When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
The action and reaction forces don’t always cancel to zero. Here’s why:
Consider two objects, A and B.
Case 1: The arrow pointing away from A represents an external force on the system.
Case 2: A exerts a force on B that may or may not affect B, but doesn’t affect A. Similarly, B exerts a force back. The action and reaction forces don’t cancel out.
Case 3: A and B exert forces that form a force pair. These forces are within and the same system, and cancel out. They do not accelerate the system.
Another example: All the interatomic forces holding a football together form action-reaction pairs, but they don’t accelerate the football.
Two opposing kicks on the football, however, would not form an action reaction pair because they’re acting on the same object, not on different objects.
When an object falls, it’s pulled towards the Earth, and it pulls the Earth towards itself.
Earth accelerates slightly when an object falls towards it.
For two bodies A and B, the acceleration of A towards B is:
A fired cannon gives us better evidence of the difference in mass.
By Newton’s second law, acceleration also depends on mass.
Common misconception → Rockets are propelled by exhaust gases from the atmosphere.
Truth → Rockets are propelled by the reaction forces exerted by the material fired out of them.
The whirling blades of a helicopter force air particles down → Action force.
There is an upward reaction force called lift → Reaction force.
Two vectors at right angles can be resolved into one.
One vector can be split into two parts perpendicular to each other.
These parts are called components.
Resolution → The process of splitting a vector into its components.
The components are horizontal and vertical.
An illustration of vector resolution:
For an object on a horizontal surface, it exerts its weight (mg) downwards, perpendicular to the surface.
For an object on an inclined surface, its weight (mg) is still exerted downwards, but it is no longer perpendicular to the surface.
Consider a monkey pulling down diagonally on a rope.
\
It pulls down with a tension T.
This tension gets split into S and mg.
Here, S is the horizontal component of the vector T, and mg is the vertical component.
Consider a stone being thrown:
Law 1 → An object at rest tends to stay at rest, and an object in motion at a constant speed along a straight line path tends to stay in motion.
This property of resisting changes in motion is called inertia.
Objects undergo changes in motion when a net force not equal to zero acts on them.
Law 2 → When a net force acts on an object, the object accelerates.
This acceleration is:
a = F/m
For an object falling in a vacuum, net force = pull of gravity. (g)
For an object falling in air, net force = force of gravity - force of air resistance.
Law 3 → When one object exerts a force on another, the second object exerts an equal and opposite force on the first.
Forces occur in pairs.
Action and reaction always occur simultaneously on an object.
Newton’s three laws are the rules of nature that connect everything around us together.