Motion And Time: CBSE Class 7 Science NCERT

Understanding Motion: Different Types

Motion is simply the change in position of an object over time. If an object is moving, it means its location is constantly shifting compared to a reference point. For example, if you are sitting on a moving bus, you are in motion relative to someone standing outside the bus, but you are at rest relative to the bus itself. This concept is called relative motion. But motion isn't just one type; it can happen in several ways:

1. Rectilinear Motion: This is the simplest type of motion, where an object moves along a straight line. Think of a car moving on a straight road, a bullet shot from a gun, or a person walking in a straight path. The direction of motion remains constant.

1. Circular Motion: When an object moves along a circular path, it's called circular motion. Examples include a stone tied to a string and whirled around, the blades of a rotating fan, or a merry-go-round. Even though the speed might be constant, the direction of motion is continuously changing, as it's always tangential to the circular path.

1. Periodic Motion: This is a type of motion that repeats itself after a fixed interval of time. A simple pendulum swinging back and forth, the oscillation of a swing, the hands of a clock moving around the dial, or the vibration of a guitar string are all examples of periodic motion. The time taken for one complete cycle (or one oscillation) is called its time period.

Essential Terms in Motion and Time

Motion

A change in the position of an object with respect to time.

Distance

The total length of the path covered by an object during its motion, regardless of direction.

Time

The duration or interval during which an event or process occurs. It is measured in units like seconds, minutes, or hours.

Speed

The rate at which an object covers distance. It is calculated by dividing the total distance traveled by the total time taken (Speed = Distance / Time).

Uniform Motion

Motion in which an object travels equal distances in equal intervals of time along a straight line. Its speed remains constant.

Non-uniform Motion

Motion in which an object travels unequal distances in equal intervals of time, or vice versa. Its speed changes over time.

Time Period

The time taken by an object undergoing periodic motion to complete one full oscillation or cycle. For a simple pendulum, it's the time for one complete swing back and forth.

Calculating Speed: Step-by-Step

1. Step 1: Identify the Distance Traveled — First, determine the total distance the object has covered. This distance should typically be in standard units like metres (m) or kilometres (km). For instance, if a car travels from Delhi to Agra, the distance would be around 200 km.
2. Step 2: Note the Time Taken — Next, measure the total time the object took to cover that distance. This time can be in seconds (s), minutes (min), or hours (h). If the car took 3 hours to reach Agra, that's your time.
3. Step 3: Apply the Speed Formula — The formula for speed is: Speed = Distance / Time. Divide the distance by the time. Make sure the units are consistent. If distance is in km and time in hours, speed will be in km/h. If distance is in metres and time in seconds, speed will be in m/s. Example: A cyclist covers a distance of 100 meters in 20 seconds. Distance = 100 m Time = 20 s Speed = 100 m / 20 s = 5 m/s.
4. Step 4: Understand Unit Conversions (Optional but Important) — Sometimes you might need to convert units. For example, to convert km/h to m/s, you multiply by (5/18), and to convert m/s to km/h, you multiply by (18/5). Example: Convert 36 km/h to m/s. 36 km/h = 36 (1000 m / 3600 s) = 36 (10/36) m/s = 10 m/s.

Exam Tip: Avoiding Common Mistakes

When solving problems related to motion and time, pay close attention to the units. Many students lose marks because they mix units, for example, using distance in kilometres and time in seconds without converting one of them. Always ensure that your distance and time units are compatible (e.g., km with hours for km/h, or meters with seconds for m/s). Read the question carefully to identify what is being asked – speed, distance, or time – and use the correct formula. Also, remember that a simple pendulum's time period depends on its length, not on the mass of the bob or the amplitude (small oscillations).

Practice Questions with Solutions

• Q: A car travels a distance of 180 km in 3 hours. Calculate its speed. A: Step 1: Identify the given values. Distance (D) = 180 km Time (T) = 3 hours Step 2: Apply the speed formula. Speed (S) = Distance / Time S = 180 km / 3 h Step 3: Perform the calculation. S = 60 km/h Final answer: The speed of the car is 60 km/h.
• Q: A boy runs at a speed of 5 m/s. How much distance will he cover in 2 minutes? A: Step 1: Identify the given values and convert units if necessary. Speed (S) = 5 m/s Time (T) = 2 minutes. Convert minutes to seconds: 2 minutes * 60 seconds/minute = 120 seconds. Step 2: Apply the distance formula (rearranged from Speed = Distance / Time). Distance (D) = Speed × Time D = 5 m/s × 120 s Step 3: Perform the calculation. D = 600 m Final answer: The boy will cover a distance of 600 meters.
• Q: A simple pendulum completes 20 oscillations in 40 seconds. What is its time period? A: Step 1: Identify the given values. Number of oscillations = 20 Total time taken = 40 seconds Step 2: Apply the formula for time period. Time Period = Total time taken / Number of oscillations Time Period = 40 s / 20 Step 3: Perform the calculation. Time Period = 2 s Final answer: The time period of the simple pendulum is 2 seconds.
• Q: An express train covers 360 km in 4 hours. What is its speed in m/s? A: Step 1: Calculate speed in km/h. Distance = 360 km, Time = 4 hours Speed = Distance / Time = 360 km / 4 h = 90 km/h Step 2: Convert speed from km/h to m/s. To convert km/h to m/s, multiply by (5/18). Speed in m/s = 90 × (5/18) Step 3: Perform the calculation. Speed in m/s = 5 × 5 = 25 m/s Final answer: The speed of the train is 25 m/s.

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