Physics IGCSE - Motion, Forces and Energy

Force

a push or pull that changes the size, shape and motion of an object

How an objects sinks or floats via density

less density than fluid → float
more density than fluid → sinks

Newton's 1st law

object remains at rest or continue to move at a constant speed unless there is a resultant force on it

Resultant of 2 or more forces acting on same straight line

Add their sizes together

Friction

the force between 2 surfaces that may impede relative motion and make heat

Friction through liquid

drags an object through liquids (e.g. water resistance)

Friction through gas

drags an object moving through gases (e.g. air resistance)

Newton's 2nd law

An object will accelerate in response to a resultant force

Resultant force equation

F (N) =ma

Direction of resultant force and acceleration

the same direction

Elastic

When objects are stretched and they return to their original shape

Tensile

to stretch

Compressive

to squeeze

Hooke's Law

force applied to a spring will cause it to extend by an amount proportional to the force

Load - extension graph

Straight line: extension of spring is proportional to load applied.

Spring constant

Force per unit extension

Spring constant equation

k (extension) = (force) F/x (extension)

Limit of proportionality

force is no longer proportional to the extension of the object.

Newton's 3rd law

When 2 objects interact, every action has an equal and opposite reaction

Pivot

the central point on which a mechanism turns or oscillates

Moment

the measure of a its turning effect (described having clockwise or anticlockwise direction)

Everyday examples

- opening doors
- seesaws
- scissors

Moment equation

Moment (Nm) = Force (N) x perpendicular distance from pivot (m)

Principle of moment

sum of clockwise moments = sum of anticlockwise moments
M = Nm

No resultant force

No resultant force + no resultant movement = object in equilibrium

Centre of gravity

average location of weight of object as centre of gravity (centre of mass)

Regularly shaped objects centre of gravity

perfectly centre (labelled 'g')

Experiment for finding centre of gravity for irregularly shaped object

hang object from several points and using a ruler to mark vertical line, intersection of 2 or more lines show centre of of gravity.

Effects of position of centre of gravity on the stability of simple objects

Higher centre of gravity → easier to topple
Lower centre of gravity → more stable