Data collection
The trolley will be set up so that the string attached to it becomes taught at the point the trolley crosses the start of the horizontal board. This will enable us to measure the stopping distance of the trolley from the point that the weights begin to raise and cause the trolley to stop.
The furthest distance the trolley has travelled before it is pulled back by the weights (the front of the trolley) will be marked on the board with a marker. This is the distance required for the trolley to stop. The experiment will be repeated with five different trolley masses. The first will be the initial weight of the trolley and then the trolley with 1,2,3 and 4 masses added.
The experiment will be repeated four times at each mass to ensure a fair representation of stopping distance is shown. This will also eliminate any anomalous results.
Safety
- Basic lab rules will be followed at all times throughout the experiment such as not running
- The experiment will be carried out with fairly low masses to ensure the string does not snap
- The pulleys will be attached to the stool securely to reduce any chances of the string coming loose
- Someone will ensure there is no possibility of the trolley going off the edge of the ramp
Repetitions
The repetition of results is crucial to gain the best and most accurate results. The experiment will be repeated 4 times, for each of the masses, and an average will be taken, so that if any anomalous results occur then it will not affect the outcome as much as if only 2 recordings were taken for example.
Variables
In order to produce a fair test and allow for comparison of results there will only be one variable in the experiment. This variable will be the mass of the trolley. If anything else is varied there can be no connection made between results because you will not be seeing what the effect of mass is. To be able to investigate how mass affects stopping distance only the mass must be changed and the stopping distance measured.
To produce a fair test the many possible variables must be controlled and kept constant so they do not have any effect on results obtained. The variables to be kept constant are as follows: -
- The length of the trolley
- The weight of the trolley (without addition of mass)
- The length of the string
- The height from which the trolley is released
- The units of measurement used
- The mass used to stop the trolley (braking force)
These are the control factors in the experiment.
Preliminary work
A preliminary experiment must be carried out ion order to determine the weight that should be used to replicate braking force. To find the most suitable weight for this particular experiment the trolley will be used without additional mass i.e. the mass of the trolley will be kept constant. If the trolley mass is kept constant we can easily see what effect the weight is having on stopping distance. This is vital in order to carry out an experiment where it is possible to make the most accurate observations and where a wide range of results is easily recorded.
By placing a variety of weights on the string and releasing the trolley down the ramp we got an indication of how far the trolley is likely to travel and how much weight will be required to stop the trolley. We experimented with weights in denominations of 1N.
Result
From the preliminary experiment it was possible to conclude that the optimum weight to be used for braking force will be 5N. This allows us to get the greatest range of accurate results.
Prediction
When a mass is raised and there are no opposing forces acting upon it, the potential energy gained will be converted to kinetic energy as it falls back to earth due to the gravitational pull. The potential energy gained must therefore be equal to the kinetic energy it produces due to the theory of energy conservation- energy can not be created or destroyed. This is shown in the formula: -
mgh = 1/2mv2
From this formula we can assume that the mass of the trolley will have a direct correlation to it’s stopping distance. It is logical to predict that if the mass of the trolley (m) is increased the potential energy it gains will be larger and consequently the kinetic energy it is converted to will be greater. Therefore it is logical to predict that the greater the mass of the trolley the greater the stopping distance will be. Because the kinetic energy will be greater as mass increases so will the force required to stop the motion and as the force used to stop the trolley is kept constant the stopping distance will increase.
Obtaining evidence
Control factors
- The length of the trolley
- The weight of the trolley (without addition of mass)
- The length of the string
- The height from which the trolley is released
- The units of measurement used
- The mass used to stop the trolley (braking force)
Safety
- Basic lab rules will be followed at all times throughout the experiment such as not running
- The experiment will be carried out with fairly low masses to ensure the string does not snap
- The pulleys will be attached to the stool securely to reduce any chances of the string coming loose
- Someone will ensure there is no possibility of the trolley going off the edge of the ramp
Apparatus
- Stool
- 2 ramps
- Trolley
- String
- Marker pen
- Weights
- 2 pulleys
- Metre stick
Diagram
Method
A wooden stool was stood on top of a desk. One end of a wooden board was placed on the cross bar of the stool. A second, longer wooden board was aligned horizontally at the foot of it.
Two pulleys were attached opposite each other on the top part of the stool. A length of string was threaded through the pulleys and the 5N mass was attached to one end. The string was attached to the rear of the trolley at the start of the horizontal board with the weights on the table and the string pulled taught.
The trolley was then released from a set point on the slope and the distance it travelled along the horizontal board was recorded. This was repeated using the 5 different trolley masses.
Results
Analysing evidence and drawing conclusions
Conclusion
As the graph clearly shows there is a strong positive correlation between the mass of the trolley and the stopping distance. This is because the potential energy of the trolley when raised is converted into the kinetic energy of the trolley as it goes down the slope. As the mass of the trolley affects the potential energy of the trolley it must affect the kinetic energy of the trolley as it goes down the slope. Both potential energy (mgh) and kinetic energy (½mv2) are affected by the mass of the trolley, if it is increased then so are the potential energy and the kinetic energy. This means that if the kinetic energy is greater then an equal breaking force being applied in each case will take a longer distance to stop the motion of the trolley as the mass of the trolley is increased. As the kinetic energy is affected by the mass this means that the mass is directly proportional to the stopping distance as clearly shown in the graph.
This was the theory on which I based my prediction and it is clearly matched here in the results, as the correlation is strong.
Evaluating Evidence
I think that the method used in this experiment was a good way carrying out the investigation because it enabled us to gain results over a range of trolley weights. This had to be tested during the pilot practical. This combined with the repetition of the experiments meant that we had a wide range of data enabling us to find any anomalous results and find a clear correlation.
What Improvements Would Be Made If Redone?
1. Reduce the friction of the system.
2. Make the trolley more aerodynamic.
3. Electronically measure the stopping point for more accuracy with results.
4. Do the experiment more times.
Accuracy of Results
The results were measured very carefully and to the best of human judgement. I would estimate the results were measured to within the nearest centimetre. This is easily accurate enough to show a clear distance between weights, accurate enough that the repeated results didn’t differ so much as a consequence of this inaccuracy and accurate enough to show the correlation and prove the theory.
