Class 7 Science Notes Chapter 13 (Chapter 13) – Examplar Problems Book

Alright class, let's focus on Chapter 13 from your Science Exemplar book, which deals with Motion and Time. This is an important chapter, not just for your regular studies, but also because concepts related to motion, speed, time, and graphs often appear in various government competitive exams. Pay close attention as we break down the key ideas.

Chapter 13: Motion and Time - Detailed Notes for Exam Preparation

1. Motion:

• Definition: An object is said to be in motion if it changes its position with respect to time and its surroundings (a reference point). If it doesn't change its position, it's said to be at rest.
• Slow and Fast Motion: We perceive motion as slow or fast based on the distance covered by an object in a given amount of time. An object covering more distance in the same time (or the same distance in less time) is considered faster.

2. Types of Motion (Based on Path):

• Rectilinear Motion: Motion along a straight line. Examples: A car moving on a straight road, a freely falling object (approximately), march-past of soldiers.
• Circular Motion: Motion along a circular path. Examples: Motion of the tip of the blades of a fan, motion of the Earth around the Sun, a stone tied to a string and whirled around.
• Periodic Motion: Motion that repeats itself after regular intervals of time. Examples: Motion of a pendulum, swing, hands of a clock, revolution of the Earth.
  • Note: Circular motion can be periodic (like Earth's revolution), but not all periodic motions are circular (like a pendulum's swing). An oscillating motion is a type of periodic motion.

3. Speed:

• Definition: Speed is the measure of how fast an object is moving. It is defined as the distance covered by an object per unit of time.
• Formula:
  Speed = Total Distance Covered / Total Time Taken
• Calculating Distance or Time:
  • Distance = Speed × Time
  • Time = Distance / Speed
• Units:
  • The basic (SI) unit of distance is the metre (m).
  • The basic unit of time is the second (s).
  • Therefore, the basic unit of speed is metres per second (m/s).
  • Other common units include kilometres per hour (km/h), metres per minute (m/min), centimetres per second (cm/s).
• Conversion: Remember common conversions, especially km/h to m/s:
  1 km/h = 1000 m / 3600 s = 5/18 m/s
  1 m/s = 18/5 km/h

4. Uniform and Non-uniform Motion:

• Uniform Motion: An object moving along a straight line is said to have uniform motion if its speed remains constant. It covers equal distances in equal intervals of time.
• Non-uniform Motion: An object moving along a straight line is said to have non-uniform motion if its speed keeps changing. It covers unequal distances in equal intervals of time. Most real-world motions are non-uniform.
• Average Speed: For non-uniform motion, we often calculate the average speed over the entire journey.
  Average Speed = Total Distance Covered / Total Time Taken (This formula is the same as the basic speed formula, but it's particularly relevant when speed varies).

5. Measurement of Time:

• Ancient Methods: Early methods relied on natural events that repeat regularly, like sunrise/sunset (day), the cycle of the moon (month), and the Earth's revolution around the Sun (year). Devices like sundials, water clocks, and sand clocks were also used.
• Modern Devices: Clocks and watches are now used. They often utilize periodic motion.
• Simple Pendulum:
  • Components: Consists of a small metallic ball (bob) suspended by a light, inextensible string from a rigid support.
  • Oscillation: One complete to-and-fro movement of the pendulum bob is called one oscillation. It goes from one extreme position (A) to the other extreme position (B) and back to A. Or, starting from the mean position (O), going to A, then to B, and back to O.
  • Time Period (T): The time taken by the pendulum to complete one oscillation. It is constant for a given pendulum (at a given place).
  • Frequency (f): The number of oscillations completed in one second. f = 1/T. Unit: Hertz (Hz).
  • Amplitude: The maximum displacement of the bob from its mean position.
  • Factors Affecting Time Period: The time period of a simple pendulum depends only on its length (longer length = longer time period) and the acceleration due to gravity (g) at that place. It does not depend on the mass of the bob or the amplitude (for small amplitudes).
  • Calculating Time Period: Measure the time taken for a large number of oscillations (say, 20 or 30) and divide the total time by the number of oscillations. Time Period = Total Time / Number of Oscillations.
• Units of Time:
  • Basic unit: second (s)
  • Larger units: minute (min), hour (h), day, year.
  • Conversions: 1 min = 60 s; 1 h = 60 min = 3600 s.

