness" of the air. Wave your hand about in the air quickly - not much resistance there. Now put your hand in a bathtub of water and try the same - it is harder work. The water has a higher drag co-efficient than air. However, air does have a significant drag co-efficient too. Simply, the higher the drag, the slower speed things will fall at.
Surface area: This is the main part on which drag takes place. The larger the surface area, the more drag is created, and the slower things will fall. E.g. Take a sheet of flat paper. Drop it, and watch it slowly fall. Now fold it in half twice and drop it again. It's the same paper, same weight, and same shape, just a smaller surface area you will notice how it falls faster. Larger surface area equatees to slower.
Shape: Different shaped objects will to fall at different speeds. The basic rule is smooth rounded objects will fall faster than flat objects. Similarly, slower again we have shapes that cup or catch air.
After looking at the options that are available to me and at the background information, I have decided to look at surface area for my investigation because I fell that this will give noticeable and reliable results and gaps between the results collected.
Prediction.
I think that the larger the surface area of the parachute the longer it will take to fall a set distance. I think this because I know that as surface area increases so does friction, which will effect the speed at which the parachute will fall to the ground. I know that if you take a sheet of flat paper. Drop it, and watch it slowly fall. Now fold it in half twice and drop it again. It's the same paper, same weight, and same shape, just a smaller surface area you will notice how it falls faster. This happens because the surface area and therefore friction levels have been decreased as the paper was screwed up into a ball.
Apparatus
Black bin bag cut into squares of the following areas
500cm
1000cm
1500cm
200cm
2500cm
We used 50 cm of string tied op corners 100-500 sr 120
0.85
1.02
111 135 173
free_ringtones_free_logos.com
Scisors selotape
Pen/pencil to mark black bin bag’ calculator.
String
Stop clock,
Weight,
Distance to drop parachute from
Ruler
Diagram. String
Tape.
Black bin bag cut to required size
Method.
Collect and set up all equipment as in diagrams above.
