Forces

A force is any influence that causes an object to undergo a specific change

Some forces are :

• Push
• Pull
• Air Resistance
• Gravity
• Friction
• Upthrust

Forces are vector quantities - they have both a magnitude and a direction, and so can be shown with arrows (e.g. 10 N -->)

Forces can be both contact (two objects touch for it to work) and non-contact (both objects don't have to touch for it to work)

Mass is the amount of matter in an object - never changes

Mass is measured in kg (like on your scales)

Weight is mass x gravitational field strength - changes on different planets etc

Weight is measured in N (like most forces)

The gravitational field strength on Earth is 9.8N/kg

Gravitational field strength can vary from planet to planet (e.g. it's 3.8 N/kg on Mars)

You could say weight and mass are directly proportional, if they're both in the same gravitational field. So if the mass doubles on Earth, the weight doubles on Earth too.

A resultant force is the overall force left (it will be the direction of the bigger arrow on a diagram)

e.g.  10N --->  and <--- 30N gives <----20N overall (30-10=20)

You can find the resultant force on a diagram by using pythagoras's theorem, or just triangle:

Force = Spring Constant x Extension

Stretching or compressing a spring requires a force 

The spring constant is measured in N/m and it shows how "stretchy" a spring is

• A higher spring constant means a "stretchier" spring
• A lower spring constant means a "less stretchy" spring

The spring however, does have a limit of proportionality, and if stretched beyond that the equation doesn't quite work :

Moment = Force x Distance from Pivot

So increased force can come from increased distance...

Levers take advantage of this, by having long handles (e.g. a crowbar)

An object will balance on a pivot if the moments on both sides are equal

Pressure = Force / Area

Pressure in Fluids = Density x Height x Gravitational Field Strength

Pressure is the force per unit area 

Several things reduce / increase pressure :

• Snow shoes have a huge surface area to spread out pressure
• "Sharper" knives have less surface area so you can cut with a greater force

Pressure increases with depth in fluids as there is the weight of other particles on top of that point, which adds to the pressure felt

Huge pressures may be written in standard form 

e.g. 1.7 x 10^6 Pa

Upthrust is the force acting up on an object in a liquid

Floating / Sinking

• An object more dense than the liquid it's in will sink
• An object less dense than the liquid it's in will float

This is due to the more dense object not being able to displace it's weight in water and the less dense object can