Pressure

Question 1

Convert 1000 kg m^-3 to g cm^-3

Answer 1

1 g cm^-3

Question 2

What is a characteristic of density of pure substance

Answer 2

The density of pure substance is unchanged, irrespective of the amount of substance

Question 3

What happens when object is placed in liquid of same density

Answer 3

The object remains wherever it is released

Question 4

What is the formula for pressure(non-liquid)

Answer 4

P = F/A where P can be in Nm^-2 or Pa. 1Pa = 1 Nm^-2

Question 5

When a man is about to jump, why is pressure exerted on ground greater than pressure exerted when standing

Answer 5

In order for the man to accelerate upwards, normal reaction force exerted on the ground has to be larger than his weight. But when standing, his normal reaction force is equal to his weight

Question 6

How to increase pressure

Answer 6

Increase the force or decrease the contact area

Question 7

What is Pascal’s Principle

Answer 7

If external pressure is applied to enclosed fluid, pressure is transmitted equally to all other parts of the fluid

Question 8

What 3 properties of liquids do hydraulic presses take advantage of

Answer 8

Liquids are incompressible
Pascal’s Principle(must state in full)
Liquid has no fixed shape, can fill up space in a container

Question 9

What is the equation for force/surface area acting on the two pistons of a hydraulic press

Answer 9

F1/A1 = F2/A2
V1 = V2
Volume of liquid pushed by both pistons are equal

Question 10

What is atmospheric pressure

Answer 10

Force due to molecular bombardment by energetic air molecules on us in all directions.
Atmospheric pressure drops by half every 5.6km(decrease w altitude)

Question 11

Why do humans bleed at high altitudes

Answer 11

At higher altitudes, atmospheric pressure is lower, breathing is difficult and nose bleeding occurs as human’s large blood pressure bursts some blood vessels in nose due to difference in pressure

Question 12

Why do aircrafts have pressurised cabins

Answer 12

Air pressure is at sufficient level to protect passengers and crew. Pressure is slightly lower than sea level to allow aircraft frame to be thinner and saves manufacturing and fuel cost

Question 13

What happens when an object experiences a pressure difference

Answer 13

When there is difference in pressure exerted on opposite sides of rigid body, there will be net force which acts in direction of region of higher pressure to region of lower pressure

Question 14

How do people suck through a straw

Answer 14

Sucking through a straw lowers air pressure in mouth/straw. Atmospheric pressure being larger than air pressure in straw, pushes on liquid surface. Pressure difference results in net force which pushes liquid up the straw and into the mouth

Question 15

How does a suction cup work

Answer 15

When a suction cup is pressed against a wall, most of air underneath suction cap is squeezed out. This creates pressure difference between atmospheric pressure and trapped space under suction cup. Net force acts inwards on suction cup to hold it against a surface

Question 16

What is equation for liquid pressure at certain depth

Answer 16

P = hpg

Question 17

Explain why pressure in water at depth of 30m is very different from that at surface, but atmo pressure 30m below surface of earth is almost same at surface

Answer 17

Density of air under the surface is much lower than the density of water

Question 18

How to find atmospheric pressure using mercury barometer

Answer 18

Height difference between top of mercury and mercury level in trough. Unit is cm Hg or mm Hg

Question 19

What happens when liquid levels(same liquid) in two connected columns are not at same height

Answer 19

Pressure at base of right arm is higher than that of left arm. Liquid in horizontal section will experience net force to the left. Causes liquid to move to the left and equalise the water levels of the two arms

Question 20

How to find difference in pressure using manometer

Answer 20

Difference in height of liquid in both arms

Question 21

One arm of manometer connected to gas supply of higher pressure and other exposed to atmo pressure. What happens to height diff of liquid when
Atmo pressure increases
Density of liquid decreases
Diameter of U-tube decreases
More mercury poured into tube

Answer 21

Difference will decrease
Difference will increase
Difference remains the same
Difference remains the same

Question 22

Explain whether a sealed empty bottle of dense material with cap will float or sink in water

Answer 22

It will float. As the density of air within the bottle has much lower density than water, the density of the empty bottle will be generally lower than that of the water and will float

Question 23

Water in manometer is replaced with mercury. Explain the height difference between the two arms(one arm connected to gas source)

Answer 23

Height difference will decrease. A shorter column of denser liquid exerts the same pressure as a taller column of less dense liquid for the same pressure difference between both arms