Experiment to find the terminal velocity of a ball travelling in glycerol

Introduction

In this piece of coursework, The subject of terminal velocity is going to be discussed, and a terminal velocity experiment will be performed.

Before I start my coursework, I will go through what velocity is and how it is different to speed.

Velocity is:

The distance travelled in unit time in a stated direction

Whereas, speed is the distance travelled in unit time, without knowing the direction.

Terminal velocity is similar to velocity. This is when an object falling in the airs air resistance (or in the case of this experiment, fluid friction) acting upward equals the weight of the object acting downwards. When this happens, the resultant force, is zero because of the opposing forces being balanced. Now, the object will be falling at a constant velocity, called its terminal velocity. The terminal velocity of an object would depend on its size, shape and weight.

For example, a small object like a raindrop, would have a low terminal velocity, and would only accelerate over a short distance before air resistance equals its weight. A denser object like a steel ball bearing, would have a high terminal velocity and would fall a considerable distance with a constant velocity before air resistance equals its weight.

Prediction

I predict that as the ball falls through the glycerol, it will slowly start to slow down in acceleration due to air resistance. Then, it will flow at a constant velocity because of the two opposing forces becoming equal, or zero.

I think that the big ball will fall at a faster speed, and will take longer to reach its terminal velocity. Whereas, the small ball will take longer to fall, as it is not as dense as the big ball, so this means that it will take a shorter time to reach its terminal velocity.

Method

Apparatus

· ...

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Prediction

Method

Apparatus

· Long tube

· Glycerol

· Rubber bung (to hold glycerol in the tube)

· Long boss and clamp

· Two steel ball-bearings (one small, and one larger)

· Meter rule

· Stop watch

· Powerful magnet

The reason I have chose to use ball bearings is because they are dense, so they will give me much better results. Also, they can be varied in size, so I can get a dense, and a less dense one to try out in my experiment.

Also, I have chose to use glycerol because it is a lot more dense than water, so the ball will flow a little slower in it, this way I can get more accurately timed results.

Now, I will go through how I will do my experiment.

First of all, set out the apparatus as shown in the diagram below. To make it a fair experiment, the ball must be dropped from the same height each time it is dropped into the glycerol. Also, the stopwatch must be started as soon as the ball hits the glycerol to make it a fair test.

After you have recorded the results for 10cm, and the ball has gone past the 10cm mark, then to get the ball out you must use the powerful magnet. Once the ball is out, you must drop the ball back in and record the time taken for it to get passed the 20cm mark, and start the procedure again for 30cm, 40cm and 50cm. You must do this at least twice with each of the balls, as this will give you a more accurate average.

You must also follow a safety procedure. One of the main safety issues would be to keep the boss and clamp with the long tube attached to it on a flat surface at ALL times. You must also move all stools and bags from the experimental area, so no one trips over any of them.

Diagram

Conclusion

In conclusion, as you can see from my graphs and results tables, my initial prediction was correct. I predicted that the ball would slow down in the glycerol, due to fluid friction, then would reach a constant velocity (terminal velocity). I have marked were I think the terminal velocity of the ball would begin on the graph. There is also a trend formed in the graphs. This is that the shapes of the graphs are both in the same sort of curve. Both graphs show the line getting steeper during the end of the line. This is the terminal velocity.

Also, I predicted that the small ball would reach terminal velocity quicker, because the two forces (fluid friction and the downward action of the ball) would become equal quicker than the larger ball. This in fact, was wrong. I have found that the larger ball did in fact reach terminal velocity before the smaller ball. This could have been due to a mistake during the experiment.

Evaluation

I have found some of the aspects of my experiment quite pleasing, however there are also some things that have not gone as well.

The graphs show that the terminal velocity of the small ball to come after the terminal velocity of the large ball. This goes against my prediction, where I said the opposite. I think this may have been due to the lack of accuracy during my experiment.

However, another thing that I predicted did go as planned. This is when I predicted that both balls would eventually slow down and reach a terminal velocity.

To make the procedure more accurate, I would, firstly, make sure that I did the experiment one or two more times to get a more accurate average at the end.

Another thing I would have done is to use other objects to drop into the glycerol. For example, I could have used small pebbles to see if the same rule applied to them. I could also use different types of liquid such as vegetable oil, to see if the same rule applied to that too.

In all, I think that the evidence I have come up with may be enough to form a firm conclusion, but certain things could be done to make this conclusion firmer.