An inverted paper plate rests on a table under chart paper. The same sheet is tried folded and unfolded. Why does the unfolded sheet resist a quick lift more?
(a)A pressure difference acting over the larger area
(b)Extra weight added by unfolding
(c)The unchanged sheet weight alone
(d)A greater upward push from air underneath
Show answer
(a) A pressure difference acting over the larger area
The sheets have equal weight. During a quick lift, air cannot immediately fill the space beneath. The resulting pressure difference across the larger area gives more downward resistance.
Question 2
The envelope of air surrounding the Earth is called the atmosphere. How thick is that envelope of air around the Earth?
(a)It is a thin skin over the ground, thinner than a tall building is high.
(b)It extends up to many kilometres above the surface of the Earth.
(c)It reaches exactly as high as the clouds and stops there.
(d)It reaches a few metres up, about as high as the tallest trees stand.
Show answer
(b) It extends up to many kilometres above the surface of the Earth.
The atmosphere extends many kilometres above Earth. The weight of the air above contributes to atmospheric pressure at the surface.
Making the area smaller on purpose
Question 3
The task uses a blunt nail and a pointed nail on the same wood. Each tool is used with the same push. How much force is applied with the sharper tool compared with the blunt tool?
(a)The same force
(b)A smaller force
(c)No force
(d)A greater force
Show answer
(a) The same force
The same push means the same applied force. A sharp edge does not supply any extra force.
What pressure is
Question 4
Two travel bags have equal total weight, including bags and contents. One has broad shoulder straps and one has narrow shoulder straps. The straps have equal lengths and lie flat on the shoulders. Which quantity differs between the bags?
(a)Contact area on the shoulders
(b)Total weight of the bag and contents
(c)Total force pressing on the shoulders
(d)Length of the straps
Show answer
(a) Contact area on the shoulders
Pressure is force per unit area. The total weight is stated to be the same in every case. The contact area changes, so the pressure changes.
Working out a pressure
Question 5
Assume passenger weight spreads evenly over each boat’s base. Ignore the boats’ own weights. Boat A has a base of 9 m² and carries 6 people. Boat B has a base of 4 m² and carries 4 people. Each person weighs 600 N. How much greater is the larger average pressure, in Pa?
(a)1200
(b)400
(c)200
(d)600
Show answer
(c) 200
Average pressure is total passenger weight divided by base area. Boat A gives 400 Pa and boat B gives 600 Pa. Compare these pressures, not just passenger weights.
Pressure in a liquid column
Question 6
A roof tank supplies a ground-floor tap. The tank is open and the tap is closed. The pipe stays unchanged. Which change increases water pressure just behind the tap?
(a)Use a smaller tank but keep its water surface at the same height above the tap.
(b)Raise the tank so its water surface is higher above the tap.
(c)Use a wider tank whose water surface remains at the same height above the tap.
(d)Lower the tank so the tap is nearer the water surface; a nearer tap has greater pressure.
Show answer
(b) Raise the tank so its water surface is higher above the tap.
With the open tank and closed tap, the pressure just behind the tap depends on the water-surface height. Tank width does not change that height.
A liquid presses in every direction
Question 7
An open bottle has three identical holes around its sides at the same height. The water surface is level and equally far above every hole. All holes open together. How do the initial jets compare?
(a)Water flowing into all holes
(b)No water from any hole
(c)Outward jets of unequal strength
(d)Equal outward jets from all holes
Show answer
(d) Equal outward jets from all holes
The holes have equal size and lie at the same depth below the water surface. Water pressure is therefore equal at every hole, so the initial jets are the same.
How large the atmospheric pressure is
Question 8
One place has 4 mb more air pressure than a nearby place. How many Pa is this difference?
(a)400
(b)4000
(c)40
(d)4
Show answer
(a) 400
One millibar equals 100 Pa. Multiply 4 by 100 to get 400 Pa.
Fast winds lower the pressure
Question 9
Two suspended balloons approach when air is blown between them. A stronger stream makes them approach faster. Which pressure change explains the observation?
