Motion and Time Class 7 Notes

Slow and Fast Motion & Concept of Speed

We see objects moving around us every day; some move slowly, while others move fast. To compare which object is moving faster than another, we use the physical quantity called Speed. Speed is defined as the total distance covered by an object divided by the total time taken to cover that distance. If an object covers a certain distance in less time, its speed is higher. The standard SI unit of speed is meters per second (m/s), but it is also commonly expressed in kilometers per hour (km/h). For everyday calculations, we use the formula: $\text{Speed} = \frac{\text{Distance}}{\text{Time}}$. Understanding these basic units and conversions (such as multiplying by $5/18$ to convert km/h to m/s) is crucial for solving numerical problems in your exams.

Comparison: Uniform vs. Non-uniform Motion

Key Terms to Remember

Speed

The distance covered by an object in a unit of time.

Simple Pendulum

A device consisting of a small metallic ball (bob) suspended from a rigid stand by a light, inextensible string that exhibits periodic motion.

Periodic Motion

A motion that repeats itself at regular, equal intervals of time (e.g., the motion of a pendulum or hands of a clock).

Oscillation

The complete to-and-fro movement of a pendulum bob from one extreme position back to the same position.

Time Period

The time taken by a pendulum bob to complete one full oscillation.

Speedometer

An instrument mounted on a vehicle dashboard that records its speed directly in km/h.

Odometer

An instrument in a vehicle that measures the total distance traveled by the vehicle.

Step-by-Step: Measuring the Time Period of a Pendulum

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Solved Examples for Numerical Practice

• {"title":"Calculating Speed","description":"A car travels a distance of 150 km in 3 hours. Calculate its speed.","solution":"Given: Distance = 150 km, Time = 3 hours.\nFormula: Speed = Distance / Time\nSpeed = 150 km / 3 h = 50 km/h."}
• {"title":"Finding Distance Travelled","description":"A cyclist rides at a speed of 5 m/s for 20 seconds. How much distance does he cover?","solution":"Given: Speed = 5 m/s, Time = 20 s.\nFormula: Distance = Speed Time\nDistance = 5 m/s 20 s = 100 meters."}
• {"title":"Calculating Time Period of Pendulum","description":"A simple pendulum takes 32 seconds to complete 20 oscillations. What is its time period?","solution":"Given: Total Time = 32 seconds, Number of Oscillations = 20.\nFormula: Time Period = Total Time / Number of Oscillations\nTime Period = 32 / 20 = 1.6 seconds."}

Key Highlights & Exam Focus Points

• The basic unit of time is second (s). Larger units of time include minutes (min) and hours (h).
• The standard symbol for speed is m/s. All symbols of units are always written in singular form (e.g., 50 km, not 50 kms).
• A simple pendulum's time period depends only on the length of the string, not on the mass or material of the bob.
• Ancient time-measuring devices included sundials, sand clocks (hourglasses), and water clocks.
• Modern clocks utilize quartz crystals that vibrate at precise frequencies to measure time accurately.
• The slope of a Distance-Time Graph represents the speed of the moving object; a steeper slope means higher speed.
• If a Distance-Time Graph is a straight horizontal line parallel to the time axis, the object is at rest (speed is zero).

Exam Trap: Unit Conversions

Many students lose marks by putting different units together in calculation formulas (e.g., using distance in km and time in seconds directly). Rule of thumb: Always convert units into the same system before applying the formula. To convert km/h to m/s, multiply by $\frac{5}{18}$ (or divide by 3.6). To convert m/s to km/h, multiply by $\frac{18}{5}$ (or multiply by 3.6). Also, remember that time is base-60; 1.5 hours is 1 hour and 30 minutes, NOT 1 hour and 50 minutes!

Quick Revision Self-Check

• What does a horizontal straight line parallel to the x-axis on a distance-time graph indicate? It indicates that the object is stationary (at rest) and its speed is zero.
• Which device measures speed and which device measures distance in a car? The speedometer measures speed (in km/h) and the odometer measures distance traveled (in km).
• If a pendulum takes 40 seconds to complete 20 oscillations, what is its frequency? Time Period = 40/20 = 2 seconds. Frequency is the number of oscillations per second, which is 1 / Time Period = 1/2 = 0.5 Hz.
• Convert a speed of 72 km/h into m/s. Multiply 72 by 5/18. Speed = 72 (5/18) = 4 5 = 20 m/s.

Frequently Asked Questions