Measurement Of Time And Motion Important Question Answer - Class 7 Science - Fully Solved Notes For Students

Short Answer Questions

Q1: What is meant by uniform linear motion?
Ans: When an object moves along a straight line with a constant speed, covering equal distances in equal intervals of time, its motion is called uniform linear motion.

Q2: State the SI unit of speed and write one other commonly used unit of speed.

Ans: 

• The SI unit of speed is metre per second (m/s).
• Another commonly used unit of speed is kilometre per hour (km/h).

Q3: What do you mean by non-uniform speed?
Ans: Non-uniform (or variable) speed means an object covers unequal distances in equal intervals of time, or equivalently, its speed changes with time.

Q4: A child is on a see-saw. What kind of motion does he have, and why? Explain.
Ans: On a see-saw the child moves up and down repeatedly about a central point. This is called oscillatory motion because the motion repeats itself about a mean position.

Q5: Explain how, in ancient times, a day, a month and a year were measured.

Ans: Ancient people measured time using natural events. A day was the interval between one sunrise and the next. A month was the time from one new moon to the next (about one lunar cycle). A year was taken as the time the Earth took to complete one revolution around the Sun.

Q6: What are quartz clocks?

Ans: Quartz clocks are timekeeping devices that use a quartz crystal and an electric circuit powered by a cell (battery). The crystal vibrates at a precise frequency when an electric current passes through it, helping the clock keep accurate time.

Q7: How do we know an object is moving faster compared to the speed of another object?

Ans: We compare the distances covered by the objects in the same unit time (for example, metres per second or kilometres per hour). The object covering the greater distance in the same time has the greater speed.

Q8: If a car is moving with a speed of 5 Km/h on highway, then find the distance travelled by the car in 4 hours?
Ans: Distance = Speed × Time = 5 km/h × 4 h = 20 km.

Q9: Sumit covers a distance of 2.4 Km from his house to reach his college on a scooter. If the scooter has a speed of 6 m/s, calculate the time taken by him to reach the college.

Ans: Total distance = 2.4 km = 2.4 × 1000 m = 2400 m.

Speed = 6 m/s.

Time = Distance / Speed = 2400 m / 6 m/s = 400 s.

You may convert 400 s to minutes: 400 s = 6 min 40 s.

Q10: How can you say that motion and rest are relative?

Ans: Whether an object is at rest or in motion depends on the chosen reference frame. For example, a person sitting in a moving train is at rest with respect to the train but in motion with respect to the ground. Thus, motion and rest are relative terms.

Q11: Differentiate between circular motion and periodic motion.

Ans: Circular motion is motion along a circular path (for example, the second hand of a clock). Periodic motion is any motion that repeats itself at regular time intervals (for example, a pendulum, a child on a swing, or the vibrating strings of a guitar). Circular motion can be periodic, but not all periodic motions are circular.

Q12: Give an example when objects undergo combinations of different types of motion.

Ans: When a ball rolls on the ground it both rotates about its axis and translates (moves forward). So it shows a combination of rectilinear (or curvilinear) motion and rotational motion.

Q13: What is a simple pendulum?

Ans: A simple pendulum consists of a small dense object called the bob suspended from a fixed point by a light, inextensible string. It can swing to and fro under the action of gravity.

Q14: What is the oscillation of a pendulum?

Ans: One complete to-and-fro motion of a pendulum from its rest (mean) position and back again is called one oscillation. The time taken for one oscillation is the time period, which depends mainly on the length of the string.

Q15: What do you mean by time?

Ans: Time is the interval between two events. Common units are the second (s) as the basic unit, and larger units like the minute (min) and hour (h). Clocks and watches are devices used to measure time.

Q16: A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

Ans: Total time for 20 oscillations = 32 s.

Time period (time for one oscillation) = Total time / Number of oscillations = 32 s / 20 = 1.6 s.

Q17: Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school.

Ans: Time = 15 minutes = 15 × 60 s = 900 s.

Speed = 2 m/s.

Distance = Speed × Time = 2 m/s × 900 s = 1800 m = 1.8 km.

Q18: The distance between two stations is 300 km. A train takes 6 hours to cover this distance. Calculate the speed of the train.

Ans: Speed = Distance / Time = 300 km / 6 h = 50 km/h.

Q19: A simple pendulum takes 15 seconds to complete 5 oscillations. What is the time period of pendulum?
​Ans: Total time for 5 oscillations = 15 s.

Time period = Total time / Number of oscillations = 15 s / 5 = 3 s.

Q20: A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is:

Ans: First part: Time = 15 min = 15/60 h = 0.25 h.

Distance = 40 km/h × 0.25 h = 10 km.

Second part: Distance = 60 km/h × 0.25 h = 15 km.

Total distance = 10 km + 15 km = 25 km.

Long Answer Questions

Q1: Explain uniform and non-uniform linear motion with definitions and examples. How can you identify them from distance–time data?

Answer: When an object moves along a straight line, its motion is called linear motion.

Uniform Linear Motion

An object moving along a straight line with a constant speed is said to be in uniform linear motion.
In this type of motion, the object covers equal distances in equal intervals of time.

Example:
A car moving on a straight highway at a constant speed of 60 km/h without slowing down.

If distance–time data show that the object covers 20 km every 10 minutes continuously, the motion is uniform.

Non-uniform Linear Motion

If the speed of an object moving along a straight line keeps changing, it is said to be in non-uniform linear motion.
In this case, the object covers unequal distances in equal intervals of time.

Example:
A car moving in city traffic where it sometimes stops, slows down, or speeds up.

If distance–time data show distances like 20 km, 15 km, 25 km in equal 10-minute intervals, the motion is non-uniform.

Q2: Describe a simple pendulum. What is meant by its time period? On what factor does the time period depend? Explain with an activity.

Answer: A simple pendulum consists of a small heavy ball called a bob suspended from a fixed rigid support by a long, light string. When the bob is displaced slightly from its mean (rest) position and released, it moves to and fro about the mean position. This type of motion is called oscillatory motion.

Oscillation and Time Period

One complete to-and-fro motion of the pendulum is called one oscillation.

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

If a pendulum takes 20 seconds to complete 10 oscillations:

Time period = Total time ÷ Number of oscillations
= 20 ÷ 10
= 2 seconds

Activity to Study Time Period

1. Take a string about 100 cm long and tie a small metal ball to one end.

2. Fix the other end to a rigid support.

3. Displace the bob slightly and release it gently.

4. Measure the time taken for 10 oscillations using a stopwatch.

5. Divide this time by 10 to find the time period.

6. Repeat the experiment several times.

You will observe that the time period is almost the same each time.

Factor Affecting Time Period

The time period of a simple pendulum depends on:

• The length of the string

• It does not depend on the mass of the bob

Pendulums of the same length have the same time period at a given place.

Q3: The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also.

Ans: Initial odometer reading = 57321.0 km.

Final odometer reading = 57336.0 km.

Distance travelled = 57336.0 - 57321.0 = 15 km.

Time taken = 08:50 AM - 08:30 AM = 20 minutes.

Speed in km/min = Distance / Time = 15 km / 20 min = 0.75 km/min.

To convert to km/h: 0.75 km/min × 60 min/h = 45 km/h.