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# 2: Comparing Model and Experiment

Learning Objectives

• Be able to estimate orders of magnitude.
• Understand units.
• Understand the process of building a model and performing an experiment.
• Understand uncertainties in experiments.

In this chapter, we will learn about the process of doing science and lay the foundations for developing skills that will be of use throughout your scientific careers. In particular, we will start to learn how to test a model with an experiment, as well as learn to estimate whether a given result or model makes sense.

Prelude

Newton’s Universal Theory of Gravity predicts that objects near the surface of the Earth will fall with an acceleration of $$9.8\:\text{m/s}^{2}$$ . Your friend reports that they have measured the acceleration of a falling ball and found that it was $$(9.0 ± 0.5)\:\text{m/s}^{2}$$. Does their result invalidate the prediction from Newton’s Theory?

1. Yes, since the range $$(9.0 ± 0.5)\:\text{m/s}^{2}$$ does not include $$9.8\:\text{m/s}^{2}$$.
2. Not necessarily, as it depends on whether your friend correctly determined the uncertainty in their measurement.
3. Definitely not, since Newton’s Universal Theory of Gravity has been confirmed by many experiments.

• 2.1: Orders of magnitude
One of the most straightforward ways to estimate if a model makes sense is to ask whether it predicts the correct order of magnitude for a quantity. Usually, the order of magnitude for a quantity can be determined by making a very simple model, ideally one that you can work through in your head. When we say that a prediction gives the right “order of magnitude”, we usually mean that the prediction is within a factor of “a few” (up to a factor of 10) of the correct answer.
• 2.2: Units and dimensions
“Dimensions” can be thought of as types of measurements. For example, length and time are both dimensions. A unit is the standard that we choose to quantify a dimension. For example, meters and feet are both units for the dimension of length, whereas seconds and jiffys1 are units for the dimension of time. When we compare two numbers, for example a prediction from a model and a measurement, it is important that both quantities have the same dimension and be expressed in the same units.
• 2.3: Making Measurements
Having introduced some tools for the modeling aspect of physics, we now address the other side of physics, namely performing experiments. Since the goal of developing theories and models is to describe the real world, we need to understand how to make meaningful measurements that test our theories and models.
• 2.4: Summary
• 2.5: Thinking about the material
• 2.6: Sample problems and solutions