Basic Ideas of Physics
You can see 3 basic ideas of physics.
These are called Newton’s 3 laws of motion.
However, here their order is changed for you to understand.
Law 1 : Velocity in a direction increases in proportion to a force applied in the direction
This is the most important law in physics.
Conversely, if the speed of an object increases, it means some force is applied to the object.
A force increases velocity of an object, so it is not needed for keeping the velocity constant.
Law 2 : Unless some force is applied, an object moves straight at a constant velocity.(An object remains at rest)
An object will move straight at a constant velocity unless some force is applied. (In reality, because some frictional forces are always applied, velocity of an object decreases.)
On the other hand, object remains at rest unless some force is applied.
This is called Law of Inertia.
We can derive another law.
Law 3 : When you exert a force on an object, you are also exerted the opposite direction force
When you exert a force on an object, simultaneously you are exerted a force equal in magnitude and opposite in direction by the object.
For example, if you push a wall, you are also pushed by the wall. This is called Law of Action-Reaction.
Assume that there are two people at rest, Mr.A and Mr.B in space.
The two people have the same weights.
Regard the two people as one group and there is no external force to apply to that group.
Therefore, the center of gravity(CG) of the two people doesn’t move (Because of Law 2).
The red cross represent the CG of the group.
If Mr.A pushes Mr.B, Mr.B flies away in the direction Mr.A pushed.
However, the CG of two people doesn’t move.
Therefore, Mr.A must fly away in the opposite direction.
The small red crosses represent the CGs of Mr. A and Mr. B.
It means Mr.A is also exerted a force by Mr.B and its direction is the opposite.