CBSE Class 7 Science Advanced Physics Kinematics Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. State whether true or false: One hour is equal to 360 second.
Answer:
False. One hour = 60 × 60 = 3600 seconds.
In simple words: An hour has 3600 seconds, not 360.

Teacher's Note:
1. Do not forget the second 60 when changing hours to seconds.
2. Minutes first (60), then seconds (60 again).

Question 2. State whether true or false: The motion of a simple pendulum is an example for oscillatory motion.
Answer:
True. The bob moves to and fro about its mean position.
In simple words: A swinging pendulum bob moves back and forth, so it is oscillatory.

Teacher's Note:
1. Any to-and-fro motion is oscillatory.
2. It is also periodic.

Question 3. State whether true or false: A nanosecond is one billionth of a second.
Answer:
True. 1 nanosecond = 10⁻⁹ s, which is one billionth of a second.
In simple words: Nano means one-billionth.

Teacher's Note:
1. Nano = 10⁻⁹.
2. Very small times are used in computers and lasers.

Question 4. State whether true or false: When an object is moving with a uniform speed, its average speed is equal to its velocity.
Answer:
False. When the speed is uniform, the average speed equals the (uniform) speed, not the velocity. Speed has no direction, but velocity has direction, so they are different quantities (their magnitudes are equal only for motion in a straight line).
In simple words: Speed and velocity are different things, since velocity needs a direction.

Teacher's Note:
1. Compare quantities of the same kind: average speed with speed.
2. Velocity must have both a magnitude and a direction.

Question 5. State whether true or false: We can use a simple pendulum to measure the time.
Answer:
True. Its motion is periodic, so the time taken for equal numbers of oscillations is the same. Old wall clocks used pendulums.
In simple words: A pendulum swings at a steady rhythm, so it can keep time.

Teacher's Note:
1. Periodic motion is the basis of all clocks.
2. Quartz clocks are now more accurate than pendulum clocks.

Question 6. State whether true or false: By using a distance-time graph we can find out the average speed of an object.
Answer:
True. From the graph, average speed = total distance ÷ total time, both of which can be read from the graph.
In simple words: The graph gives the distance and the time, so we can divide to get the speed.

Teacher's Note:
1. The slope of a distance-time graph gives the speed.
2. The graph also gives the distance at any instant.

Question 7. State whether true or false: Speedometer is used to measure the distance travelled by a vehicle.
Answer:
False. A speedometer measures the speed of a vehicle. The distance travelled is measured by an odometer.
In simple words: The speedometer shows speed, not distance.

Teacher's Note:
1. Speedo-meter: speed. Odo-meter: distance.
2. Both are fitted on the dashboard.

Question 8. State whether true or false: The distance-time graph of the motion of an object moving with a constant speed is a straight line.
Answer:
True. For constant speed the distance covered increases by equal amounts in equal times, so the graph is a straight line.
In simple words: Steady speed gives a straight sloping line on the distance-time graph.

Teacher's Note:
1. The steeper the line, the greater the speed.
2. A curved line means that the speed is changing.

Teacher's Note:
1. Do the sure matches first and leave the doubtful ones for the end.
2. Note that 1 m s⁻¹ = 18/5 km h⁻¹.

Teacher's Note:
1. Rectilinear means a straight path and curvilinear a curved path.
2. T ∝ √ℓ is the link between time period and length.

Teacher's Note:
1. For the seconds pendulum use T = 2 s in ℓ = gT²/4π² with π² = 10.
2. D is a rearranged form of v = u + at.

Teacher's Note:
1. Learn the example of each kind of motion along with its definition.
2. Distance = speed × time is the most used relation in this chapter.

Teacher's Note:
1. Average speed can be matched with both its formula and its unit.
2. A falling body keeps gaining speed, so it is non-uniform.

Teacher's Note:
1. A ball rolling on the ground slows down, hence non-uniform.
2. Acceleration = change in velocity ÷ time.

Question 15. When can the speed and average speed of a vehicle be equal?
Answer:
They are equal when the vehicle moves with uniform (constant) speed.
If the speed does not change, total distance ÷ total time always gives that same constant speed.
In simple words: Steady speed means the average is the same as the speed at every moment.

Teacher's Note:
1. Average speed matters only when the speed keeps changing.
2. Check by taking any two stretches of the journey at the same speed.

Question 16. While determining the time period of a simple pendulum, why is the time for 20 or more oscillations found?
Answer:
One oscillation is over very quickly, and the small error in starting and stopping the stop watch would be a large part of a very short time. If the time for 20 or more oscillations is measured and divided by the number of oscillations, the error is divided too, so the time period is found more accurately.
In simple words: A longer time is easier to measure correctly, and dividing gives the time of one swing.

Teacher's Note:
1. The same stop watch error matters less in a long measurement.
2. Take an average of repeated readings for even better accuracy.

Question 17. Define amplitude and frequency. Write their SI units.
Answer:
Amplitude: the maximum displacement of the bob of a pendulum from its mean position. SI unit: metre (m).
Frequency: the number of oscillations made in one second. SI unit: hertz (Hz).
In simple words: Amplitude is how far it swings; frequency is how many swings in a second.

Teacher's Note:
1. Frequency n = 1/T.
2. Amplitude is a distance, so its unit is the unit of length.