Measurement of Time and Motion — Class 7 Science

Name some devices used to measure time before modern clocks.

Sundials, water clocks (including the sinking-bowl Ghatika-yantra), hourglasses, and candle clocks were used to measure time.

What did Galileo discover about a pendulum?

Galileo found that a pendulum of a given length always takes the same time to complete one oscillation, regardless of how it is swung.

What is the time period of a pendulum?

The time period is the time taken by the pendulum to complete one oscillation.

Does the time period of a pendulum depend on the mass of the bob?

No. The time period depends on the length of the pendulum but not on the mass of the bob; it stays constant at a given place.

What is the SI unit of time and its relation to minutes and hours?

The SI unit of time is the second (s); 60 s = 1 min and 60 min = 1 h.

Define speed and give its SI unit.

Speed is the distance covered in unit time, calculated as total distance divided by total time; its SI unit is metre per second (m/s).

Write the formula relating speed, distance, and time.

; also distance = speed times time and .

Why do we usually use average speed?

Because an object rarely moves at the same speed throughout a journey — it speeds up and slows down — so we use the average speed over the whole journey.

What is uniform linear motion?

Uniform linear motion is motion along a straight line at a constant speed, covering equal distances in equal intervals of time.

How does non-uniform linear motion differ from uniform linear motion?

In non-uniform linear motion the speed keeps changing, so unequal distances are covered in equal intervals of time, unlike the constant speed of uniform motion.

1. Calculate the speed of a car that travels in . Express your answer in .

Speed is total distance divided by total time. To convert to multiply by : So the car moves at which is .

2. A runner completes in . Another runner completes the same distance in . Who has a greater speed and by how much?

Speed of first runner Speed of second runner The second runner is faster. Difference which is about faster.

3. A train travels at a speed of and covers a distance of . How much time does it take?

Convert distance to metres: . Time = distance / speed So it takes , i.e. .

4. A train travels in . Find its speed in: (i)

4. A train travels in . Find its speed in: (ii)

Convert to :

4. A train travels in . Find its speed in: (iii) What distance will it travel in if it maintains the same speed throughout the journey?

Distance = speed time Using : .

5. The fastest galloping horse can reach the speed of approximately . How does this compare to the speed of a train moving at ?

Convert train speed to : The horse at is slightly slower than the train at . The train is faster by which is . Equivalently horse speed in is versus .

6. Distinguish between uniform and non-uniform motion using the example of a car moving on a straight highway with no traffic and a car moving in city traffic.

Uniform linear motion is motion along a straight line with constant speed, covering equal distances in equal time intervals. A car on an empty straight highway can maintain steadily, e.g. each hour, so it is approximately uniform. Non-uniform linear motion is motion along a straight line where speed keeps changing, covering unequal distances in equal intervals. A car in city traffic repeatedly speeds up, slows down, stops at signals, so in successive hours it might cover , , , showing non-uniform motion.

7. Data for an object covering distances in different intervals of time are given in the following table. If the object is in uniform motion, fill in the gaps in the table. Time (s) 0 10 20 30 50 70 Distance (m) 0 8 24 32 40 56

For uniform motion speed is constant. From the given pair in : So distance . Filled values are: at , (so the printed would correspond to ); at , ; at , ; at , ; at , ; at , . In general every adds . If the table as printed leaves blanks, they are filled by accordingly.

8. A car covers in the first hour, in the second hour, and in the third hour. Is the motion uniform? Justify your answer. Find the average speed of the car.

The motion is non-uniform because equal time intervals (each ) have unequal distances (). Uniform motion would require equal distances in equal times. Average speed = total distance / total time which is .

9. Which type of motion is more common in daily life— uniform or non-uniform? Provide three examples from your experience to support your answer.

Non-uniform motion is much more common in daily life because speed usually changes due to traffic, turns, starts and stops. Examples: 1) A school bus stopping at bus stops and slowing in traffic. 2) A person walking in a crowded market speeding up and slowing down. 3) A bicycle going uphill slowly and downhill faster. Truly uniform motion for a long time, like a train cruising at exactly constant speed on a straight track, is rare and is an idealisation.

10. Data for the motion of an object are given in the following table. State whether the speed of the object is uniform or non-uniform. Find the average speed. Time (s) 0 10 20 30 40 50 60 70 80 90 100 Distance (m) 0 6 21 29 35 42 45 55 60

Check distances in equal intervals: , , , , , , , etc. They are not equal, so speed is non-uniform. Average speed = total distance / total time. Using the full table up to and (as visible): If the last entry is higher, use total values shown: . In any case it is not constant, confirming non-uniform motion.

11. A vehicle moves along a straight line and covers a distance of . In the first , it moves with a speed of and in the next , it moves with a speed of . With what speed should it move the remaining distance so that the journey is complete in ? What is the average speed of the vehicle for the entire journey?

Total distance . Time for first : Time for next : Time used so far . Time left for remaining : Required speed for remaining part Average speed for entire journey