6. Measuring Speed in Vehicles:

• Speedometer: An instrument in vehicles that indicates the speed at which the vehicle is moving at that instant (usually in km/h).
• Odometer: An instrument that measures and records the total distance covered by the vehicle (usually in km).

7. Distance-Time Graphs:

• Purpose: A graphical representation showing how the distance covered by an object changes with time. Time is usually plotted on the X-axis and Distance on the Y-axis.
• Plotting: Choose appropriate scales for both axes based on the data. Mark points corresponding to distance covered at different times and join them.
• Interpretation:
  • Object at Rest: The graph is a straight line parallel to the time axis (X-axis). Distance does not change with time.
  • Uniform Motion (Constant Speed): The graph is a straight line inclined to the time axis. The slope (steepness) of the line represents the speed. A steeper slope indicates higher speed.
  • Non-uniform Motion: The graph is a curved line. The speed is changing.
• Calculating Speed from the Graph: For uniform motion, the speed can be calculated from the slope of the distance-time graph.
  Slope = (Change in Distance) / (Change in Time) = Speed.
  Example: If the graph passes through points (t1, d1) and (t2, d2), then Speed = (d2 - d1) / (t2 - t1).

Key Takeaways for Exams:

• Know the definitions of motion, rest, speed, uniform/non-uniform motion, oscillation, time period.
• Be thorough with the formula Speed = Distance / Time and its rearrangements.
• Master the units of speed and time and their conversions (especially km/h to m/s).
• Understand the working principle of a simple pendulum and what its time period depends on (length, not mass or amplitude).
• Be able to interpret distance-time graphs for objects at rest, in uniform motion, and in non-uniform motion.
• Know how to calculate speed from a distance-time graph for uniform motion.
• Differentiate between speedometer and odometer.

Multiple Choice Questions (MCQs):

1. Which of the following is an example of rectilinear motion?
   a) Motion of a child on a swing
   b) Motion of the tip of a clock's hand
   c) Motion of a car on a straight highway
   d) Motion of the Earth around the Sun

2. A simple pendulum takes 40 seconds to complete 20 oscillations. What is the time period of the pendulum?
   a) 40 s
   b) 20 s
   c) 2 s
   d) 0.5 s

3. The basic unit of speed in the SI system is:
   a) km/h
   b) m/min
   c) km/min
   d) m/s

4. A car travels a distance of 120 km in 2 hours. Its average speed is:
   a) 120 km/h
   b) 60 km/h
   c) 240 km/h
   d) 30 km/h

5. Which instrument in a car measures the instantaneous speed?
   a) Odometer
   b) Speedometer
   c) Manometer
   d) Barometer

6. A distance-time graph for an object at rest is:
   a) A straight line inclined to the time axis
   b) A curved line
   c) A straight line parallel to the time axis
   d) A straight line parallel to the distance axis

7. If the distance-time graph of an object is a straight line inclined to the time axis, it represents:
   a) The object is at rest
   b) The object is in non-uniform motion
   c) The object is in uniform motion
   d) The object is accelerating uniformly

8. The time period of a simple pendulum depends on:
   a) The mass of the bob
   b) The amplitude of oscillation
   c) The length of the string
   d) The material of the bob

9. A bus travels at 54 km/h. Its speed in m/s is:
   a) 10 m/s
   b) 15 m/s
   c) 20 m/s
   d) 54 m/s

10. Which of the following motions is not necessarily periodic?
    a) Motion of a swing
    b) Motion of Earth around the Sun
    c) Motion of a bouncing ball
    d) Motion of the hands of a clock

Answer Key for MCQs:

1. c) Motion of a car on a straight highway
2. c) 2 s (Time Period = Total Time / No. of Oscillations = 40s / 20 = 2s)
3. d) m/s
4. b) 60 km/h (Speed = Distance / Time = 120 km / 2 h = 60 km/h)
5. b) Speedometer
6. c) A straight line parallel to the time axis
7. c) The object is in uniform motion
8. c) The length of the string
9. b) 15 m/s (54 km/h = 54 * (5/18) m/s = 3 * 5 m/s = 15 m/s)
10. c) Motion of a bouncing ball (It loses energy and height with each bounce, so the time between bounces may not be perfectly regular)

Study these notes carefully, understand the concepts behind the formulas and graphs, and practice the Exemplar problems. Let me know if any part needs further clarification. Good luck with your preparation!