Newton’s third law of motion
In I/GCSE Physics, force comes in pairs.
The third law states that, If body A exerts a force on body B, then body B exerts a force of the same size on body A but in the opposite direction.
These forces act on different bodies (i.e. a trolley and the person)
The ‘missing’ force is the force of the ground on the person (which would be shown on a free-body diagram)
The push of the ground upwards o our feet is not a member of three thirds law pair, involving the pull of the earth downwards on us. The two third law pairs in this case are different types of force pairs. One is a gravitational pair, the other is caused by contact between two surface (so if you jump, the contact pair doesn’t exist but the gravitational pair does).
Third law pairs of forces are always of the same type – gravitational, electrostatic, contact etc.
Statics
In I/GCSE Physics,
when two forces are equal, they cancel each other out and the object they’re acting on is stationary – or in EQUILIBRIUM.
VELOCITY diagrams are used to add forces, and to get them into or out of a triangle, then use Pythagoras’ theorem.
Great Job!
