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.

Long Answer Questions

Q1: 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.

Q2: 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.

Q3: 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.

Distance1 = 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.