Motion and Time

Slow or Fast

In Class VI, you learned about different types of motions:

• Straight Line Motion
• Circular Motion
• Periodic Motion

Slow or Fast

Observations:

• Some objects move slowly; others move fast.
• Example: Different vehicles moving at various speeds on the road.

Speed

Understanding Speed:

• Speed shows how fast an object moves.
• Higher speed means covering a given distance in less time or more distance in the same time.

How to Compare Speeds:

• Check distances covered in a unit of time (e.g., kilometers per hour).
• Example: A car moving at 50 km/h covers 50 kilometers in one hour.

Calculating Speed:

• Formula: Speed = Total distance covered / Total time taken
• Example: If a car covers 100 km in 2 hours, its speed is 100 km / 2 hours = 50 km/h.

Uniform vs. Non-Uniform Motion:

• Uniform Motion: Constant speed along a straight line. The average speed equals the actual speed.
• Non-Uniform Motion: Changing speed along a straight line.

Key Points to Remember

• Speed helps us compare how fast different objects are moving.
• Average speed is used to simplify the understanding of speed.
• Understanding and measuring both distance and time are crucial to calculate speed accurately.

Conclusion

Knowing the speed and type of motion helps us understand how different objects move and compare their movements.

Measurement of Time

How do we tell time without a clock?

• Elders used shadows to tell the approximate time of day.

Ancient Methods:

• Day: Time between one sunrise and the next.
• Month: From one new moon to the next.
• Year: Time taken by the Earth to complete one revolution around the Sun.

Using Clocks and Watches

• Need for shorter intervals: We need clocks for shorter time measurements.
• Periodic Motion: Clocks use periodic motion like a simple pendulum.

Simple Pendulum

• Components:
  • A small metallic ball (bob)
  • Suspended from a rigid stand by a thread.
• Oscillation:
  • To and fro motion.
  • One oscillation: Bob moves from the mean position (O) to one extreme (A), to the other extreme (B), and back to mean position (O).
  • Time Period: Time taken for one complete oscillation.

Modern Clocks

• Quartz Clocks:
  • Use electric circuits.
  • More accurate than older clocks.

Interesting Fact

• Galileo’s Discovery:
  • Noticed that a swinging lamp in a church had a constant time period matching his pulse.
  • Led to the development of pendulum clocks.

Key Points

• Time Measurement: Crucial for daily activities and scientific experiments.
• Consistency: Slight changes in displacement don’t affect the pendulum’s time period.

Units of Time and Speed

Basic Units of Time:

• Second (s): The basic unit.
• Larger Units: Minutes (min) and hours (h).
• Example: 1 minute = 60 seconds, 1 hour = 60 minutes.

Basic Unit of Speed:

• Formula: Speed = Distance / Time.
• Unit: Meter per second (m/s).
• Other Units: Meters per minute (m/min), kilometers per hour (km/h).

Writing Units:

• Rule: Always use singular form.
  • Example: 50 km (not 50 kms), 8 cm (not 8 cms).

Interesting Calculations:

• Seconds in a Day: 24 hours * 60 minutes * 60 seconds = 86,400 seconds.
• Hours in a Year: 365 days * 24 hours = 8,760 hours.

Understanding Time Intervals:

• One Second: Time to say “two thousand and one”.
• Pulse Rate: Normal adult pulse beats about 72 times per minute (12 times in 10 seconds).

Historical Time Measuring Devices:

• Examples: Sundials, water clocks, sand clocks.

Modern Precision:

• Small Intervals: Special clocks can measure microseconds (one millionth of a second) and nanoseconds (one billionth of a second).
• Use in Sports: Clocks measure intervals as small as one-tenth or one-hundredth of a second.
• Historical and Astronomical Times: Measured in centuries, millenniums, or billions of years.

