Air exerts pressure –  ( perform the given activity 8.1 )

Q1.  What happens  to the shape of the can?
Ans:
The shape of the can is distorted.

Q2.  Can you guess why the shape of the can gets distorted?
Ans:
As water is poured over the hot can, some steam in the can condense  into water reducing the amount air inside. The outer pressure is higher than the inner pressure. As a result the can gets distorted.

High speed winds are accompanied by reduced air pressure (Activity 8.2 – do the activity)

Q1.  Why is it difficult to force the paper ball into the bottle?
Ans:
When air is blown into the mouth of the bottle, the pressure around it decreases. The air pressure inside the bottle being higher pushes the ball out.

Activity 8.3 –  (do the activity)

Q2.  What did you expect? What happen .
Ans:
I expected the balloons to move apart, but they moved towards each other.

Activity 8.4 –  (do the activity)

Q3.  What do you think will happen to the paper?
Ans:
The paper will move upwards.

Air expands on heating Activity 8.5 –  (do the activity)

Q1.  What makes the balloon inflated when the boiling tube is placed in hot water?
Ans:
The balloon inflates because air expands on heating.

Q2.  Why is the same balloon deflated when the tube is kept in cold water?
Ans:
The balloon deflates because air contracts on cooling.

Activity 8.6 – (do the activity)

Q1.  Why is the balance of the bags disturbed?
Ans:
The bag below which the candle is lighted, is pushed up by the warm air created by the lighted candle.

Q2.  Can you now explain why smoke always rises up.
Ans:
Smoke is hotter than air around it. And so is lighter than air. Therefore smoke always rises up.

Activity 8.4

a) I wonder why winds shown in the figure are not in the exact north – south direction.
Ans:
This is caused due to the rotation of the earth.

b) Uneven heating of land and water .

Q1.  I want to know what these winds do for us.
Ans:  The winds from the oceans carry water and bring rain.

TEXTUAL EXERCISES

Q1.  Fill in the missing word in the blank spaces in the following statements:

a) Wind is _______  air .

b) Winds are generated due to __________  heating on the earth.

c) Near the earth’s surface  _________  air rises up whereas ________ air comes down.

d) Air moves from a region of __________  pressure to a region of ________  pressure.

Ans:
a) moving               b) uneven               c) warm, cold               d) high, low   .

Q2.  Suggest two methods to find out wind direction at a given place.
Ans.
i) Seeing the direction of dry leaves released in air.
ii) Seeing the direction of wet clothes put for drying.

Q3.  State two experiences that made you think that air exerts pressure.
Ans.

i) When we overfill air in the balloon, it will burst.
ii) Wind causes curtains, doors and windows to move with the direction of the wind.

Ans:
No, warm air is lighter than cold air so it will move out from the ventilators. Without ventilators the house will remain warm.

Q5.  Explain why holes are made in hanging banners and hoardings.
Ans:
Holes help the air to pass through it. Hence banner can withstand air pressure and remain intact.

Ans:

i) Warn everyone about oncoming cyclone.
ii) Help the neighbours to shift to a safer place.
iii) Calling emergency services.
iv) Helping to shift essential household goods, domestic animals etc.

Q7.  What planning is required in advance to deal with the situation created by a cyclone?
Ans:

i) A cyclone forecast and warning service.
ii) Keeping ready phone numbers of emergency services.
iii) Shift along with household essentials to a safer place.
iv) Setting up of cyclone shelters.

Q8.  Which one of the following place is unlikely to be affected by a cyclone.
i) Chennai                   ii) Mangaluru (Mangalore)                    iii) Amritsar                    iv) Puri

Ans:   iii) Amristar

Q9.  Which of the statements given below is correct?

i) In winter the wind flow from the land to the ocean.
ii) In summer the wind flow from the land towards the ocean .
iii) A cyclone is formed by a very high – pressure system with very high – speed winds revolving around it.
iv) The coastline of India is not vulnerable to cyclone .

Ans:  i) In winter the wind flow from the land to the ocean .