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3.B Making Claims from Theory & 3.C Justifying Claims

Science Practice 3 is about scientific questioning and argumentation. Skill 3.B is to apply an appropriate law, definition, theoretical relationship, or model to make a claim and Skill 3.C is to justify or support a claim using evidence from experimental data, physical representations, or physical principles or laws.

Skill 3.B will 20-25% of the multiple-choice section, while 3.C will only be 5-10% of the this section of the AP Physics 1 exam. Both will be only part of 35-45% of the free response dedicated to all of science practice 3. The individual skills do not have a % breakdown provided for the free-response section.

How will you recognize when to use this skill?

This type of question is essentially the miscellaneous category designed to test your ability to make a claim about or based on the Physics of the situation (3.B) and justify it by connecting it to the Physics principles (3.C).

For skill 3.B the question will just be a general Physics question and there is no pattern other than not being the other styles.

For multiple choice questions, you can recognize 3.C is being tested if the prompt includes something to the effect of "and gives a correct reason" in it and/or you see the answer choices list "because" in the choice, often times with duplicate claims.
For example, a question might ask "Which of the following correction describes the magnitude of the acceleration of the cart during the time interval shown and gives the correct reason?
A) less than g because air resistance causes less net force,
B) less than g because the inertial mass of the system has been increased,
C) equal to g because the object is in freefall,
D) equal to g because m cancels out in the equation a = mg/m"

On the free response section, these will often be tested together where a question is asked, and you are also required to justify your response. There may be a few times where the justification from 3.C will not be a scored point, but you will still need to go through that justification yourself in order to be sure you are right, and you will not know which questions will not evaluate that point, so let's just use the following process on all general Physics questions.

Take a minute and laugh with me!

When students don't understand how to justify their claims in Physics they end up with big holes in their logic, like this magic comedian does in his explanation of what happened to this woman's rings.

How to approach making and justifying claims

Any time you attempt to make a claim about a physical situation you should use a the ABCD method to ensure your claim is correct and that you justify it in a way others will understand. Go through these steps mentally for a multiple-choice question and document them clearly on an open-ended response.

A. Answer the question (these are the claims in Science Skill 3.B)

Do the BCD first so you know what answer to choose. Very often choosing the answer first makes you force your way through an incorrect justification. Then you can either add the answer to the beginning of your argument or it at the end.
Box the simplest, most direct form of your answer.
If the FRQ has you place a checkmark next to your answer that is sufficient - and don't forget to do that!
examples: "the momentum of A is greater," "No," or "the mechanical energy decreased."

B. Basic Physics

Basic Physics can include facts, principles, definitions, relationships, and, if not prohibited, equations that should be part of any Physics notebook because they are statements that are true about the universe in general and we are bringing up because we will apply them to this situation.
Select what physics to bring up based on what will connect the answer to the terms and data in the question prompt.
If you don't remember the name of the law, that is OK. Just describe the important part of it.
examples: "average speed = distance divided by time," "area under a velocity (time) graph is equal to displacement," "total linear momentum in an isolated system remains constant," or "Static friction adopts the value required to prevent an object from sliding on a surface."

C. Cite Evidence

Evidence comes from the question information given about the scenario. It can be data from a data table, trends or values from a graph, details or descriptions from the prompt or a diagram, etc.
If you are unsure if it is evidence, try starting your statement with "In this situation you can see . . . "
See the section below on using correct descriptions of trends in graphs. Students often make mistakes with these.
Select what evidence to bring up by thinking about the question is asking and what physics principles can apply to the situation.
examples: "the graph shows that v = 6 m/s at t=3 s," "the slope of the graph for A is greater than for B," "in this situation there are no external forces," or "the velocity of the 2nd trial is triple the first trial."

D. Draw it Together (these are the justifications in Science Skill 3.C)

Connects the basic physics and evidence to the answer.
Sometimes this can involves connecting several elements of basic physics with several bits of evidence.
For calculations this can be the manipulation and substitution steps.
This step might not be needed for simple questions where the physics and evidence lead naturally and easily to the correct answer.

See examples below.

Example scenario

The graph at right shows the velocity as a function of time for a cart of 0.5 kg moving in one dimension, where positive is defined as to the right.

a) Does the cart end to the right, left or at the location it began?

b) Is the energy of the cart increasing, decreasing, or remaining constant at t=40 s?

c) Is the net force at t=5 greater, lesser, or equal to the net force at t=15?

d) What is the impulse exerted on the cart from t=0 to t=35s?

Try these and expand to see an example justifications. Notice that the order of ABCD is not the same in any of these examples.

a) The cart ends to the left of where it started. Displacement on a velocity (time) graph is equal to the area under the graph and the graph shows there is more negative area (trapezoid under X axis) than there is positive area (triangle above x axis). Since the total area is negative, the displacement is negative. Displacement is equal to the final position minus the initial position, and since negative is to the left in this situation, then a negative displacement means the final position is to the left.

b) Kinetic energy is directly related to the magnitude of speed squared by K = 1/mv^2 and at t=40 s the speed is decreasing, since v is approaching 0. Since the speed is decreasing and the kinetic energy is directly related to the speed squared (this drawing it together is optional, but good) then the kinetic energy must be decreasing.

c) Since slope of a velocity (time) graph is acceleration and ΣF=ma then whichever line segment has a greater magnitude of slope must have a greater net force acting on it (since the mass is constant). The slope of the line segment t=5 is part is less steep than the slope t=15s is part of. Thus, the net force is less at t=5s than at t=15s.

d)
m = 0.5 kg, J = FΔt = Δp = m(v_final - v_initial)
v = -3 m/s at 35 s, J = (0.5 kg) (-3 m/s - 0 m/s) = -15 kg m/s
v= 0m/s at 0 s

Describing trends in graphs

You should be able to describe graphs relationships by looking at them and matching them to the following trends

- A dependent relationship just means that one changes the other. This can describe all relationships except constant (not shown above, but where the y value does not change the X or vice versa).
- A positive correlation is a general term that indicates that when one value increases the other does as well. Use this term only if you are simply claiming if a value increased or decreases. Whenever possible use these more specific terms:
  - A linear relationship is represented by a linear function y=mx+b
    - A direct, also known as directly proportional, relationship is a special kind of linear relationship in that the b value is zero. so, y=mx. It must pass through the origin. This is the type where doubling one value will also double the dependent variable.
- A negative correlation indicates that when one value increases the other decreases. Use this term only if you are simply claiming if a value increased or decreases. Whenever possible use thes more specific terms:
  - An inverse relationship (also called inversely proportional) follows the equation y=m/x or y=m(1/x).
    - There is debate about what indirect means and I would avoid it's use. Some people consider it the same as inverse while others use it for any negative correlation.
  - An inverse square relationship is tough to tell from a simple inverse relationship - you need to linearize it to tell the difference.

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