Parachutes fall by trapping air underneath them and increasing the force of air resistance. This makes the force of air resistance larger or equal to the force of gravity so that the parachute now falls at a constant rate rate. The constant rate of descent is slow enough to stop objects that are very aerodynamic, as long as the object is not too heavy for the size of parachute, such as weights or human bodies. The structure of a parachute means that it has to displace air in order to float, otherwise, the edges would flap around and there would be little reduction in air resistance. This is achieved by making a small hole in the centre of the parachute, which lets the air escape.
Apparatus
- Stop clock
- Plastic for parachute
- Plasticine
- Strings
- Measuring tape
Method
First of all, the metre ruler should be used to measure the height of the area being used (in this case the stairs). The first parachute can now be made. A square of plastic should be cut, measured to a surface area of 100cm², 200cm², 300cm², 400cm² or 500cm². Three small holes should then be cut into the parachute. The holes should be the diameter of the string being used. The holes should also be 0.5cm in from the edge of the parachute. The string can then be tied around the holes and the edge of the parachute. The string should be tied so that it leaves 3mm of string after the knot has been tied. The weight (plasticine) can then be tied on to the strings that are attached to the parachute. The parachute can now be taken to the 5.5 metre height and dropped. The parachute should be timed as it falls, and the horizontal distance from the point it was dropped should also be measured. Following the first experiment, the second parachute can be made to similar proportions, but bigger.
Diagram
Results
Analysis
I think that the graph showed that as the surface area got bigger the time the parachute took to reach the ground increased because as the surface area got bigger so did the air resistance and as the air resistance got bigger it equalled the force pushing down on it (gravity).
When the air resistance equalled the force of gravity, the falling object had no resultant force. Having no resultant force means that the object will not speed up any further during its descent. If a mass has no resultant force it is moving at a constant speed in a straight line a. So the quicker the point of no resultant force is reached the slower the resultant force is because a mass is speeding up all the time that it is descending until it has no resultant force.
Looking at the graph of velocity-surface area we can see the trends of the results show that the speed increases at what seems to be an exponential rate, as shown on the graph by the curve of best fit. There seems to be proportionality between the surface area and the time as the line of best fit seems to show a linear relationship.
Evaluation
I think that our results are quite accurate, we tried to ensure that they are accurate by timing each parachute 5 times and then taking an average this helps to make the results accurate because when an average is taken it is supposed cancel out any little timing errors made, so the more times the average is taken, the more accurate the results will be. We also did everything the same so that the experiment was a fair test e.g. using the same weight of plasticine on each parachute. The overall experiment was fairly precise and accurate. I had no anomalous results. The problems encountered were minor. If I had more time, I would have re-done the experiment at least 10 times in order to get an accurate result from that particular parachute area. I could extend my investigation by using bigger surface areas. If I did do this I think that eventually the line on the surface area-average time graph would straighten off because eventually the parachutes wouldn't be able to reach the point of no resultant force any quicker.
To make our experiment a fair test I dropped the parachutes from the same height each time, I kept the weight on the end of the parachute the same, I kept the strings on the parachutes the same length, I put the plasticine at the same point on the strings on the parachute and I secured the strings in the same position on each parachute. I closed the windows and made as little movement as possible while conducting the experiment in order to prevent the wind from blowing the parachute. This was quite effective, as the wind could have easily blown the parachute off course or blown it up or down. I also used the same weight each time, in case the other weights were different weights, either by manufacturing fault or by usage of the weights, getting chips out of them or being otherwise damaged. This was useful because it made the test fairer, because lighter weights would make the parachute fall slower than normal, and bigger weights would make the parachute fall faster than normal. There was the problem of not have very accurate measuring instruments that could measure to the nearest millimetre accurately. The plastic was also very easy to fold and crumple, so it was not always flat, which could have changed the rate of descent again.
As a further experiment to show that gravity is not the only force acting on the parachute is, if I were to conduct this same experiment in a vacuum because there is no air in a vacuum which means that there is no air resistance so all the parachutes no matter what there surface area would fall at the same rate and speed.
