The variables
The variables are the actual pulling force, the weight of the box, the type of surface, the angle of the surface and the speed at which you will pull it.
Air friction is another possible variable that could be considered but air friction at this sort of speed is not important. The surface area in contact might affect things, it could change the resistance.
The variables I will keep constant and the way I will keep them constant
The variables that I will keep constant are the actual pulling force, the weight of the box, the type of surface, the surface area and the speed at which you will pull it.
I’ll need to keep the small surface otherwise the force won’t be big enough and it needs to be horizontal otherwise gravity will have an effect on it. I will always use the same surface, with the same wooden box; same surface area and I’ll pull it at the same speed by trying to get the same velocity horizontally each time. You could adopt a situation where you count to maintain a reasonably constant speed.
Diagram of the apparatus
See separate sheet.
The basic idea of the method
Start with a weight on the wooden block and then start to pull the spring balance so it will then start to slip or you could just give it a gentle push to release it and then you take the reading straight away.
The range of values for the weight of the box and how I decided on this range
I did a preliminary experiment, I put 30N on the wooden block but I couldn’t measure the pulling force with a 10N metre so I decided the range will go from 5N to 25N, going up in 5N Newton each time.
The measurements to be taken
Observations
Results table and calculation of averages
(table from excel or a hand drawn one)
Graph of pulling force against load
See attached sheet
(Either a full page hand drawn or detailed computer drawn one)
Conclusions
This graph is a straight line but it does not go through the origin.
This shows that it looks quite close to go through the origin.
The rule of the experiment is equal changes in load, gives you equal changes in pulling force. This will apply wherever the line goes.
10N on my graph gives me 3.7N.
20N I end up with a force of 6.8N.
The results roughly show double one, double the other.
10N 3.7N 7.4N
20N 6.8N 0.6N
0.6 x 100 = 8.8%
6.8
If difference was less than 5% I am reasonably certain it is the rule.
My results show the difference is 8.8%, it shows it is possible but not certain.
I make the conclusion that if you double the load, you double the pulling force. It is not that reliable on my graph because it doesn’t go through the origin so if you double the load it doesn’t exactly double the pulling force.
The conclusions do agree with my original predictions because the graph I predicted is a straight line, the graph is a straight line therefore it suppors my predictions.
Evaluation
In carrying out the experiment I found it difficult to get the reading because it kept on wobbling. I found that the boards weren’t long enough. If you pulled it along you don’t have much time to take the reading.
I tried to overcome this by taking a lot of repeat readings and the taking an average.
I could improve my procedure by redesigning the apparatus.
Comments about the reliability of the evidence
The results on the graph are closely grouped along a straight line and there is no other line that can be drawn. I think that my evidence is reliable and that my results support the conclusion I have made.
The results on the graph so a wide variation. Although a straight line can be drawn which passes through the origin can be drawn, there are others which can be drawn which do not. I think it is likely that the line passes through the origin but it is not absolutely certain.
