Simply CBSE

Time and Motion — Class 7 Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Time has been measured using sundials, water clocks, sand clocks, pendulum clocks, quartz clocks, and atomic clocks.
  • The SI unit of time is the second (s). Other units include minutes, hours, and days.
  • The atomic clock is the most accurate time-measuring device, losing or gaining only about one second in millions of years.
  • A simple pendulum consists of a bob (heavy metal ball) suspended from a rigid support by a light, inextensible string.
  • One oscillation is one complete to-and-fro motion of the pendulum bob.
  • Time period (T) is the time taken for one complete oscillation: T = Total time / Number of oscillations.
  • Frequency (f) is the number of oscillations per second: f = 1 / T. The SI unit of frequency is hertz (Hz).
  • The time period of a pendulum depends only on its length, not on the mass of the bob or the amplitude.
  • A longer pendulum has a longer time period (swings slower); a shorter pendulum swings faster.
  • Speed is the distance covered per unit time: Speed = Distance / Time. SI unit is m/s.
  • To convert km/h to m/s, multiply by 5/18. To convert m/s to km/h, multiply by 18/5.
  • A speedometer shows instantaneous speed; an odometer shows total distance travelled.
  • Uniform motion: equal distances covered in equal time intervals. The distance-time graph is a straight line.
  • Non-uniform motion: unequal distances covered in equal time intervals. The distance-time graph is a curved line.
  • Average speed = Total distance / Total time (not the average of individual speeds).
  • In a distance-time graph, a horizontal line means the object is at rest (not moving).
  • The slope (steepness) of a distance-time graph represents speed. Steeper slope means higher speed.

📘 Important Definitions

Oscillation
One complete to-and-fro motion of a pendulum bob, starting from a position and returning to the same position while moving in the same direction.
Time Period
The time taken by a pendulum to complete one full oscillation. Measured in seconds. Formula: T = Total time / Number of oscillations.
Amplitude
The maximum displacement of the pendulum bob from its mean (rest) position to an extreme position.
Frequency
The number of oscillations completed by a pendulum in one second. Measured in hertz (Hz). Formula: f = 1 / T.
Speed
The distance covered by an object per unit time. Formula: Speed = Distance / Time. SI unit is metre per second (m/s).
Uniform Motion
Motion in which an object covers equal distances in equal intervals of time. The speed remains constant throughout.
Non-Uniform Motion
Motion in which an object covers unequal distances in equal intervals of time. The speed keeps changing.
Average Speed
The total distance travelled divided by the total time taken for the entire journey. Average speed = Total distance / Total time.
Speedometer
An instrument in a vehicle that shows the instantaneous speed at any given moment.
Odometer
An instrument in a vehicle that records the total distance travelled. Its reading only increases.

🔢 Formulas & Laws

Speed

Speed = Distance / Time

SI unit: m/s. Also used: km/h. Rearranged: Distance = Speed x Time, Time = Distance / Speed.

Time Period of Pendulum

T = Total time / Number of oscillations

Measured in seconds. For accuracy, count many oscillations and divide.

Frequency

f = 1 / T (or f = Number of oscillations / Total time)

SI unit: hertz (Hz). Frequency and time period are inversely related.

km/h to m/s Conversion

Speed in m/s = Speed in km/h x 5/18

Example: 36 km/h = 36 x 5/18 = 10 m/s.

m/s to km/h Conversion

Speed in km/h = Speed in m/s x 18/5

Example: 20 m/s = 20 x 18/5 = 72 km/h.

Average Speed

Average Speed = Total distance / Total time

Do NOT simply average two speeds. Always use total distance divided by total time.

⚠️ Common Mistakes

✗ Wrong: Counting one swing of a pendulum (from one side to the other) as one full oscillation.

✓ Correct: One complete oscillation means the bob goes from one point, swings to the other side, and returns to the starting position moving in the same direction. One swing is only half an oscillation.

✗ Wrong: Calculating average speed as the average of two speeds: (speed1 + speed2) / 2.

✓ Correct: Average speed = Total distance / Total time. This is not the same as averaging the speeds. For example, if a car goes 120 km at 60 km/h (2 h) and 120 km at 40 km/h (3 h), the average speed is 240/5 = 48 km/h, not 50 km/h.

✗ Wrong: Mixing units when calculating speed (e.g., distance in km and time in seconds).

✓ Correct: Always ensure consistent units. If distance is in km and time in hours, speed is in km/h. If distance is in metres and time in seconds, speed is in m/s. Convert units before calculating.

✗ Wrong: Thinking the time period of a pendulum depends on the mass of the bob.

✓ Correct: The time period of a simple pendulum depends only on its length. It does NOT depend on the mass of the bob or the amplitude (for small swings).

✗ Wrong: Swapping axes on a distance-time graph (putting distance on X-axis).

✓ Correct: In a distance-time graph, time is always on the X-axis (horizontal) and distance on the Y-axis (vertical).

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Calculate time period and frequency of a pendulum given number of oscillations and total time.
3m★★★
Calculate speed, distance, or time using the formula Speed = Distance / Time.
3m★★★
Convert speed from km/h to m/s or vice versa.
2m★★★
Differentiate between uniform and non-uniform motion with examples.
3m★★★
Describe the distance-time graph for uniform motion, non-uniform motion, and rest.
3m★★★
Differentiate between speedometer and odometer.
2m★★
Calculate average speed for a journey with two or more parts.
3m★★
List different devices used for measuring time from ancient to modern.
3m★★
Define oscillation, amplitude, time period, and frequency of a pendulum.
2m★★

Diagram to practice: Practice drawing: (1) A simple pendulum showing bob, string, mean position, extreme positions, and amplitude, (2) Distance-time graph for uniform motion (straight line), non-uniform motion (curved line), and object at rest (horizontal line), (3) Labelled diagram showing how to measure oscillations of a pendulum.

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!