(a)Pressure cannot change, because the balloons hang from the same strings.
(b)The pressure difference across the balloons increases, with lower pressure in the gap.
(c)Faster air raises pressure in the gap, so the balloons must move apart.
(d)Blown air always drives the balloons apart, whatever the observed motion.
Show answer
(b) The pressure difference across the balloons increases, with lower pressure in the gap.
In this activity, the faster stream is associated with lower pressure in the gap. The larger pressure outside gives a greater inward force, matching the faster approach.
Question 10
Strong storm winds blow over a house with a weakly fixed roof. Why can the roof be lifted off?
(a)It cannot; the fast wind presses the roof down onto the house.
(b)The rain on the roof makes it lighter.
(c)The fast wind lowers the air pressure above the roof, so the air below pushes it up.
(d)The fast wind raises the air pressure above the roof.
Show answer
(c) The fast wind lowers the air pressure above the roof, so the air below pushes it up.
Fast wind over the roof lowers the air pressure above it. The air below the roof still pushes up with the same pressure. If this difference is large and the roof is weak, the roof is lifted off.
How wind forms
Question 11
A bicycle pump holds compressed air at higher pressure than room air. Its outlet opens. What is the net airflow when the opening is made?
(a)There is no net airflow, because air moves only when a person keeps squeezing it.
(b)The net airflow is into it, from lower pressure in the room to higher pressure inside.
(c)The net airflow is out into the room, from higher pressure inside to lower pressure outside.
(d)The net airflow is equal both ways, because equal amounts of air must be on both sides.
Show answer
(c) The net airflow is out into the room, from higher pressure inside to lower pressure outside.
Air moves from higher pressure to lower pressure. The open balloon and punctured tube are chapter examples of this rule.
Staying safe from lightning
Question 12
Lightning begins near a swimmer, a walker with an umbrella, and a passenger inside an enclosed hard-topped car. A house is nearby. Where is each person safer?
(a)Swimmer: in water; walker: indoors; passenger: inside vehicle
An umbrella is not lightning shelter. Leave water and seek a substantial building or enclosed hard-topped vehicle. Stay inside such a vehicle with windows closed.
Storms and thunderstorms
Question 13
A warm cloud contains tiny liquid water droplets. How can these droplets grow into raindrops?
(a)The cloud forms because rising air warms, and further warming turns the droplets into rain.
(b)Every newly formed tiny droplet immediately falls to the ground as rain.
(c)They collide and join into larger drops that can fall as rain.
(d)Droplets cannot grow until the cloud moves over land.
Show answer
(c) They collide and join into larger drops that can fall as rain.
In a warm cloud, liquid droplets can collide and join. Larger drops fall more readily. Hail and snow involve ice processes and are not described by this liquid-only example.
What causes lightning
Question 14
In which places can lightning occur?
(a)Only between two clouds
(b)Within one cloud, between clouds, or between a cloud and the ground
(c)Only within one cloud
(d)Only between a cloud and the ground
Show answer
(b) Within one cloud, between clouds, or between a cloud and the ground
Lightning is a sudden flow of electric charges. It can occur within one cloud, between clouds, or between a cloud and the ground.
How a cyclone forms
Question 15
How does a cyclone’s strength usually change after it moves from the ocean onto land?
(a)Stays unchanged
(b)Gradually decreases
(c)Gradually increases
(d)Drops to zero immediately
Show answer
(b) Gradually decreases
A cyclone is fed by warm moist air rising from the sea. Over land, that supply is cut off and the cyclone gradually loses strength. It can still cause great damage while weakening.
Common misconception in Pressure, Winds, Storms, and Cyclones
Does the same force always create the same pressure?
Wrong
A sharp and blunt end press equally because I use the same force.
Why it feels right. I compare only the effort of pushing.
Right
Pressure increases when the same force acts over a smaller area.
Worked example
Both presses use the same force. The narrow edge applies it over less clay surface.
Press the broad face and the narrow edge of the same block into soft clay with similar force.
The narrow edge concentrates the force on less area.
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