CBSE Class 7 Science Advanced Physics Our Universe Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. Fill in the blank: Substances whose density is ___ the density of water, sink in water.
Answer:
greater than
In simple words: A thing denser than water cannot be held up by it, so it sinks.

Teacher's Note:
1. Greater density means sinking, smaller density means floating.
2. Water’s density is 1 g cm⁻³.

Question 2. Fill in the blank: __________ is responsible for the motion of planets around the sun.
Answer:
Gravitational force (the force of gravitation).
In simple words: The sun’s pull keeps the planets going round it.

Teacher's Note:
1. Newton showed that the same force that pulls an apple down also holds the planets in their orbits.
2. Name it as the force of gravitation.

Question 3. Fill in the blank: For a freely falling body, its acceleration ________ as it falls.
Answer:
remains constant (it stays equal to g).
In simple words: A falling body speeds up by the same amount every second.

Teacher's Note:
1. Velocity changes, acceleration does not.
2. The value is about 9.8 m s⁻².

Question 4. Fill in the blank: For a freely falling body the distance covered is directly proportional to the _______.
Answer:
square of the time.
In simple words: Fall for twice as long and you cover four times the distance.

Teacher's Note:
1. This is Galileo’s third law of free fall.
2. Write s ∝ t².

Question 5. Fill in the blank: Development of cyclone depends on the wind _______ and _______.
Answer:
speed and direction.
In simple words: How fast the wind blows and which way it blows both matter for a cyclone.

Teacher's Note:
1. Temperature and humidity also play a part.
2. Name speed first, then direction.

Question 6. Fill in the blank: At higher altitudes the height of mercury column in a barometer will be _______.
Answer:
low (less than 76 cm).
In simple words: Higher up there is less air above, so the pressure is smaller and the column is shorter.

Teacher's Note:
1. Pressure decreases with altitude.
2. The column at sea level is 76 cm.

Teacher's Note:
1. Do the sure matches first: rocket with Newton’s third law.
2. The new moon tide is stronger because the sun and moon pull from the same side.

Teacher's Note:
1. Link each name to its discovery.
2. The spinning earth gives rise to centrifugal force.

Teacher's Note:
1. g is larger at the poles because they are nearer the centre of the earth.
2. Wind is caused by a change in air pressure.

Question 10. What happens to the pressure when the speed of air is increased?
Answer:
The air pressure decreases.
In simple words: Faster moving air has lower pressure.

Teacher's Note:
1. High speed wind always goes with reduced pressure.
2. This is why a roof can be lifted in a storm.

Question 11. What is a tornado?
Answer:
A tornado is a dark funnel shaped cloud that reaches from the sky to the ground.
In simple words: It is a spinning funnel of air that touches the ground.

Teacher's Note:
1. Its width can range from about 1 m to 1 km.
2. A violent one can move at about 300 km h⁻¹.

Question 12. What is ‘anemomter’?
Answer:
An anemometer is a device used to measure the speed of the wind.
In simple words: It tells how fast the wind is blowing.

Teacher's Note:
1. Do not confuse it with the barometer, which measures air pressure.
2. Wind speed matters in forecasting storms.

Question 13. Why are wind currents generated?
Answer:
Wind currents are generated due to the uneven heating of the earth. Warm air rises, which lowers the pressure there, and cooler air rushes in to take its place.
In simple words: The sun heats places unequally, and air moves from the cooler to the warmer regions.

Teacher's Note:
1. Moving air is called wind.
2. Air always moves from high pressure to low pressure.

Question 14. What is gravitation?
Answer:
Gravitation is the force of attraction between any two material bodies in the universe.
In simple words: Every object pulls every other object.

Teacher's Note:
1. It acts everywhere in the universe.
2. It is unaffected by heat, pressure, electric and magnetic fields.

Question 15. What is weight of a body?
Answer:
The weight of a body is the force of attraction exerted by the earth on the body. W = m × g.
In simple words: Weight is how hard the earth pulls you down.

Teacher's Note:
1. Weight changes with g, mass does not.
2. The SI unit of weight is the newton.

Question 16. What is a freely falling body?
Answer:
A freely falling body is one that falls under the force of gravity alone, with no other force acting on it.
In simple words: It falls with nothing but gravity acting, and so it feels weightless.

Teacher's Note:
1. Air resistance must be absent.
2. Its acceleration is g.

Question 17. What is acceleration due to gravity? What is its average value?
Answer:
The constant acceleration produced in a freely falling body by the earth’s gravity is the acceleration due to gravity, denoted by g.
Its average value on the earth is 9.8 m s⁻² (980 cm s⁻²).
In simple words: It is how much a falling body’s speed increases every second: about 9.8 m s⁻¹ each second.

Teacher's Note:
1. The value is 9.8 m s⁻² in SI and 980 cm s⁻² in CGS.
2. It is larger at the poles than at the equator.

Question 18. What are different types of tides?
Answer:
There are two main types: high tides and low tides.
High tides are of two kinds: direct (primary) tides and antipodal (secondary) tides.
Spring tides and neap tides are the tides formed by the relative positions of the sun and the moon.
In simple words: The sea water rises to make high tides and falls to make low tides.

Teacher's Note:
1. Primary tides face the moon, secondary tides are on the opposite side of the earth.
2. Do not mix the two groups: high/low and spring/neap.