Forces, Pressure & Moments Key Notes
Mass & Weight
All objects have a force that attracts them towards each other - this is gravity
The mass of an object is the amount of matter or "stuff" it contains. The more
matter an object contains, the greater its mass
An elephant contains more matter than a mouse, so it has a greater mass
Mass is measured in kilograms (kg)
An object's mass stays the same wherever it is in the universe (if you have two
arms and legs on Earth, then these will be present on the moon, on Saturn, in
deep space, in a black hole etc…)
Weight is a force caused by the pull of gravity acting on a mass
weight (N) = mass (kg) x gravitational pull (N/kg)
An elephant has more mass than a mouse, so it has a greater weight (if the
mouse and elephant are influenced by the same gravity)
Weight is measured in Newtons (N) and has both magnitude and direction
Weight has different values depending on where you are in the Universe. An
object's weight can change if it goes into space or to another planet, because
gravity may be weaker or stronger
Satellites & The Solar System
A satellite is any object that orbits another object. Satellites must travel at a
given speed to stay in their orbits
There are two types of satellite:  Natural satellites
 Artificial satellites
The moon is the natural satellite of the Earth
A satellite has to travel at the correct speed to stay in orbit at a particular
distance from a planet. It takes the moon 28 days to make a complete orbit of
the Earth (as it orbits, we see different amounts of the moon (different
phases))
An artificial satellite is an object made by and put into orbit by humans
Artificial satellites can have different type of orbits:  Geostationary satellites
 Polar orbit satellites
Geostationary Satellites - Stay above the same point on Earth
Polar Orbit Satellites - Low orbit around the Earth, passing over North and
South poles
The universe contains over 100 billion galaxies. A galaxy is a group of billions of
stars. Our own galaxy is called the Milky Way, containing ~100 billion stars
(including our Sun)
The Sun is at the centre of our solar system, which contains many different
types of objects including:  A star (the Sun)
 Planets, orbiting the Sun
 Satellites, orbiting planets
 Smaller objects such as asteroids and comets
Our Sun is a star - it seems much bigger than other stars, but this is because it
is so much closer than the others
Most stars have planets orbiting them
Galaxies contain many millions of stars (our galaxy is the Milky Way)
The universe contains everything - billions of galaxies, each with millions of
stars (which usually have planets orbiting them)
The idea that fits scientific observations, allowing us to predict the movement
of the planets is called the heliocentric model
The Sun is at the centre of the solar system, and the Earth and other planets
orbit the Sun in ellipses
The planets in our solar system are: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune and Pluto
The order of the planets can be remembered by the phrase: “my very efficient
memory just stores up nine planets”
Speed, Streamlining & Friction
Speed = Distance  Time
Whenever an object moves against another object, it is likely to experience
frictional forces - forces that act in the direction opposite to the direction of
movement
Friction can be useful - friction between our shoes and the floor stop us from
slipping; friction between tyres and the road stop cars from skidding; friction
between the brakes and wheel help bikes and cars slow down
Friction can also be unhelpful - friction between moving parts causes energy loss
to the surroundings (heat): rub your hands together quickly
Vehicles experience air resistance as they move - caused by the frictional
forces of the air against the vehicle.
The faster the vehicle moves, the bigger the air resistance becomes (top speed
is reached when the force from the engine is balanced by air resistance)
A bigger mass takes longer to accelerate. It takes longer to accelerate as more
force is needed to move the larger object
Acceleration = Force  Mass
Friction always tries to slow moving objects down – it opposes motion
Friction is created whenever two touching objects or surfaces move past each
other, as well as when objects move through air (known as air resistance or
drag)
Forces & Movement
A force is a push or a pull upon an object
 Contact forces - two objects in contact with each other
 Non-contact forces - a force that acts over a distance
Arrows are drawn to represent forces
These arrows always work in pairs - if there is a force one way, there will be an
opposite force the other way
The bigger the arrow, the bigger the force
Pressure
Pressure is exerted when a force is applied over an area
Pressure is measured in Newtons per square meter (N/m2) also known as Pascals
(Pa) – it can also be measured in N/cm2
Pressure = Force  Area
Pressure can be extremely useful - drawing pins have a large round end for you
to push the sharp end into a notice board
The round end applies a low pressure to your thumb, but the sharp end applies a
high pressure to the notice board, so it pushes in
There are 3 states of matter – solids, liquids, and gases (only gases can be
compressed (squashed))
In liquids and gases the particles are moving around – as they bump into the
surface the force of the particles hitting it causes pressure
Pressure is caused by the force of particles hitting the side of the container
The greater the depth, the greater the pressure (the weight of the water
above compresses the water below)
Pressure is transmitted through a liquid
Hydraulic systems can be used to increase the size of a force (master piston
applies the force which is transmitted to the slave piston)
The slave piston has a larger area than the master piston, so the force exerted
by the slave piston on the brakes is greater than the force exerted at the
master (i.e. you get much more force from slave when you gently press the
master)
This is how you can stop a very heavy object, travelling at high speed, by simply
using your foot / your fingers
The larger force at the slave piston comes at a price – it only moves the slave
piston half the distance (so the energy out is the same as the energy in)
Levers & Moments
žLevers are simple machines – they can make work easier by increasing the size
of a force
žA longer lever makes lifting the load easier
Work done = force x distance
Forces can make objects turn if there is a pivot
Think of a see-saw – when no-one is on it, the see-saw is level, but when someone
sits on the end it tips. The see-saw can be balanced again if someone sits on the
other end
To work out moments we need to know:  The force (weight) applied
 The distance from the pivot where the force is applied
Moment (Nm) = Force (N) x Distance (m)