NEET Physics - Chapter 17
Determination of Unknown Frequency – Complete Notes, Revision, Important Questions & Downloads
This topic focuses on the TOC subtopic Finding Tuning Fork Frequency Using Beats, where an unknown fork is compared with a known fork and beat count is measured before and after loading or filing. The page logic is decision-based: from x and x' behavior, you identify whether n_B was initially above or below n_A and then select n_B = n_A +/- x correctly. NEET tests this topic through short lab-style statements such as wax loading, beat increase, beat decrease, or beat becoming zero. A reliable solve path is to write both possibilities first, then use the post-loading trend to remove the wrong branch.
Application TopicBeats MethodNCERT-Aligned
Expected QuestionsQ
1
Usually one direct or mixed MCQ appears from beats-based unknown frequency determination, often coupled with loading or filing conditions.
Time Required⏱
1.0 h
About 25 minutes to lock case logic, 20 minutes for branch-selection drills, and 15 minutes for timed mixed wave MCQs.
Difficulty⚡
Medium
Formulas are short, but branch selection mistakes are common when students ignore how loading or filing changes the frequency gap.
NRI USA Curriculum GapUS
Bridge Needed
AP Physics treatment is usually broad on beats, while NEET expects fast conditional decisions from x' > x, x' < x, x' = x, and x' = 0 style stems.
3Subtopics
16Practice Questions
4Free Downloads
1.0 hPrep Time
Determination of Unknown Frequency Weightage and Trend
Waves and Sound - Topic 24| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2020 | 1 | 4 | |
| 2021 | 1 | 4 | |
| 2022 | 0 | 0 | |
| 2023 | 1 | 4 | |
| 2024 | 1 | 4 | |
| 2025 | 0 | 0 | |
| Estimated topic-linked asks in recent NEET-style papers | 4 | 16 |
Most questions test whether the student can decide between n_B = n_A - x and n_B = n_A + x after applying a frequency change operation.
High-error stems are those where beats remain same or become zero after loading; students often assume one fixed branch for all cases.
If filing is used instead of wax loading, direction of frequency change flips and the branch logic must be reversed accordingly.
High-error stems are those where beats remain same or become zero after loading; students often assume one fixed branch for all cases.
If filing is used instead of wax loading, direction of frequency change flips and the branch logic must be reversed accordingly.
0.7
Avg Questions / Year
16
Total Marks (6 yrs)
Mixed
Pattern
Medium
Difficulty
5-Step Branch-Selection Strategy
1
Start with both candidate equations Write n_A - n_B = x and n_B - n_A = x before touching any modified beat value; this prevents early sign errors.
2
Tag the operation direction Mark whether fork frequency is decreased (loading with wax) or increased (filing), and identify which fork was changed.
3
Compare x' with x structurally Use the trend x' > x, x' < x, x' = x, or x' = 0 to infer whether the modified fork moved farther from or closer to n_A.
4
Resolve original branch only then compute After trend inference, pick only one branch for initial n_B and evaluate final numeric value once.
5
Run one consistency check Substitute the inferred initial n_B into post-modification story to verify that the beat trend in the stem is reproduced.
Determination of Unknown Frequency Download Kit
PDF · Cheat Sheet · MCQ Set · PYQFull Notes
Theory notes for beats method, two-possibility setup, and all four x' conditions with branch-wise unknown-frequency selection.
Download PDFFormula Sheet
Quick relation sheet for beat frequency, n_B branch equations, and loading/filing direction rules used in objective solving.
Download PDFMCQ Practice
Practice set emphasizing case discrimination when x' is greater, smaller, unchanged, or zero after controlled frequency modification.
Download PDFPYQ Workbook
Year-tagged beats and tuning-fork objective questions with branch-selection annotations and sign-check checkpoints.
Download PDFSubtopics in Determination of Unknown Frequency
2-Column Table| Column A | Column B |
|---|---|
Rapid Revision Cards
Concept → Trap → Example1) Finding Tuning Fork Frequency Using Beats
Branch logic from beat changeWith known fork frequency n_A and unknown n_B, first use |n_A - n_B| = x. After loading or filing one fork, use x' trend to decide sign and infer initial n_B as n_A +/- x.
