10.2 Mass, Volume & Surface Area

10.2 Mass, Volume & Surface Area

Quick Answer: After the solid is complete, material is applied, and units match the question, run Evaluate → Mass Properties. Report mass (usually grams, two decimal places under MMGS) as the graded answer. Treat volume and surface area as diagnostics. Stay inside the exam’s tight multiple-choice band (about 1% when choices are close). No partial credit—one wrong digit path (missed cut, wrong density, wrong units) yields zero.

If Section 10.1 made density automatic, this section is how you read and trust the Mass Properties dialog under exam pressure. Nearly every basic, intermediate, and advanced part creation item ultimately asks for a mass (sometimes after a key-dimension modification). Assembly blocks often ask for assembly mass as well. Precision here converts modeling skill into points.

Opening Mass Properties

Typical path (wording varies slightly by version):

1. Complete features; Rebuild (Ctrl+B / traffic-light green).
2. Confirm material and document units.
3. Evaluate tab (or Tools menu) → Mass Properties.
4. If prompted, select the part, bodies, or assembly components to include (default is usually the active part or full assembly).
5. Read the listed Mass, Volume, Surface area, and later Center of mass coordinates (Section 10.3).
6. Optionally click options related to coordinate system (for CoM relative to a custom system—next section).

On assemblies, ensure the configuration and suppressed/resolved states match the problem before measuring.

What each primary output means

You almost never type volume or surface area as the final CSWA answer, but you should glance at them. A volume that barely changed after a Through All hole pattern is a red flag that cuts did not fire.

Units displayed must match the question

The dialog shows values in the document unit system. The question stem or answer blank specifies what to report:

Unit disasters:

• Model in inches, answer expects grams of a steel part → mass in lb looks “small,” conversion forgotten → wrong choice.
• Report kilograms when the blank says grams (divide/multiply by 1000 error).
• Change units mid-problem without rebuilding mental scale of expected mass.

Before submitting, re-read the unit in the question one more time. CSWA is merciless on unit mismatch because keys are absolute.

Decimal places and numeric entry

Prep materials and common CSWA practice emphasize:

• Report mass to two decimal places when using metric mass (e.g., 245.67 grams), unless the interface specifies otherwise.
• Do not round aggressively early; build accurately, then round only for the final reported value per standard rounding if needed.
• For fill-in numeric answers, type exactly what Mass Properties shows at the required precision (or the nearest value consistent with exam instructions).
• For multiple choice, pick the option within the acceptance band of the calculated value—not the option that matches a previous attempt’s wrong material.

About the ~1% multiple-choice idea

When answers are multiple choice, distractors are often spaced so that small modeling errors (wrong fillet radius, one missed pattern instance) land on a wrong choice, while the correct solid+material lands uniquely on one option. Industry prep discussion often cites staying within roughly 1% of the key for credit on choice items. Practically:

• If your mass sits halfway between two choices, do not average and pick—re-check geometry and material; something is incomplete.
• If you are within a fraction of a percent of one choice and far from others, that choice is likely correct.
• Numeric entry items may require still tighter agreement with the internal key; treat two-decimal display discipline seriously.

There is no partial credit: a mass that is 5% high because one hole pattern was short is still a full miss on that question’s points (often 15 or 30 points).

Procedure after a modification step

Many part questions are two-stage:

1. Build to the initial drawing → Mass1.
2. Change a key dimension (length, thickness, hole Ø, pattern count, etc.) → rebuild → Mass2.

Rules:

• Edit the driving dimension or feature parameter; do not rebuild a parallel wrong model.
• Resolve any rebuild errors before measuring.
• Re-open or recalculate Mass Properties—do not submit the pre-modification number by habit.
• Material usually stays the same; units stay the same; only volume (and thus mass and CoM) should change in the expected direction.

If the trend is opposite your expectation, you edited the wrong parameter or a feature failed silently.

Using volume and surface area to debug

Mass too high vs answer key / choices:

1. Wrong or missing material? (lighter material applied than specified, or default weirdness)
2. Missed cut, hole, pattern instance, or mirror of a cut
3. Blind cut not Through All
4. Suppressed feature that should be active
5. Fillet/chamfer smaller than specified (less removal) or missing internal removal features
6. Units: reading g but thinking the choice is in another unit

Mass too low:

1. Extra cut or pattern instance
2. Shell removed too much / wrong faces
3. Fillet radius too large on external edges
4. Material denser than specified? (less common if library matched) or wrong heavier→lighter mistake inverted
5. Multibody with a body excluded, or base not merged so measurement set is incomplete (context-dependent)

Volume unchanged after a cut you thought you made: cut feature failed, sketch open, wrong contour selected, or feature suppressed.

Surface area huge vs expectation: many extra faces—patterns of holes, shell inside/out, unmerged thin bodies. Use section view to confirm hollow regions.

Worked numeric discipline example

Setup: MMGS, material Alloy Steel, finished part Mass Properties shows:

• Mass = 1283.457… g (dialog may show more digits)
• Volume = some mm³ consistent with size
• Surface area = …

Question: “Enter the mass of the part (grams) to two decimal places.”

Report 1283.46 g (standard half-up rounding from 1283.457) or exactly as the exam software and dialog convention require—follow on-screen Mass Properties rounded display if the exam instructs to use displayed values. The habit is: correct material + correct solid + correct unit + two decimals.

If choices are 1205.20 g / 1283.46 g / 1350.10 g / 2566.92 g, select 1283.46 g. 2566.92 might be roughly double density or double volume error; 1205.20 might be wrong alloy or missed feature volume.

Pre-submit Mass Properties checklist

1. Rebuild with no unresolved errors on required features.
2. Material matches drawing; no mass override.
3. Document units match answer units.
4. All cuts, patterns, mirrors, shells, holes present and unsuppressed as drawn.
5. Modification step applied if this is stage two.
6. Mass Properties recalculated on the correct part/assembly/config.
7. Value copied with correct decimals and unit interpretation.
8. Sanity: is the mass plausible for the size and material (steel fist-sized part is thousands of grams, not 5 g unless tiny).

Link forward

Mass is a scalar. Many items also grade where that mass balances—center of mass X, Y, Z—which depends on the same solid and material but on the coordinate frame. Section 10.3 covers origin choice, custom coordinate systems, signs, and the classic traps that flip one coordinate while mass still looks perfect.