Physics DENSITY AND PRESSURE

What is density?

The mass per unit volume of a material

What is the equation for density?

Density (kg/m^3) = mass (kg) / volume (m^3)

p = m/v

What mass do objects with low density often have?

Low mass

What mass do objects with high density often have?

High mass

How does density change for a susbtance in different states?

A gas is less dense than the same substance in liquid or solid form

Volume of a sphere

4/3πr³

Volume of a cylinder

πr²h

How do you measure the density of regularly shaped objects?

- Place object on a digital balance for more accuracy + record results

- Use a ruler to measure the object's dimensions

- Repeat measurements and take the average

- Calculate volume of the object using the correct formula

- Divide mass by volume to get density

How do you measure the density of irregular shaped objects?

- Place the object on a digital balance for accuracy and record the results

- Place an empty measuring cylinder below the eureka can's spout

- Fill the eureka can with water until the water overflows + empty the measuring cylinder

- Carefully lower the object into the eureka can

- Measure the volume of displaced water in the measuring cylinder

- Repeat + take the average

- Divide mass by volume to get the density

How do you measure the density of liquids?

- Place an empty measuring cylinder on a digital balance + record results

- Fill the cylinder with liquid + record the change in mass

- Record the volume of the liquid

- Repeat + take the average

- Divide the mass by the volume to get the density

What is Pressure?

The concentration of a force

What is the equation for pressure?

Pressure (Pa) = Force (N) / Area (m²)

Explain why heels cause more damage than trainers

- Heels have a smaller area than trainers

- The smaller the area, the higher the pressure

- P = F/A

- Heels exert more pressure than trainers

- More pressure = more damage

Air Pressure: Collapsing can

- Can with a bit of water is heated up + put upside down into cold water

- Causes an implosion

- As the can + water are heated, water molecules gain enough energy to break bonds + change state

- Water vapour exerts a high pressure on the walls of the can as it is very hot -> a lot of KE + great frequency of collisions

- When placed in cold water, the vapour condenses

- Droplets don't exert as much force as the vapour did so the pressure outside the can is greater than inside the can

- Can implodes due to an imbalance of pressure + force pushing on its outside walls

Air Pressure: How does a straw work?

- There is high atmospheric pressure (100,000Pa) on the cup and a low pressure inside the mouth because the mouth acts as a vacuum

- Pressure goes from high to low when using a straw

- pressure balances out

How is pressure exerted by a gas in a sealed + fixed container?

- Molecules are in a constant state of random motion

- They collide with the walls of the container

- During the collision, there is a change in momentum + direction

- This change in momentum exerts a force because: F= M(v - u)/t

- The force exerted on the wall is equal in magnitude and acts in the opposite direction, as stated by Newton's 3rd Law

- As P = F/A, the gas exerts a pressure on the container

How does pressure change as temperature increases?

- As the temp increases, the kinetic energy of the gas increases -> speed of molecules increases

- This means that collisions occur more frequently

- There is a greater change in momentum during the collision

- This means a larger force is exerted on the container and therefore a larger pressure is exerted as the temperature increases

What is fluid pressure?

- A fluid pressure (liquid or gas) exerts a pressure at a point below the surface due to the weight of the fluid above that point

- Acts in all directions

- Acts perpendicular to the surface it is acting on

what factors affect fluid pressure?

- Depth

- Density of the Fluid

-The gravitational field strength of the planet

What is the equation for the pressure difference at different depths?

Pressure (Pa) = Height (m) x Density (kg/m^3) x Gravitational field strength (N/kg)

P = h x p x g