Physics: Post your doubts here!

Thought blocker

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I Believe said:
no no...I disagree, in fact totally disagree :p
If a person's coming into car decreased it's speed then his going away must increase the speed
why shouldn't it?
Click to expand...
Close ur eyes xD Just imagine a small truck u kept send in it, when the sand entred the speed decreased, then when the doors or something opened at the bottom of truck, the sand leaves the truck or trolley and speed will be the same.. what u did to increase the speed?
 
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Some important tips for physics Paper 5.
Question 1 (Planning Experiment)
- When answering, read the question given carefully. You have to design a suitable experiment based on the question, either to test whether the suggested relationship between 2 variables is valid or to determine the value of a constant. Your answer must include independent variable, dependent variable, variables to be kept constant, diagram of set-up apparatus, procedure, data analysis, safety precautions and additional details.
- For the independent variable and dependent variable, they can be obtained easily from the question itself and you should give one of each. For the variables to be kept constant, there is often more than one and you should give all of them that you can think of. Note that the term 'kept constant' or 'keep constant' must be used in your answer and no other terms are acceptable. You need not answer in complete sentences.
- For the diagram of set-up apparatus, show all the apparatus and material used in the experiment and label them correctly. If a diagram is provided by the question, you can use the same diagram, but it is usually not complete and you have to add other apparatus or materials to it.
- When writing the procedure, you must include the steps to measure the manipulated variable, responding variable and every variable to be kept constant. For each measurement, you should state the method and apparatus used. For some variables, you may have to measure other quantities in order to calculate them. You are advised to answer in complete sentences so that your answer can be easily understood. You should also include other steps required to set up the apparatus if you did not show them in the diagram.
- For the analysis of data, you should explain how a suitable graph should be plotted that enables you to test whether the suggested relationship between the 2 variables is valid or to determine the value of the constant. Sometimes, the graph may be a log-log graph or ln-ln graph. You should also show a sketch of the graph. Then, explain how to determine whether the suggested relationship is valid or to determine the value of the constant based on the graph. This often involves the gradient, y-intercept and rearranging of equation.
- For the safety precaution, you should state at least 2 potential safety hazards when carrying out the experiment and the ways to avoid each of them.
- For the additional details, 4 marks will be given and examples of them are how variables are kept constant, calibration of the measuring instruments and additional steps to improve the accuracy and reliability. You may have already stated some of them in other parts of your answer and it is not compulsory to write them again in this part. In this section, you should state the additional details that are not stated in other parts of your answer.
- For all parts of the question, you are allowed to write extra points in your answer, but you must be careful not to write any points with wrong facts. Marks will only be given for the correct points. For points that are irrelevant but does not contain wrong facts, no mark will be given or deducted. However, for points that contain wrong facts, marks may be deducted.
- You should use the correct experimental and Physics terms in your answer. Do not replace them with other terms that are inappropriate, even if their meaning are the same. You should spell all experimental and Physics terms correctly. If you can't do so, try to spell it in such a way where it sounds the same as the actual term when read out. Marks are usually not deducted for spelling errors in experimental and Physics terms as long as it still sounds the same and that it is not easily confused with other terms. If you spell other terms wrongly or if you make grammatical errors in your answer, marks will not be deducted for as long as the examiner can understand what you are writing. You are allowed to use suitable short forms in your answer, especially for representing physical quantities or their units.

6. Paper 5 Question 2 (Analysis, Conclusions and Evaluation)
- When answering, read the question and all information given carefully. Make sure that you understand the experiment given and know what the question is asking for. Answer the questions based on the experiment and its observations and results. You may need to apply your Physics experimental skills to answer some questions. Certain questions may require your knowledge and understanding in Physics to answer them.
- For the graph, plot all points on the graph accurately. The points should be accurate to half a small square. Then, draw the error bars based on the uncertainties of the values given in the question. After that, draw line of best fit. The line should pass through as many points on the graph as possible, all the points should be close to it and the number of points above and below the line should be almost equal. Next, draw the worst acceptable line. It should be either the steepest possible line which passes from the top of top error bar to the bottom of bottom error bar or or the shallowest possible line which passes from the bottom of top error bar to top of bottom error bar. The line must also pass through all error bars.
- When determining the gradient of line of best fit, choose 2 points on the line and draw a triangle. The distance between the 2 points chosen should be at least half the length of the line. For the uncertainty in gradient, you should also determine the gradient of worst acceptable line using the same method. The uncertainty in gradient is equal to the difference between the 2 gradients.
- When determining the y-intercept of line of best fit, usually you cannot read it off directly from the y-axis of graph since the x-axis does not start from 0. Instead, you should choose a point on the line, preferably one of the points that you used to calculate its gradient, and substitute its x and y values as well as the gradient into the equation y=mx+c to determine the value of c which is the y-intercept. For the uncertainty in y-intercept, you should also determine the y-intercept of worst acceptable line using the same method. The uncertainty in y-intercept is equal to the difference between the 2 y-intercepts.
- When calculating the uncertainty in a calculated value, use your knowledge in uncertainties that you learnt in Chapter 2 for AS Level. However, if the calculation is too complicating or if it involves logarithms, there is an alternative method. For this method, you have to determine both the maximum value and minimum value of the calculated value by using the maximum values and/or minimum values of the raw values used in the calculation of the calculated value based on their uncertainties. You have to be very careful to ensure that the maximum value and minimum value you determined are correct. The absolute uncertainty is equal to half of the difference between the maximum and minimum value.
- For any questions involving calculation, the number of significant figures of your answer should be equal to or one more than the number of significant figures of the raw value used in the calculation with the least number of significant figures. For absolute uncertainties, the number of decimal places should be equal to the number of decimal places of the value. You should show all workings and do not skip any important steps. You must also write the correct unit for the final answer if it is not provided. You are not allowed to write extra solutions or answers. If you do so and any of the answers or solutions is wrong, marks will be deducted. For a calculation question which requires you to use your answer from the previous question, even if your answer for the previous question is wrong and you use it for this question causing your answer for this question to be wrong, usually you will still get full marks for this question as long as your calculation for this question is correct. This is known as 'error carried forward'.

Hope this helps! :)
 
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Kashaf Ali said:
View attachment 61484
could someone please do this....
Click to expand...
Ep = GMm/R
Change in Ep = GMm/change in R
That is Delta Ep = GMm*(1/R1 - 1/R2)
Thus coming to the question,
above earth surface is the TRICKY part of the question so the informatuon we have :
Earth radius : R
We have to find change in EP from R to 2R above earth surface now if u add earth surface R to this two Rs so
R1 = 2R and R2 = 3R

Use the formula ,
Delta Ep = GMm(1/2R - 1/3R)
Delta Ep = GMm/6R
 
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