First 264 words of the document:
P4 Explaining Motion
Forces are needed for motion. Forces act in a particular DIRECTION (push or pull). Forces are
measured in newtons, N. forces always come in pairs. Each force in a reaction pair is 1) equal in size 2)
opposite in direction 3) of the same sort ( gravity). Forces are shown on diagrams as arrows. Force is
a vector which means you need its size and direction.. The size of the force can be written as a
number or indicated by the size of the arrow.
This is newtons 3rd law of motion-
Every action (force) has an equal and opposite reaction (force)
The resultant force on an object is the TOTAL force acting on it.
All forces acting in the same direction add up
All forces acting in opposite directions cancel out.
Resultant force and motion
Without a resultant force acting on an object, it's motion will not change. We do not need a forcer to
keep something moving, only to change it's motion.
We need a resultant force to
Start something moving
Stop something moving
To speed something up
Slow something down.
This is Newtons 1st law: Zero resultant force means zero change in motion.
Exam tip : Look for the words "constant speed" or "at rest" or "constant velocity" in exam questions-
this means there is 0 resultant force on the object (all forces acting on it are balanced).