Topic 1: Physics and Physical Measurements
In Physics we are concerned with observing the world around us so we can develop theories and test those ideas to deepen our understanding of the universe and how it works.
First step is to become better observers. Good observers can make good estimates based on physical intuition. This is difficult to teach, but gets better with practice. The best observations are ones that can be measured, thus allowing us to quantify our theories and make precise predictions which can be accurately tested.
WORDS, words, words. It’s important to get them right:
Accuracy and Precision and What do examiners mean
Once you have read these documents, write answers to the following questions:
1. What is the difference between a precise measurement, and an accurate measurement?
2. How does this diagram illustrate precision and accuracy?
You will do some simple measurements, but you will need to pay particular attention to the accuracy and precision of your measurements. Ignore the uncertainties for the moment.
Why do you think it is important for us to be able to make good estimates of the results we expect to get in experiments?
Now look at your results. How many significant figures have you quoted your answers to? Is this an appropriate level of precision given the accuracy of your measurements? This is always something to think about when you are making measurements and doing calculations with them. Remember to be appropriate in the number of significant figure (precision) you use for your results. You should beable to justify them.
Whenever you make a measurement you must quote an associated uncertainty to make the measurement meaningful, otherwise all you have is a number. At IB, this is as important in practical work as including units with your numbers.
If you are combining measurements to determine a quantity (like density or velocity), then you must also combine their uncertainties. This is much easier than most people think…
Now go back and estimate uncertainties for all the measured values in the previous exercise, and determine the uncertainties in the calculated values.
Now try these: Errors questions
Ref: Hutchings Chapter 1
We use graphs to visually analyse data. Drawing them well is an important skill to master. Graphs should have:
- Sharp points
- Titles, labels and units
- Line of best fit (not dot-to-dot)
- Error bars
What does this graph tell you?
Lines of best fit are drawn to go as close as possible to your data points. They can be used to assess if a relationship is linear or not. Lines of best fit are not necessarily straight, although when they are we can easily get a formula for the relationship from y= mx +c (the equation of a straight line).
Remember that m = gradient of the line (Dx/Dy) and c = intercept of the line with the y axis of the graph.
How could you draw a straight line graph for the data collected from the pendulum experiment?
Error bars are another property of graphs that you will draw in IB Physics. They give you the opportunity to analyse how good your experiment is and the range of uncertainty you may have in your calculated results.
For every data point you must add "error bars" to represent the experimental uncertainty you have estimated for that value...
This means a graph that originally looked like this...
Becomes...
You can be discerning about the line of best fit you apply to the data, and potentially identify anomolous results to retake or disregard.
You can also see how "accurate" your experiment is by considering the available variation in an acceptable line of best fit.
Graphing physical data exercises
Units are also very important in Physics, so as to give numbers meaning. We use standard units in physics which are governed by Le Systeme International d’Unites.
Research project on the SI system.
Fundamental units are the units of the 7 fundamental quantities. We can derive units for all other quantities from these (derived units).
Eg: The Newton, the Joule, the Pascal
Sometimes we have to quote quantities that are very large or very small in terms of the units used to define them. In this situation we use a system of prefixes...
Illustration of the range of quantities we have to deal with in physics
Introduction to PSOW: Who killed Miss Carter?
- A Scalar has size only. eg: speed, mass, distance
- A vector has size and direction. eg: velocity, force, displacement
We can deal with vector quantities in two ways...
-
We can add vector quantities using the law of parallelograms.(www.walter-fendt.ac.de)
- We can deal with more complex systems by defining a horizontal and vertical direction, and then dividing each vector into its constituant parts acting in each of these directions. All the components in the same direction can add directly, and the resultant can be deduced from the resulting horizontal and vertical comonents.
A mathematical bit...
A picture frame has forces acting on it in different directions because of vectors...
Hutchings pp23-33 q3.1-3.5, pp28-29:Analysis; Forces in Frameworks and pp32 q3.9-3.12
anrophysics 2007