
物理降落伞速率实验报告.docx
15页MYP ScienceClaudia ZhangScience 10-AMay 14,2015Motion Laboratory ReportBackground information / Introduction: The investigation is to test whether the increase of the surface area of an object would cause a change in the velocity since there is more air resistance for a larger surface area. The theory will be tested through the experiment of dropping different sizes of parachutes attaching to a juice box. Purpose: To observe a free falling object (with a parachute made of plastic bags attached to it), in case that the influential factors (the size of the object, the height of the releasing point, the vertical distance between the parachute and the object etc.) stay constant, how does changing the surface area of the parachute affect the velocity of the falling object?Hypothesis: If the surface area of the parachute increases, then the downward average velocity of the object would decrease due to the increased air resistance.If the object is falling down with the force of gravity, applying Newton’s Second law : Force = Mass x Acceleration, the falling object has “mass·gravity” of force. Since the earth is not a vacuum, then there would be air resistance acting upward as the mg of force is pulling the object downward: “Air resistance is a friction-like force that opposes the motion of objects that move through the air… The amount of the air resistance force depends on the speed, size, shape [and cross-sectional area] of the object” (BC Science, 399) and “how thick the fluid is that is going through” which is the air itself.Air resistance is velocity dependent, because “air resistance is the result of collisions of the object's leading surface with air molecules” (The Physics Classroom). If the object has a greater speed of velocity, the object would hit the particles harder and would interact with more particles in the air per second. This creates greater frictions between the object and the air and thus creating more air resistance to slow down the falling object. In addition, the formula for calculating the force of air resistance is “Fair = (1/2)CρSV2”(Baidu) where C represents drag coefficient, p represents air density, S represents the surface area, and V represents velocity. In the case of the investigation, where the drag coefficient, and the air density are constant, the relationship between the force of air resistance and velocity can approximately be written as “Fair = S·V2”. The V in the investigation is always negative because the object is falling downward. Therefore, from the equation, it can be predicted that Fair is directly proportional to S, and to the square of V. MYP ScienceClaudia ZhangScience 10-AMay 14,2015However, since V is negative, as the number decreases (which means there is greater velocity downwards), the square of it will still be positive, therefore it would increase Fair. “Eventually, the force of air resistance becomes large enough to balance the force of gravity·mass. At this instant in time, the net force is zero Newton; the object will stop accelerating” (The Physics classroom), and reach to terminal velocity. Because the larger surface area of a parachute would increase the amount of air resistance, which means that it would reach terminal velocity in a shorter amount of time, thus decreasing the average velocity of the falling object.Therefore, it is assumed that if the surface area of the parachute increases, it would create more force of air resistance, reaching to terminal velocity in a shorter time period, and thus decreases the average downward velocity. Variable: Independent Surface area of the garbage-bag parachutes30cm x 30cm; 40cm x 40cm; 50cm x 50cm; 60cm x 60cm, 70cm x 70cmDependent Velocity of the falling objects (cm/s)Constant Size of the object (a cut-open juice box with 4.7cm in length, 3.8cm in width, 6cm in height) 。












