Motion is everywhere and we know motion very well in our daily life but everything we know about motion is our common sense which is not enough. In Kinematics we learn motion without knowing cause of motion. The cause of motion is always a force, that is force causes motion to a body at rest or changes the motion of a body already in motion.
It's more accurate to say the force causes the change in motion of a body not simply motion. If a body moves with constant velocity, the body is already in motion but there is no force (you'll see this in Newton's first law of motion), so the force causes the change in motion.
But it's true that force causes motion to a body at rest and we can still say the force causes change in motion from rest to motion as the state of rest can be considered as the special case of motion, that is the motion at zero velocity. You'll see this in Newton's first law of motion later where we also define force using Newton's first law.
The dynamics part of Mechanics, that is the study of motion in relationship with force which causes motion to a body at rest or the change in motion to a body already in motion begins with understanding the Newton's laws of motion.
You may have been through a lot of real world activities involving motion without knowing what causes it. You may have developed your own basic understanding of motion and its cause such as the push or pull. For example, our common sense understands push and pull as forces but what about friction forces? Friction forces are hidden and to a common sense it seems there is no force at all.
Here we take the basic understanding (don't let basic understanding of force fool you) aside and understand the real experimental facts based on Newton's laws of motion. In Kinematics all we consider is motion but in dynamics we relate motion with force.
There are three Newton's laws namely Newton's first law, Newton's second law and Newton's third law. We simply state them here.
The Newton's first deals with the case when the net force on a body is zero. The statement of Newton's first law is
NEWTON'S FIRST LAW: When viewed from inertial frame of reference, a body has constant velocity if no net external force acts on it.
Now the second law called Newton's second law deals with the case what happens when there is nonzero net force on a body. The ultimate statement of Newton's second law considering how Newton originally stated is (based on momentum)
NEWTON'S SECOND LAW: When viewed from inertial frame of reference, the net force on a body is equal to the rate of change of momentum of the body.
You saw that Newton's first law dealt with the case of zero net force on a body and second law dealt with the case of nonzero net force on a body. Now Newton's third law simply allows us to identify what forces act on a body and can be stated as
NEWTON'S THIRD LAW: If a body \(A\) exerts a force on body \(B\) (action), the body \(B\) exerts the same magnitude of force on body \(A\) (reaction) in opposite direction.
Newton's laws require modification for particles of very small sizes such as in subatomic level and motion near the speed of light. These Newton's laws are vital in all realms of Physics and considered as the heart of Mechanics. These law all combined together tell us everything about the forces on a body.
