ACT Science: What is an “Assumption”?

In an argument, an assumption is an underlying basis for a conclusion. It is something that needs to be true in order for a certain conclusion to be based on specific evidence. We can think of this relationship like a math problem: Evidence + Assumption = Conclusion. The assumption is the linking factor. On the ACT Science Test, assumption ACT test questions will either ask you to apply a given set of information to a new situation or ask you to correctly identify the basis for a conclusion.

Assumption Practice Question

Let’s look at an example Conflicting Viewpoints passage.
The velocity with which a small body A orbits a much more massive body B depends on the mass of body B and the distance between the two bodies: If body B is twice as massive, body A will orbit it at twice the velocity, and if body B is more distant from body A, it will orbit more slowly.
These relationships allow astronomers to predict the velocities at which stars in a galaxy should orbit the center of the galaxy, based on the total mass of the visible stars.  In certain cases, however, the measured velocities of the stars in a galaxy are greater than predicted.  This indicates that the galaxy must contain additional material (often termed “dark matter”) which astronomers are unable to see.
Figure 1 shows how dark the presence of dark matter alters the velocities of the stars in a hypothetical galaxy:  The lower “no dark matter” curve shows the theoretical velocities in a galaxy containing 100 million stars similar to the Sun in a sphere 5 kiloparsecs (kpc) in radius, and the upper “dark matter” curve shows the theoretical velocities if the stars are embedded in the center of a dark matter cloud that is much larger and much more massive than the visible galaxy.

Study 1 : Astronomers have measured the velocities of stars in the galaxy NGC 3198.  The results are shown in Figure 2.  The “no dark matter” model in Figure 1 would have predicted a decline in the velocities at distances greater than the 5 kpc radius of the galaxy, but instead they remain approximately constant.
Study 2 : Astronomers have measured the velocities of stars in 100 additional galaxies of approximately the same size (diameter and mass) as NGC 3198.  The velocities of the stars are approximately the same as in NGC 3198.

Question 1

Which of the following statements correctly describes a possible reason why many additional galaxies were examined in Study 2?
A  To determine whether they showed the same evidence of dark matter as NGC 3198.
B  To determine the theoretical relationship between velocity and distance from the center of the galaxy
C  To confirm that the velocities of the stars in these galaxies also match the theoretical “no dark matter” predictions
D  To determine whether the same velocity changes occur in much larger galaxies
We know this is an Assumption question because of the phrase “a possible reason.” Think back to the passage – what is a logical basis for the set-up of Study 2? Notice how Figure 1 shows that the theoretical curves for the “no dark matter” and “dark matter” cases differ greatly. The experimental observations for the NGC 3198 galaxy, shown in Figure 2, obviously match the shape of the theoretical “dark matter” curve and obviously do not match the shape of the theoretical “no dark matter” curve. This is clear evidence for the presence of dark matter in this particular galaxy, and a study of other galaxies with similar characteristics using the same technique could indicate whether dark matter is a typical feature of such galaxies. The answer is A.

Question 2

Which of the following statements can best explain why the velocities of the stars in NGC 3198, as shown in Figure 2, are different from the velocities of the stars in the hypothetical galaxy in Figure 1?
A  The stars in NGC 3198 have higher velocities because the mass of NGC 3198 is larger
B  The stars in NGC 3198 have higher velocities because the mass of NGC 3198 is smaller
C  The stars in NGC 3198 have lower velocities because the mass of NGC 3198 is larger
D  The stars in NGC 3198 have lower velocities because the mass of NGC 3198 is smaller
Again, we know we are being asked about an unspoken assumption because of the phrase “best explain why.” Here we have to “see through” to the core  of what the data actually means. Select any distance from the center of the galaxy, and compare Figures 1 and 2 at that distance. The velocity in Figure 2 (NGC 3198) will be higher than the value on either curve in Figure 1 (the hypothetical galaxy) so only answers A or B can be correct. The passage notes that the velocity of an orbiting body is higher when the mass of the body it orbits is greater. Thus, the higher velocities in NGC 3198 can be explained if the mass of NGC 3198 is larger. The answer is A.

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