2.2.5Forces, Space and Radioactivity
Investigating Terminal Speed — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 11 of 37
Section 1 of 6Aim and variables
Aim
- Aim To investigate how the speed of a falling object depends on its mass when it falls at terminal speed.
Variables
- Variables Independent variable: the mass, changed by stacking 1 to 6 cake cases.
- Dependent variablethe speed, found from distance divided by time.
Control variables
- Control variables Surface area (use the same type of cake case each time), drop height, the timing method, and draughts or air flow.
Section 2 of 6Equipment
| Equipment | Purpose |
|---|---|
| Paper cake cases | Stacked to give different masses |
| Stopwatch | Measures the time of fall |
| Two metre rulers | Measure the drop height |
| Clamp stand, boss and clamp with pointer | Mark the height at which timing begins |
| Electronic balance | Measures the mass of the cake cases |
Section 3 of 6Method
1
Set up the pointer on the clamp stand at a measured height of about 150 cm above the floor, and record the height.
2
Find the mass of one cake case on the balance and record it.
3
Release the cake case from about 20 cm above the pointer so that it has already reached terminal speed when it passes the pointer.
4
Start the stopwatch as the case passes the pointer and stop it when it hits the floor. Record the time.
5
Repeat four more times for the same mass, giving five trials, and calculate the mean time.
6
Repeat the process with stacks of 2, 3, 4, 5 and 6 cake cases, finding the mass each time.
7
Calculate the terminal speed for each mass using speed = distance ÷ mean time and plot a graph of speed against mass.
Section 4 of 6Results and conclusion
Analysis
Calculation
- Calculation With a fall of 1.50 m and a mean time of 0.71 s, the terminal speed is 1.50 ÷ 0.71 = 2.1 m/s.
Pattern
- Pattern Terminal speed increases as the mass increases, because the weight is larger, so a greater air resistance is needed before the forces balance.
Not directly proportional
- Not directly proportional The graph curves, so doubling the mass does not double the speed, since air resistance rises faster than speed.
Improving the data
- Improving the data Use repeated trials, discard anomalies, and record times to the resolution of the stopwatch (about 0.01 s).
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Terminal speed | The constant speed reached when the air resistance on a falling object equals its weight. |
| Air resistance | The frictional force that opposes the motion of an object moving through air. |
| Weight | The force of gravity on an object, equal to mass × gravitational field strength. |
| Resolution | The smallest change that a measuring instrument can detect. |
| Anomalous result | A reading that does not fit the pattern of the other results. |
Section 6 of 6Exam technique
1
Starting the timer at the moment of release: the cake case must already be at terminal speed when timing starts.
2
Changing the type or size of cake case: this changes surface area, so the test is unfair.
3
Saying heavier objects accelerate faster in a vacuum: here air resistance causes the difference.
4
Saying air resistance stops the object accelerating: it is that weight and air resistance become equal, so the resultant force is zero.
5
Using only one trial for each mass.
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