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Physics · Chapter 2: Physical Quantities and Measurement
Measuring Mass and Time
Measuring mass with physical and electronic balances, measuring time with stopwatches, atomic clocks and reducing reaction-time error.
Key termRememberExampleWatch out
01Concept
Mass
Mass is the quantity of matter in a body. Its SI unit is the kilogram (kg). Mass does not change from place to place: a 1 kg bag of sugar has the same mass in Karachi, on Nanga Parbat or on the Moon.
| Instrument | How it works | Least count (typical) |
|---|---|---|
| Physical (beam) balance | Compares the object with standard masses on a beam | 0.01 g – 0.1 g |
| Electronic balance | A sensor measures the force and shows the mass on a screen | 0.01 g or 0.001 g |
| Lever balance / shopkeeper's balance | Compares with standard weights | a few grams |
- Before using an electronic balance, press tare (zero) so that it reads 0.00 g with the empty container on it.
- Keep the balance on a flat, steady surface away from fans and drafts.
- Never put hot objects or chemicals directly on the pan.
Time
The SI unit of time is the second (s). Today, the second is defined by atomic clocks, using vibrations of caesium atoms: one second is exactly 9,192,631,770 of these vibrations. Atomic clocks lose less than one second in millions of years.
| Instrument | Least count | Use |
|---|---|---|
| Mechanical stopwatch | 0.1 s (some 0.2 s) | School experiments |
| Digital stopwatch | 0.01 s | School experiments, sports |
| Electronic timers / light gates | 0.001 s or less | Accurate timing in labs and races |
Human reaction time
A digital stopwatch shows 0.01 s, but you cannot start and stop it that accurately. A person takes about 0.2–0.3 s to react. That is why, to find the time of one swing of a pendulum, we time 20 swings and divide by 20: the reaction-time error is shared over 20 swings and becomes much smaller.
Example: 20 swings take 40.2 s. Time for one swing = 40.2 ÷ 20 = 2.01 s.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Key Formulas
Time period of a pendulum from many swings
Timing 20 swings shares the reaction-time error over 20.
04Real World Examples
Shops and pharmacies
Sweet shops weigh mithai on electronic balances accurate to 1 g; pharmacies and jewellers use balances accurate to 0.01 g or better.
Cricket and athletics
Sprint races are timed electronically to 0.01 s, because hand timing with a stopwatch is slowed by the timekeeper's reaction time.
Mobile phone clock
Your phone's clock is corrected from atomic clocks through the mobile network and GPS satellites, so it stays exactly on time.
05Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define mass. Write its SI unit.Show answer
Answer
Mass is the quantity of matter in a body. Its SI unit is the kilogram (kg).
Q2.Name three instruments used to measure mass.Show answer
Answer
- Physical (beam) balance
- Electronic balance
- Lever (shopkeeper's) balance
Q3.What is the least count of a mechanical and a digital stopwatch?Show answer
Answer
- Mechanical stopwatch: 0.1 s
- Digital stopwatch: 0.01 s
Q4.Why do we time 20 oscillations of a pendulum instead of one?Show answer
Answer
Because human reaction time (about 0.2–0.3 s) causes an error in starting and stopping the watch. Timing 20 oscillations makes this error small compared with the total time, so the time for one oscillation is more accurate.
Q5.A pendulum makes 20 oscillations in 36.4 s. Find its time period.Show answer
Answer
- T = total time ÷ number of oscillations
- = 36.4 ÷ 20
- = 1.82 s
Long questions
Q6.Describe how mass and time are measured in the laboratory. Mention the precautions.Show answer
Answer
Mass: an electronic balance is switched on, set to zero (tare) with the empty container, and the object is placed on the pan. The mass is read from the screen, e.g. 25.36 g. Precautions: flat surface, no drafts, no hot objects.
Time: a stopwatch is used. Start and stop it at a clear point, such as the pendulum passing the centre. Time many oscillations (e.g. 20) and divide, to reduce reaction-time error. Repeat and take the mean.