TEST! Next Monday Topic 2 “Matter and Forces” Use the “Can you…..?” sheet to help you We’ll do some practice questions on Friday.

Can you continue the pressure questions you started last lesson? Do now! Can you continue the pressure questions you started last lesson?

This lesson – 2.4 Pressure Relate (without calculation) pressure to force and area. Recall and use the equation P = F/A

Oh no! YouTube - falling through ice YouTube - Jason Falls Through Ice

Spread the force over a greater area

Camels

Snow shoes Yoost

Caterpillar tracks

House foundations

My sofa!

I had a dream

Death?

Sharp = small surface area

Drawing pins

Pressure N Pressure = Force Area N/m2 or Pa m2

Can you copy this please? Pressure F N Pressure = Force Area P A x Can you copy this please? N/m2 or Pa m2

An example A woman of weight 600N has a total shoe area of 150 cm2 and a man of weight 750 N has a total shoe area of 360 cm2. What is the pressure beneath their feet?

An example A woman of weight 600N has a total shoe area of 150 cm2 and a man of weight 750 N has a total shoe area of 360 cm2. What is the pressure beneath their feet? Angelina pressure = force/area = 600/150 = 4 N/cm2 Brad pressure = force/area = 750/360 = 2.1 N/cm2

Bunny Suicides involving pressure

Bunny Suicides involving pressure

Bunny Suicides involving pressure

Bunny Suicides involving pressure

Calculating your own pressure Can you do the calculations carefully and neatly on the graph paper to show your pressure (don’t forget you have two feet!) and stick it in your book

Pressure under a car

Pressure under a car Mass = 2000kg Weight = mg = 2000x10 = 20000N Area = ? Pressure = ?

Let’s try some questions!

Gas pressure Phet http://phet.colorado.edu/sims/ideal-gas/gas-properties_en.jnlp

Temperature and pressure With a fixed volume and mass of gas, the pressure increases with temperature Can you copy this please?

Atmospheric pressure

Atmospheric pressure

Atmospheric pressure

Atmospheric pressure

Can you copy this please? Atmospheric pressure The atmospheric pressure caused by air particles colliding with objects is about 110000 Pascals (this is called ‘1 atmosphere’) It is this pressure that pushes drink up a straw or causes suction cups to work. Can you copy this please?

U571 YouTube - Ocean Odyssey - Ocean Pressure YouTube - Styrofoam cup vs. deep sea YouTube - U-571 Trailer

Pressure and depth Pressure acts in all directions and pressure increases with depth.

That’s it! Let’s remind ourselves of what we have done in this unit.

Gravity Gravity is a force between ALL objects! Gravity

Gravity The size of the force depends on the mass of the objects. The bigger they are, the bigger the force! Small attractive force Bigger attractive force

Gravity The size of the force also depends on the distance between the objects.

Gravity The force of gravity on something is called its weight. Because it is a force it is measured in Newtons. Weight

Gravity 800 N On the earth, Mr Porter’s weight is around 800 N. I love physics! 800 N

Gravity On the moon, his weight is around 130 N. Why? 130 N

Mass Mass is a measure of the amount of material an object is made of. It is measured in kilograms.

Mass Mr Porter has a mass of around 77 kg. This means he is made of 77 kg of blood, bones, hair and poo! 77kg

Mass On the moon, Mr Porter hasn’t changed (he’s still Mr Porter!). That means he still is made of 77 kg of blood, bones, hair and poo! 77kg

Weight (N) = mass (kg) x gravitational field strength (N/kg) Calculating weight To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are. Weight (N) = mass (kg) x gravitational field strength (N/kg)

Forces Force is measured in Newtons

Friction opposes motion!

Newton’s 1st Law Remember? If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

Newton’s 1st law If there are no forces, or the forces are balanced – constant velocity Pushing force friction NO RESULTANT FORCE

Newton’s 2nd law There is a mathematical relationship between the resultant force and acceleration. Resultant force (N) = mass (kg) x acceleration (m/s2) F = ma

An example What will be Mr Porter’s acceleration? Mass of Mr Porter and bike = 100 kg Pushing force (100 N) Friction (60 N)

An example Resultant force = 100 – 60 = 40 N FR = ma 40 = 100a a = 0.4 m/s2 Mass of Mr Porter and bike = 100 kg Pushing force (100 N) Friction (60 N)

Density (g/cm3) = mass(g) volume(cm3) Or using the formula triangle; m You have to LEARN this! kg/m3 kg Density (g/cm3) = mass(g) volume(cm3) Or using the formula triangle; m D x V m3 http://www.youtube.com/watch?v=TRkCz3zG7w0&feature=related

Density of regular shapes volume = length x width x height density = mass/volume mass using a scale height width length

Density of liquids Volume Mass of liquid Mass of cylinder Mass of liquid and cylinder Density = mass/volume

Density of irregular shapes (1) mass Difference in level gives the volume of the shape Density = mass/volume

Density of irregular shapes (2) mass Displacement can volume of object Density = mass/volume

Adding forces For example; Resultant force 2 N 6 N 4 N 62

Hooke’s law (F = kx) Force (N) Extension (x) (cm) Limit of proportionality Force (N) The extension of a spring is proportional to the force applied (until the limit of proportionality is reached). The gradient of the graph is equal to k, the spring constant. Extension (x) (cm) Hooke's Law and springs - Physics – YouTube http://www.youtube.com/watch?v=pVdGUTRI49E 63 63

What does k mean in F=kx? k is called the spring constant and is a measure of the stiffness of the spring or material It has units of N/m (newtons per metre) The higher the k the stiffer the spring Materials with a high k need a large force to for a given extension

More info on Hooke at http://www.roberthooke.org.uk/ A material is said to obey Hookes Law if its extension is directly proportional to the applied force More info on Hooke at http://www.roberthooke.org.uk/

Steel, glass and wood! Force Even though they don’t stretch much, they obey Hooke’s law for the first part of the graph Extention 66 66

Rubber Force Extension 67 67

Pressure N Pressure = Force Area N/m2 or Pa m2

Learning profile/Test Use the learning profile to help you study! Test on Tuesday 29th November www.mrsimonporter.wikispaces.com

Let’s try some practice questions