- Apply this in stems where a fork is loaded with wax (frequency decreases) or filed (frequency increases) before recounting beats.
- Always map the operation to frequency direction first, then compare whether frequency gap with n_A grows or shrinks.
- Trap: assuming x' < x always means n_B = n_A - x without checking which fork was modified and in what direction.
Example (NEET-style)If n_A = 256 Hz and initial beats x = 4, then n_B could be 252 or 260 Hz. If unknown fork B is loaded and new beats become x' = 2, B moved closer to n_A after decrease, so initial B had to be above n_A. Therefore n_B = 260 Hz.
US Curriculum Gaps - Determination of Unknown Frequency
Two concrete preparation gaps to bridge for NEET readinessAP Physics 1 often treats beats qualitatively, NEET demands conditional branch resolution
Many US classroom problems stop at beat frequency calculation, but NEET asks inverse reasoning from beat change after controlled loading or filing to recover the original unknown frequency.
- Practice stems where the same x appears before and after modification, then justify why one branch still survives.
- Train with four-condition tables: x' > x, x' < x, x' = x, x' = 0 for both known-fork and unknown-fork modification.
AP Physics C students compute well but lose time on sign logic in objective papers
Even strong algebra users miss marks in NEET when they do not explicitly track whether the modified frequency moved toward or away from the known fork during the second beat count.
- Use a fixed three-line scratch routine: operation direction, gap trend, initial branch decision.
- After solving, replay the story mentally to check whether predicted x' trend matches the question statement.
Concept IQ Check
2 MCQs1A known tuning fork A has frequency 300 Hz. With unknown fork B, it gives 6 beats per second. Fork B is loaded with wax and new beat frequency becomes 2 beats per second. The original frequency of B is:Finding Tuning Fork Frequency Using Beats
294 Hz
306 Hz
298 Hz
312 Hz
Initial beat relation gives |300 - n_B| = 6, so n_B can be 294 Hz or 306 Hz. Loading B decreases its frequency. If B were 294 Hz, loading would move it farther from 300 and beats would increase above 6, which contradicts x' = 2. If B were 306 Hz, loading moves it toward 300 and beats can decrease from 6 to 2, exactly as stated. Hence original n_B = 306 Hz. Option 294 fails the trend test, 298 does not satisfy initial beat count 6, and 312 implies initial beat count 12.
2A known fork A of frequency 256 Hz is sounded with unknown B and 3 beats per second are heard. B is filed slightly and then no beats are heard. The initial frequency of B was:Finding Tuning Fork Frequency Using Beats
253 Hz
259 Hz
256 Hz
250 Hz
From first observation, |256 - n_B| = 3 so candidates are 253 Hz and 259 Hz. Filing increases frequency of B. After filing, no beats means new B frequency equals 256 Hz. That can happen only if initial B was below 256 and moved upward to meet it, so initial B must be 253 Hz. If initial B were 259 Hz, filing would increase it further and the beat frequency would not become zero. Option 256 is impossible because initial beats are non-zero, and 250 gives beat 6, not 3.
Practice Questions
Click "Reveal Answer" after attempting1A fork of known frequency 512 Hz gives 5 beats/s with an unknown fork. Unknown fork is loaded and beats become 9 beats/s. Find unknown frequency before loading.
507 Hz
517 Hz
503 Hz
521 Hz
👁 Reveal Answer
Correct option: A. Initially unknown can be 507 Hz or 517 Hz because |512 - n_B| = 5. Loading decreases unknown frequency. If unknown were 517 Hz, loading would move it toward 512 and beats should reduce, not rise to 9. Since beats increase to 9, unknown must have been below 512 so loading moved it farther away. Therefore n_B = 507 Hz.
2Known fork A = 400 Hz gives x = 4 beats/s with unknown B. A is loaded with wax and now x' = 0. What is initial B?
