NEET Physics Trick: Speed of Light = 1 Unit | Distance = Time Shortcut Explained

 - Dr.Sanjaykumar Pawar 

When speed of light becomes 1 unit/s, distance turns equal to time in seconds — the fastest NEET shortcut!

🔗 INTERNAL LINKS 

Units and Measurements Full Chapter Notes

NEET Physics Formula Sheet PDF

Speed, Velocity & Time MCQ Practice Set

Assertion Reason Questions NEET Physics

Dimensional Analysis Tricks

NEET Physics Important Shortcuts (All Chapters)

Previous Year Questions (PYQ) Physics Class 11

CBSE Class 11 Physics

Chapter: Units and Measurements

Question

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in terms of the new unit if light takes 8 min 20 s to cover this distance?

Step-by-Step Solution (CBSE Toppers Style)

Given:

• Speed of light in the new system = 1 unit/s
• Time taken by light = 8 min 20 s

Step 1: Convert time into seconds

8 minutes = 8 × 60 = 480 seconds

Total time = 480 + 20 = 500 seconds

Step 2: Apply the formula

Distance = Speed × Time

Since,

Speed = 1 unit/s

Therefore,

Distance = 1 × 500 = 500 new units

Final Answer

Distance between the Sun and the Earth = 500 new units.

Short Notes (Easy for Beginners)

• A unit is a standard way of measuring a physical quantity.
• Scientists sometimes define new units to simplify calculations.
• In this question, the speed of light is taken as 1 unit per second.
• This means light travels 1 new unit in 1 second.
• The basic formula is:

Distance = Speed × Time

• If speed = 1 unit/s, then

Distance = Time (in seconds)

How to Solve Similar Questions

1. Convert the given time into seconds.
2. Use the formula: Distance = Speed × Time.
3. If speed = 1 unit/s, then distance equals the time in seconds.

Example

Time = 8 min 20 s

= (8 × 60) + 20

= 480 + 20

= 500 seconds

Therefore,

Distance = 500 new units

Exam Tip

• Always convert minutes into seconds first.
• Remember the formula: Distance = Speed × Time.
• If speed is 1 unit/s, then distance is simply the time in seconds.
• This shortcut helps solve such questions quickly in CBSE examinations.

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NEET Smart Learning Structure

1. Concept Snapshot

Topic: New Unit of Length (Speed of Light = 1 unit/s)

Key Idea: When speed = 1, distance = time (in seconds)

Formula: Distance = Speed × Time

Special Case: If Speed = 1 → Distance = Time

2. Smart Table

Parameter	Value	Memory Trick
Speed of Light	1 unit/s (new system)	"Light = 1 Rule"
Time Given	8 min 20 s	Convert to seconds
Total Time	500 s	8×60 + 20
Distance	500 units	Time = Distance

3. Mnemonic

"LIGHT = ONE, TIME = DONE"

If speed is 1 unit/sec → Distance is always time in seconds.

4. Flow Chart

Question ↓ Convert minutes → seconds ↓ Apply formula D = S × T ↓ S = 1 unit/s ↓ Distance = Time (sec) ↓ Final Answer

5. NEET Shortcut

If Speed = 1 → Directly write Distance = Time (seconds)

No calculation of velocity needed.

6. Exam Trap

❌ Mistake: Using minutes directly as distance

✔ Correct: Always convert into seconds first

7. Practice

Q: Light takes 10 min. Find distance.

Solution: 10 × 60 = 600 sec → 600 units

NEET Practice Questions

Topic: Speed of Light & New Unit System

1. Direct MCQs (Single Correct Option)

Q1 (Easy): If speed of light is 1 unit/s, then distance equals:
A) Speed × Time
B) Time (seconds)
C) Speed only
D) Zero

Answer: B

Q2 (Moderate): Light takes 3 min to reach a point. Distance is:
A) 180 units
B) 3 units
C) 60 units
D) 1800 units

Answer: A (3×60=180 s)

Q3 (Hard): If time is doubled, distance becomes:
A) Half
B) Double
C) Same
D) Zero

Answer: B

PYQ Type: In a system where c = 1, unit of length is:
A) Meter
B) Time unit × c
C) Second
D) Joule

Answer: B

2. Statement-Based Questions

Q: Statement I: In new unit system, speed of light is 1.
Statement II: Distance becomes numerically equal to time in seconds.

Answer: Both I and II are true, and II is correct explanation of I.

3. Assertion and Reason (A & R)

Assertion (A): Distance between Sun and Earth becomes 500 units.
Reason (R): Light takes 500 seconds to reach Earth.

Answer: Both A and R are true, and R explains A.

Assertion (A): If speed = 1, conversion is not required.
Reason (R): Distance equals time directly.

Answer: Both true, R explains A.

4. Match the Columns

Match Column A with B:

Column A	Column B
Speed of light = 1	A. 500 units
8 min 20 s	B. New unit system
Distance	C. 500 s

Answer:
Speed = 1 → B
8 min 20 s → C
Distance → A

5. Diagram / Flow Based Questions

Study the flow chart:

Time (min) → Convert to seconds → Multiply by speed → Distance

Q: If speed = 1 unit/s, what is final step?

Answer: Distance = Time (in seconds)

Graph Type Question: If distance vs time graph is straight line passing through origin, speed is:
A) Increasing
B) Constant
C) Decreasing
D) Zero

Answer: B (Constant speed)