NCERT solutions for Class 7th Science Motion and Time

The SI unit of speed is meters per second (m/s).

Speed is defined as the distance traveled per unit of time. In the SI (International System of Units), distance is measured in meters (m) and time is measured in seconds (s), so the unit of speed becomes meters per second (m/s).

For example, if a car moves 100 meters in 5 seconds, its speed is:

speed = distance/time

speed = 100/5 = 20 m/s

Uniform motion is a type of motion in which an object travels in a straight line and maintains a constant speed throughout. This means there is no change in the magnitude of velocity over time. Since the speed remains the same, the object covers equal distances in equal intervals of time, regardless of how small or large the time intervals are.

In uniform motion:

• There is no acceleration (acceleration = 0).
• The path is straight.
• The object’s velocity/speed is constant.

Example of Uniform Motion:

1. A train moving at a constant speed of 80 km/h on a straight track. If the train covers 80 kilometers every hour without speeding up or slowing down, it is said to be in uniform motion.
2. A car moving at a constant speed of 60 km/h on a straight highway is in uniform motion. It covers 60 kilometers every hour without changing speed or direction.

Stopwatch, the instrument used to measure time in a laboratory is called a stopwatch.

A stopwatch is a precise time-measuring device used to record the time interval of an event. It can start, stop, and reset with the press of a button, making it ideal for timing experiments, chemical reactions, or any activity that requires accurate measurement of time. Digital stopwatches are commonly used in modern laboratories for their accuracy and ease of use.

Uses of a Stopwatch in a Laboratory:

1. Measuring Reaction Time: To record the duration of chemical reactions or physical processes.
2. Timing Experiments:  Used to track the exact time an experiment is started and stopped, ensuring accurate results.
3. Rate Calculations: Helps in calculating the speed, rate of reaction, or frequency by measuring time intervals.
4. Physics Experiments: Essential in experiments involving motion, such as measuring the time taken for a pendulum to complete oscillations.
5. Biology Studies: Used to time biological processes like heartbeat rate or enzyme activity duration.
6. Accuracy & Precision:Allows scientists to perform repeated trials and ensure consistency in time-based observations.

Different Types of Motion with Examples:

1. Linear Motion (Translational Motion): When an object moves in a straight line or along a curved path.
   Example: A car moving on a straight road.
2. Rotational Motion: When an object moves around a fixed axis.
   Example: Blades of a fan rotating.
3. Oscillatory Motion:When an object moves to and fro about a fixed point.
   Example: A pendulum swinging back and forth.
4. Periodic Motion:Motion that repeats itself after regular intervals of time.
   Example: Revolution of the Earth around the Sun.
5. Random Motion:When an object moves in an unpredictable or irregular path.
   Example: Movement of dust particles in the air.

Non-uniform motion is a type of motion in which an object does not travel equal distances in equal intervals of time. This means the speed of the object keeps changing as it moves. The motion can be accelerated (speeding up) or decelerated (slowing down).

This type of motion is irregular and can happen due to various factors like changes in force, resistance, or obstacles in the path.

Characteristics of Non-uniform Motion:

1. The speed is not constant.
2. The motion may be in a straight line or along a curved path.
3. Acceleration may be present.

Example:

1. A bus traveling on a crowded city road — it slows down at traffic lights, speeds up on empty stretches, and stops for passengers.
2. A ball rolling down a slope — it gains speed as it moves due to gravity.

An odometer is an instrument used to measure the total distance traveled by a vehicle or any moving object. It is typically found on the dashboard of vehicles, alongside the speedometer, and provides the driver with a reading of how far the vehicle has gone since its last reset or the total distance traveled since the vehicle was manufactured.

What It Measures:

• The odometer measures the accumulated distance that a vehicle has traveled, usually in kilometers (km) or miles (mi).
• Unlike a speedometer, which shows the current speed, the odometer tracks the cumulative distance and does not change with speed.

Types of Odometers:

1. Mechanical Odometer (Analog):
   1. Uses a set of rotating gears and dials to record distance.
   2. The wheels or gears inside the odometer rotate with the movement of the vehicle, and each rotation corresponds to a specific distance traveled.
   3. As the vehicle moves, the dials turn to display the total distance.
2. Digital Odometer (Electronic):
   1. Uses sensors and electronic circuits to track the distance traveled.
   2. The vehicle's wheel rotation is detected by sensors and the data is sent to a computer, which records and displays the total distance digitally.
   3. Modern vehicles often feature digital odometers that are more accurate and less prone to mechanical wear.

Importance of the Odometer:

• Vehicle Maintenance:The odometer helps determine when maintenance tasks, such as oil changes, tire rotations, and brake inspections, should be performed based on the distance driven.
• Buying and Selling Vehicles: Odometer readings are used to assess the mileage of a vehicle, which can influence its resale value. A lower mileage generally increases a vehicle’s value, as it suggests less wear and tear.
• Tracking Fuel Efficiency: By monitoring the odometer and calculating how far the vehicle has traveled between refuels, drivers can estimate their fuel efficiency and adjust driving habits accordingly.

Modern Features:

• Trip Odometer: Many vehicles have a trip odometer that allows drivers to track the distance traveled for a specific trip. This can be reset at any time to measure the distance for short trips or specific journeys.
• Odometer Reading for Legal and Safety Checks: Some countries have laws requiring an odometer reading during vehicle inspections, and they may require the recording of the reading during the purchase or sale of a vehicle.

First, convert the distance to meters:
120 kilometers = 120 × 1000 = 120,000 meters.

Now, convert time to seconds:
3 hours = 3 × 3600 = 10,800 seconds.

Now, use the formula for speed:
speed = distance / time
speed =  120000 / 10800 = 11.11 m/s

The speed is approximately 11.11 m/s.