396 Hz
404 Hz
400 Hz
392 Hz
👁 Reveal Answer
Correct option: A. Initial possibilities are 396 Hz or 404 Hz. A is loaded, so known frequency decreases from 400. If x' becomes zero, new known frequency must match fixed unknown B. That is possible only when B is below 400 so A can decrease to it. Hence B = 396 Hz. If B were 404 Hz, decreasing A would move away further and never produce zero beats.
3A known 250 Hz fork and unknown fork produce 2 beats/s. Unknown is filed and beats become 6 beats/s. Initial unknown frequency is:
248 Hz
252 Hz
244 Hz
256 Hz
👁 Reveal Answer
Correct option: B. Initial candidates from |250 - n_B| = 2 are 248 Hz and 252 Hz. Filing raises unknown frequency. If unknown starts at 248 Hz, filing moves it toward 250 and then beyond; a slight filing generally first reduces beats toward zero, not directly to a much larger value. If unknown starts at 252 Hz, filing moves farther from 250, so beats increase from 2 to 6 naturally. Therefore initial n_B = 252 Hz.
4With known fork n_A, unknown B gives x beats. After loading B, x remains unchanged. Which initial relation is valid in textbook case logic?
n_B = n_A - x
n_B = n_A + x
n_B = n_A
Cannot be determined
👁 Reveal Answer
Correct option: B. In the page's listed conditions for loading unknown B, x' = x implies the new frequency differs from n_A by the same amount as before but on opposite side after decrease. That is possible when initial B was higher than A. So initial relation is n_B = n_A + x. Option A belongs to different trend logic, option C contradicts nonzero beats, and option D is incorrect because condition table resolves it.
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Frequently Asked Questions
Notes · Downloads · Revision · Important QuestionsWhy do we start with two possible values for unknown frequency?
Beat frequency gives only magnitude of difference, not direction. From x beats/s, we only know |n_A - n_B| = x, which permits n_B = n_A - x or n_B = n_A + x. The second experiment after loading or filing provides directional information. That second trend is what removes one branch and gives the true initial value.
How does loading a tuning fork change its frequency?
Loading with wax increases effective mass of a prong and lowers its natural frequency, while filing removes material and raises the frequency. In this topic, that direction change is the key control input. Without marking this step first, students often pick the wrong branch even when arithmetic is otherwise correct.
What does x' = 0 physically mean in this method?
x' = 0 means no beats are heard after modification, so both forks now have the same frequency. This does not automatically mean the unknown was equal initially. It means the modified fork moved exactly to the other fork's frequency. You still need to infer initial sign by checking whether the applied operation could have moved it in that direction.
Why is x' = x still informative instead of ambiguous?
Equal beat count before and after modification can still reveal initial branch because the modified frequency can cross to the other side while preserving absolute difference. The chapter's case table explicitly treats this condition. In objective questions, this is a favorite trap because many learners assume unchanged beats means no useful information.
Can I use only formulas and skip case reasoning?
Not safely for NEET. The method is formula plus directional logic. Formula alone gives two candidates; case reasoning from operation direction and beat trend selects one. Fast and accurate solving comes from a fixed decision sequence rather than memorizing isolated final statements.
What is the most common error in this topic?
The most frequent error is applying a memorized branch like n_B = n_A + x for all stems without checking which fork was modified and whether its frequency increased or decreased. A close second is ignoring whether the post-modification beat count moved up or down. Both errors produce sign mistakes that look numerically plausible.
Does this method apply only when frequencies are very close?
Yes, practical beat counting requires slightly different frequencies so that distinct waxing and waning are audible. The chapter notes that frequencies should be nearly equal for distinct beats. In exam problems, this condition is assumed unless a stem explicitly indicates that beats are not clearly observed.
How should I revise this topic one day before exam?
Revise it as a four-case decision drill, not as a paragraph. Practice short stems for loading known fork, loading unknown fork, filing known fork, and filing unknown fork, each with x' trends. For each stem, force yourself to write one-line direction logic before selecting branch. This process reduces avoidable sign errors in timed sections.
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