CBSE Class 7 Science Advanced Physics Kinematics Very Short Answer Questions Set 03

Very Short Answer Questions

Question 1. Name the physical quantity that measures fastness or slowness of a moving object.
Answer:
Speed.
In simple words: The speed tells how fast or slow something is moving.

Teacher's Note:
1. Speed = distance ÷ time.
2. Higher speed means more distance in the same time.

Question 2. Two cars A and B move on a straight road for the same time. Car A covers 80 m and car B covers 100 m. Which one of the two is faster?
Answer:
Car B is faster. In the same time B covers more distance (100 m against 80 m), so B has the higher speed.
In simple words: The car that goes farther in the same time is faster.

Teacher's Note:
1. Compare distances only when the time is the same.
2. If the distances were equal, the one needing less time would be faster.

Question 3. What are quartz clocks? Which one is more accurate, the clock using a pendulum or the quartz clock?
Answer:
Quartz clocks are clocks and watches that work with an electric circuit built around a quartz crystal.
The quartz clock is more accurate than a pendulum clock.
In simple words: A quartz clock keeps time electrically and is more accurate than a pendulum clock.

Teacher's Note:
1. Most modern wrist watches are quartz watches.
2. Pendulum clocks can be affected by changes in length and place.

Question 4. If the speed of an object is known, how can we find the distance covered by it in a given time?
Answer:
Distance covered = speed × time.
In simple words: Multiply the speed by the time travelled.

Teacher's Note:
1. Keep the units matching: m s⁻¹ with seconds, km h⁻¹ with hours.
2. This formula comes from speed = distance ÷ time.

Question 5. Name the different types of graphs used to represent the motion of a body.
Answer:
Motion is shown with line graphs: the distance-time graph (or displacement-time graph) and the speed-time graph (velocity-time graph).
In simple words: Line graphs of distance against time or speed against time describe motion.

Teacher's Note:
1. The chapter mentions bar graphs, pie charts and line graphs, but motion is drawn with line graphs.
2. A distance-time graph is a line graph.

Question 6. Give an example where translatory and rotatory motion occur simultaneously.
Answer:
A moving bicycle: the cycle as a whole moves forward (translatory) while its wheels also turn about their axles (rotatory).
In simple words: A rolling wheel moves forward and spins at the same time.

Teacher's Note:
1. A car's wheels and a rolling ball are other examples.
2. Check whether the body moves from place to place as well as turns.

Question 7. Define rest and motion.
Answer:
Motion: a body is in motion if its position changes with respect to its surroundings.
Rest: a body is at rest if its position does not change with respect to its surroundings.
In simple words: A moving body changes its place; a body at rest does not.

Teacher's Note:
1. Both depend on the observer, so they are relative.
2. Always mention the reference object.

Question 8. Define acceleration and state its SI unit.
Answer:
Acceleration is the rate of change of velocity, that is, the change in velocity per unit time: a = (v − u)/t.
SI unit: m s⁻².
In simple words: Acceleration tells how quickly the velocity is changing.

Teacher's Note:
1. Acceleration is zero when the velocity is constant.
2. Negative acceleration is called deceleration.

Question 9. What is a seconds pendulum? What is its approximate length?
Answer:
A seconds pendulum is a simple pendulum with a time period of 2 s.
From ℓ = gT²/4π² = 9.8 × 4/39.5 ≈ 0.99 m, its length is approximately 1 metre (about 100 cm).
In simple words: It takes one second for each swing, and it is about a metre long.

Teacher's Note:
1. Use T = 2 s in the formula for the length.
2. With g = 10 m s⁻² and π² = 10, ℓ is exactly 1 m.

Question 10. Find the relationship between the time period of the pendulum and the length of the pendulum.
Answer:
The time period is T = 2π√(ℓ/g). At one place g is constant, so T ∝ √ℓ (T² ∝ ℓ): the longer the pendulum, the greater the time period.
In simple words: A longer pendulum swings more slowly.

Teacher's Note:
1. If the length is made four times, T becomes two times.
2. The mass of the bob does not appear in the formula.

Question 11. In a 100 m race, A touches the finishing line in 10.5 second and B touches it in 11 second. Who is faster, A or B?
Answer:
Both cover 100 m, but A needs less time, so A is faster.
Speed of A = 100 ÷ 10.5 = 9.52 m s⁻¹; speed of B = 100 ÷ 11 = 9.09 m s⁻¹.
In simple words: A takes less time for the same distance, so A is faster.

Teacher's Note:
1. For equal distances, less time means more speed.
2. Work out the speeds to confirm.

Question 12. What is meant by average speed? How is it calculated?
Answer:
Average speed is the single steady speed with which a body would cover the same total distance in the same total time when its actual speed keeps changing.
Average speed = total distance covered ÷ total time taken.
In simple words: It is the whole distance divided by the whole time.

Teacher's Note:
1. Do not just average the individual speeds.
2. Add the times of all parts for the total time.

Question 13. Mention the different units of time and speed.
Answer:
Time: second (basic unit), minute and hour (also day, month, year).
Speed: metre per second (m s⁻¹, SI unit), metre per minute (m min⁻¹) and kilometre per hour (km h⁻¹).
In simple words: Time is measured in seconds, minutes or hours, and speed in m s⁻¹ or km h⁻¹.

Teacher's Note:
1. Choose the unit that fits the situation.
2. Convert km h⁻¹ to m s⁻¹ by multiplying by 5/18.

Question 14. What is an odometer and a speedometer?
Answer:
An odometer is an instrument in a vehicle that measures the distance moved by the vehicle.
A speedometer is an instrument that shows the speed of the vehicle in km h⁻¹.
In simple words: One shows how far you have gone; the other shows how fast you are going.

Teacher's Note:
1. Both are on the dashboard.
2. The difference of two odometer readings gives the distance of a trip.

Question 15. When do we say that a pendulum has completed one oscillation?
Answer:
When the bob goes from the mean position O to one extreme position B, back through O to the other extreme position A and returns to O, it has completed one oscillation. Going from one extreme position to the other and back to the same extreme position is also one oscillation.
In simple words: One full to-and-fro trip back to the starting point is one oscillation.

Teacher's Note:
1. Going from one extreme to the other is only half an oscillation.
2. Time for one oscillation is the time period.

Question 16. Give an example of a motion which is rotatory as well as periodic.
Answer:
The rotation of the earth about its own axis (or the motion of the earth around the sun) repeats after equal intervals of time, so it is rotatory and periodic.
In simple words: The earth spinning on its axis turns and repeats every 24 hours.

Teacher's Note:
1. Not every rotatory motion is periodic; a fan that is slowing down is not.
2. A periodic motion repeats at equal intervals.

Question 17. What is the difference between an oscillatory and a vibratory motion?
Answer:
Oscillatory motion is the to-and-fro motion of a body about a mean position, for example a swing.
Vibratory motion is a to-and-fro motion at a much faster rate, in which the vibrating body also undergoes a small change in size or shape, for example the string of a guitar.
In simple words: Vibration is a very fast oscillation in which the body slightly changes shape.

Teacher's Note:
1. All vibratory motions are oscillatory, but not all oscillations are vibrations.
2. Strings and drum skins vibrate.