Investigate the effect of air resistance to the speed of a trolley moving down a ramp in relation to the area of a sail attached to the trolley.

Introduction

We are trying to investigate the effect of air resistance to the speed of a trolley moving down a ramp in relation to the area of a sail attached to the trolley.

The prediction I have made is that the larger the sail area, the longer the trolley will take to descend, i.e. it will be slower.

The variable I will be changing is the height of the sail, and so, the area of the sail. Some variables cannot be changed, but may affect the result of the experiment, and so should be noted. These include:

Atmospheric Pressure - Obviously, if there is no air, there can be no air resistance. The effect of this on our experiment is incredibly small, and so can be discounted, as there is always atmospheric pressure of approximately 960-1050 bars. This can be measured with a barograph/barometer.
Humidity – If there is lots of water in the air, it will increase drag on the trolley by a minimal amount. In an air-conditioned room, the humidity is on average, approximately 50%. The humidity in the experiment room was approximately 60%. This can be measured with a hygrometer or a wet and dry thermometer set-up.

Other variables that can be controlled and must be kept constant are:

Mass of trolley – If extra mass is added or removed, it will change the velocity of the descent. To keep this constant, every time the sail is trimmed, the removed paper will be placed into the trolley.
Ramp surface condition – If there is a very smooth or lubricated surface, the trolley will slide quicker than on a rough surface. To keep this constant, the ramp must have the same surface condition all the way down, i.e. any bumps halfway down must be smoothed.

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Other variables that can be controlled and must be kept constant are:

Mass of trolley – If extra mass is added or removed, it will change the velocity of the descent. To keep this constant, every time the sail is trimmed, the removed paper will be placed into the trolley.
Ramp surface condition – If there is a very smooth or lubricated surface, the trolley will slide quicker than on a rough surface. To keep this constant, the ramp must have the same surface condition all the way down, i.e. any bumps halfway down must be smoothed.

The following apparatus will be required.

Clampstand
Ramp (plank)
Trolley (butter tub)
Sail (paper)
Sellotape
Scissors
Stopclock
Pen & paper to record results

Method

Set up apparatus in the following manner.

Place the trolley at the top of the ramp, and time the descent. Record the length x and the time of descent in a table. Repeat three times, recording each attempt. Obtain an average, and record. If you obtain a set of results that you believe to be completely anomalous (e.g. 5secs, 10secs, 18secs, average=11secs) make a note to disregard these results from the final graph, then repeat.

3.Trim two centimetres off from the top of the sail, then add the trimmings to the tub.

4.Repeat steps two and three until no sail is left.

Preliminary Work

To work out the best height of the ramp for this experiment, we did some preliminary tests with a trolley with no sail. We recorded the time taken for the trolley to traverse the ramp:

Based on these results, I conclude that 40 cm is the best height for the ramp.

To give an adequate descent time we made the length one metre. This will also make it easier to work out velocity.

Results

N.B. All temporal values are rounded to the nearest second.

* The highlighted values would appear to be anomalous, as they don’t fit with general trend of the data, see the following graph.

Conclusion

In conclusion, this experiment has shown that, as I predicted, the larger the sail area the greater the time of descent and so the lower the velocity.

When the trolley is placed at the top of the ramp, static friction is acting on it, stopping it from moving, when it is released, the downward force trying to pull the trolley to the ground becomes greater than the static friction (which as the trolley starts to move becomes dynamic friction) and so the trolley accelerates. When the sail is put up, there is now a larger area, where more air particles will hit, causing more dynamic friction, which means that the trolley will accelerate at a lower rate. As the sail area is reduced, the amount of dynamic friction is reduced, and so the acceleration rate will be greater.

Evaluation

The experiment was quite good, although the data collection techniques were inaccurate. As the trolley was released, another person would start the stopclock. As soon as the stem crossed our finish line, the timer was stopped. The accuracy of this method is entirely dependent on the reactionary speed of the experimenter, which is comparatively slow (light from a source bounces from the trolley into our eye. Brain interprets light patterns as pictures and makes sense of them. Neurons send messages from brain to motor neurons in hand muscles, muscles contract to press button and start clock). To make this more accurate, a new ramp could be set up, using automated lightgates and a laptop to collect highly accurate data.