CBSE Class 6 Science Advanced Physics Motion and Measurement of Distances Long Answer Questions Set 01

Long Answer Type

Question 1. What are the advantages of wheel?
Answer:
1. The wheel replaced dragging, so heavy loads can be moved with much less effort.
2. Rolling causes far less friction than sliding.
3. It made carts, cycles, cars and trains possible, so travel became faster and easier.
4. Wheels are used in many machines, such as gears and pulleys.
5. Goods can be carried over longer distances in less time, which helped trade.
In simple words: The wheel lets things roll instead of drag, saving effort and time.

Teacher's Note:
1. Make a short list of 4 to 5 points.
2. Link with friction for full marks.

Question 2. What is the SI system of units and explain why such a system is needed?
Answer:
The SI system (International System of Units) is a set of standard units agreed upon all over the world. It was adopted in 1960 by the General Conference on Weights and Measures. Its units of length, mass and time are the metre, the kilogram and the second.
Why it is needed:
1. Earlier, body parts were used for measuring, and these differ from person to person.
2. Even standard units differed from country to country, for example gram and pound for mass.
3. With one common system, scientists of different countries can compare and share their results easily, and trade becomes fair.
In simple words: SI is the one set of units that the whole world uses.

Teacher's Note:
1. State the three base units.
2. Give the reason with the example of gram and pound.

Question 3. While measuring the length of a knitting needle, the reading of the scale at one end is 3 cm and at the other end is 33.1 cm. What is the length of the needle?
Answer:
The needle is not placed at the zero mark, so subtract the two readings.
Length = 33.1 cm − 3 cm = 30.1 cm.
In simple words: Subtract the first reading from the second reading.

Teacher's Note:
1. Length = final reading − initial reading.
2. Check that both readings are in the same unit.

Question 4. How are the motions of a wheel of a moving bicycle and a mark on the blade of a moving electric fan different? Explain.
Answer:
Wheel of a moving bicycle: the wheel spins about its axle and the axle itself moves forward along with the bicycle. So the wheel has two motions together (circular and linear), which is combined motion.
Mark on a fan blade: the fan stays in its place and the blade only spins about a fixed axis. The mark goes round in a circle, which is circular (rotational) motion only.
In simple words: The bicycle wheel spins and travels; the fan blade only spins.

Teacher's Note:
1. Look for whether the axle moves.
2. Moving axle plus spinning means combined motion.

Question 5. While travelling in a train, it appears that the trees near the track are moving whereas co-passengers appear to be stationary. Explain the reason.
Answer:
Motion is relative. It depends on the observer (the reference).
The trees are fixed to the ground, but our train changes its position with respect to them, so the trees seem to move backward.
The co-passengers do not change their position with respect to us, because they move along with us in the same train, so they appear to be at rest.
In simple words: Motion depends on who is looking: we move with respect to the trees but not with respect to the passengers.

Teacher's Note:
1. Use the phrase 'with respect to'.
2. Rest and motion are relative terms.

Question 6. Write down the similarities (in motion) between:
(i) Movement of the earth about its axis and the movement of the rotating top.
(ii) The birds in the sky and fish in a pond.
Answer:
(i) Both the earth and a top spin about an axis, so both show rotational motion. Both repeat the turn after a fixed time, so both are periodic.
(ii) Both birds and fish move without a fixed path or pattern, changing direction as they like, so both show random motion.
In simple words: Earth and top both spin; birds and fish both wander randomly.

Teacher's Note:
1. Use the name of the motion in your answer.
2. The earth also revolves round the sun, but the question asks about its axis.

Question 7. If an object moves with the speed 18 km/hr in 20 minutes, find the distance covered by the object in SI unit.
Answer:
Speed = 18 km/h = 18 × 1000 ÷ 3600 = 5 m/s.
Time = 20 min = 20 × 60 = 1200 s.
Distance = speed × time = 5 × 1200 = 6000 m (= 6 km).
In simple words: Change everything into metres and seconds, then multiply.

Teacher's Note:
1. SI units here are metre and second.
2. Check: 18 km/h for 1/3 h gives 6 km.

Question 8. Find the speed of Vijay if he covers a distance of 20 km in 0.5 hour.
Answer:
Speed = distance ÷ time.
Speed = 20 km ÷ 0.5 h = 40 km/h (about 11.1 m/s).
In simple words: Dividing 20 km by half an hour gives 40 km in a full hour.

Teacher's Note:
1. Write the unit with the answer.
2. Half an hour means the speed is double the distance.

Question 9. Convert the following:
(i) 54 km/hr = ? m/s
(ii) 15 m/s = ? km/hr
Answer:
(i) 54 km/h = 54 × 1000 m ÷ 3600 s = 54 × 5/18 = 15 m/s.
(ii) 15 m/s = 15 × 18/5 = 54 km/h.
In simple words: Multiply by 5/18 to go to m/s and by 18/5 to go back to km/h.

Teacher's Note:
1. 1 m/s = 3.6 km/h.
2. Learn the factor 5/18.