Density & Pressure

define density

mass per unit volume

method- find the density of a regular object

1. find the mass of the solid using a balance

2. set to zero to avoid zero error

3. measure the length width and height of the object using a ruler

4. calculate volume (lxwxh)

5. calculate density using formula density=mass over volume

method- find the density of an irregular object

1. measure mass first using a balance

2. (bc wet rock weighs more)

3. set to zero to avoid zero error

4. fill eureka can to the brim with water

5. play measuring cylinder under spout of the can

6. gently place in object, measure volume of displaced water

7. = volume of steel ball

8. read volume at eye level and at bottom of meniscus to avoid parallax error

9. repeat each mass and volume readings

10. calculate density using density =mass over volume

what error could have been made which means the density is too high

mass was too high- didn’t measure the mass of the rock before volume, wet rock weighs more

OR

volume too low- didn’t fill eureka can to the brim or didn’t place rock in gently therefore water splashes out

define pressure

force per unit area

what happens when a drawing pin is pushed downwards

• pushed into surface not ur finger

• bc sharp point has a much smaller area than top disc

• creating a high pressure point

what influence on pressure do tractors large tires have

• spreads weight force over a large surface area

• reducing pressure

• stopping heavy tractor from sinking into mud

what’s pressure measured in

Pa

what are the units for force

N

how can you calculate force when given mass

W=mg

if force is spread over a larger area, it will result in a…pressure

lower

if force is spread over a smaller area, it will result in a…pressure

higher

how do you calculate the pressure you exert on the floor- method

1. stand on a piece of squared paper

2. draw around shoe

3. count up number of 1cm squares your footprint covers

4. give you area of foot, multiply by 2

5. stand on balance to measure ur mass

6. calculate your weight using W=mg

7. calculate the pressure you exert on the floor by using the equation P=F over A

what 2 things can a fluid be l

liquid or gas

explain why pressure in a liquid increase with depth

1. pressure is equal to the force divided by the area

2. as depth of the liquid increases, there is a greater mass of liquid above a point

3. therefore there is a greater weight of liquid above that point

4. greater weight means greater force acting over the same area

5. therefore greater pressure

what is atmospheric pressure

101×10³

water pressure=

total pressure- atmospheric pressure

explain the pattern of the paths of water from the tubes

• pressure increases with depth

• hence the force on the water at the bottom is greatest

explain why the water fills the vases all to the same height

• pressure = row x g x h

• liquid is the same therefore has the same density

• pressure only depends on depth

what does a barometer measure

atmospheric pressure

what is at the top of a barometer and why is this significant

• vacuum

• no particles = no collisions

• no pressure exerted on top of fluid in tube

• so the pressure in the column is just the fluid pressure which is proportional to depth

what happens in the barometer if the atmospheric pressure is high

• high pressure pushing down onto fluid that is open to the air

• causing fluid to rise further up the tube

• the atmospheric pressure equals the height of the column of fluid in the tube

hat happens in the barometer if the atmospheric pressure is low

• low pressure pushing down on fluid that is open to air

• causing fluid to fall down the tube and move out into the main container

• atmospheric pressure equals the height the of the column of fluid in the tube

how is pressure transmitted in a liquid

• evenly

• in all directions

what is pressure only determined by

1. height

2. gravitational

3. density- will be the same if the same liquid is used

what is a manometer used for

to measure difference in gas pressures

what level are the liquids in a manometer if both sides are open to air and why

• equal level

• both sides o fluid have the same atmospheric pressure being exerted on top of the fluid

what level are the liquids in a manometer if the left side is covered and the right open to air when ATM pressure > gas pressure and why

2. how do we use this to calculate the pressure of the gas

• level of fluid is lower on the side open to air (right)

• bc the atm pressure is pushing more on the fluid surface than the gas

2. ATM pressure - row x g x h

with the height being the difference in height of the fluid

what level are the liquids in a manometer if the left side is covered and the right open to air when ATM pressure < gas pressure and why

• level of fluid is higher on the open side of the tube

• because the gas is pushing more on the fluid than the air is

to calculate pressure of gas = ATM pressure + pressure of fluid that is different in height

with height being the difference in height of the fluid

explain the path of the water leaving the bottle from the top hole

• pressure is lower at A

• force acting on water at A is less

• water leaves bottle at lower speed / has less KE

Hole C is at the same depth in the bottle as hole B but on the opposite side of the bottle, explain the shape of the path of the water leaving the bottle from hole c

• path from C is identical to path from B

• because the pressure is the same

• so the force on the water is the same

• so speed of the water is the same

• pressure acts equally in all directions

suggest why there is a hole in the cap of the bottle

to allow air in to maintain equal pressure between air inside and air outside the bottle