Key Points

• Basic Units: Second, minute, hour.
• Speed Units: m/s, m/min, km/h.
• Precision: From seconds to billionths of a second.
• Historical Devices: Various designs like sundials and sand clocks.
• Large Time Intervals: Used for historical events and ages of stars and planets.

Measuring Speed

Calculating Speed:

• Formula: Speed = Distance / Time.
• Example: Find the speed of a ball rolling on the ground.

Comparing Speeds

Speeds of Various Objects

• Examples:
  • Rockets: Up to 8 km/s.
  • Tortoises: About 8 cm/s.
• Exercise: Calculate how much faster a rocket is compared to a tortoise.

Using Speed to Calculate Distance and Time

• Formulas:
  • Distance Covered: Distance = Speed × Time.
  • Time Taken: Time = Distance / Speed.

Vehicle Speedometers and Odometers

• Speedometer: Measures speed in km/h.
• Odometer: Measures total distance traveled by the vehicle.
• Example: Note the odometer readings of a bus during a picnic to calculate speed and distance.

Plotting Distance-Time Graphs

• Purpose: Helps to visualize and solve problems related to distance and speed over time.
• Example: Use the readings from the odometer to plot a distance-time graph and determine the travel distance at a specific time.

Key Points

• Measurement: Learn to measure time and distance accurately.
• Comparison: Compare speeds to understand differences.
• Visualization: Use graphs to interpret data easily.

Distance-Time Graph

Types of Graphs:

• Bar Graph: Shows data with bars.
• Pie Chart: Represents data in a circular format.
• Line Graph: Shows data points connected by lines.
• Distance-Time Graph: A type of line graph showing distance over time.

Drawing a Distance-Time Graph

1. Setup:
   • Use graph paper.
   • Draw two perpendicular lines: x-axis (horizontal) and y-axis (vertical).
   • The intersection is called the origin (O).
2. Axes:
   • x-axis: Represents time.
   • y-axis: Represents distance.
3. Choosing Scales:
   • Decide scales for both axes. For example:
     • Time: 1 min = 1 cm.
     • Distance: 1 km = 1 cm.
4. Plotting Points:
   • Use given data to mark points on the graph. For example:
     • At 0 min, distance is 0 km (origin).
     • At 1 min, distance is 1 km.
   • Draw lines to locate these points on the graph.
5. Connecting Points:
   • Join all points with a line.
   • A straight line indicates constant speed.

Example: Motion of a Car

• Data Collection: Record distance and time.
• Table: Create a table with time and distance (Table 9.6).
• Graph: Plot points using the data and draw the line.

Interpreting the Graph

• Constant Speed: A straight line indicates the object is moving at a constant speed.
• Variable Speed: If the line curves, the speed is changing.

Practical Example: School Picnic

• Data: Distance covered by a bus (Table 9.5).
• Choosing Scales:
  • Distance: 5 km = 1 cm.
  • Time: 6 min = 1 cm.
• Plotting: Use the scales to draw the graph.
• Finding Distance: Use the graph to find the distance at any given time (e.g., 8:15 AM).

Benefits of Distance-Time Graphs

• Detailed Information: Provides continuous data, unlike tables which show data at specific intervals.
• Instant Calculation: Helps find distances or speeds at any given time by plotting points on the graph.

Chapter Summary:

• The distance moved by an object in a unit time is called its speed.
• Speed of objects helps us decide which one is moving faster than the other.
• The speed of an object is the distance traveled divided by the time taken to cover that distance.
• The basic unit of speed is meter per second (m/s).
• Periodic events are used for the measurement of time.
• Periodic motion of a pendulum has been used to make clocks and watches.
• Motion of objects can be shown in pictorial form by their distance-time graphs.
• The distance-time graph for the motion of an object moving with a constant speed is a straight line.

Keywords

Serial No.	Keyword	Serial No.	Keyword
1	Bar graph	6	Speed
2	Graphs	7	Time period
3	Non-uniform motion	8	Uniform motion
4	Oscillation	9	Unit of time
5	Simple pendulum