Kepler's first law:
Planet orbits are ellipses with the sun at one focus of the ellipse.
Kepler's second law:
A line joining a planet to the sun sweeps out equal areas in the ellipse over equal times.
Kepler's third law:
The square of the orbital period of a planet equals the cube of its semimajor axis.
Newton's first law
An object at rest stays at rest. An object in motion stays in motion at a constant speed in a straight line unless acted upon by an unbalanced force.
Newton's second law
The net force on an object is equivalent to the product of object's mass and its acceleration
Newton's third law
Forces come in pairs. For every force, there exists and equal and opposite force.
Acceleration (a) is the change of velocity with respect to time. Velocity is like the speed of the object, but with one important difference. Velocity also considers the direction that the object is moving. For example, an object's speed might be five meters per second (m/s). This tells you how fast it is going, but not its direction. The velocity might be five meters per second, headed east.
Acceleration is a measure of how the velocity changes. It carries units of m/s2. This really means meters per second, per second. For example, if an object moving in a straight line has an acceleration of 10 m/s2 its speed changes by ten meters per second, every second. Starting from rest, after the end of one second, it would be going 10 m/s. At the end of two seconds, it would be going 20 m/s. At the end of three seconds, it would be going 30 m/s, etc.
An object can have constant speed and still have nonzero acceleration if it changes direction. Since velocity has a direction, if the direction changes, the velocity changes, even if the speed does not.
Quantities that have magnitude and direction, like velocity and acceleration, are called vectors. Quantities that just have magnitude, like speed, are called scalars.
Force is a vector. It has a magnitude and a direction. In the simplest terms, you can think of a force as a push or a pull. An object can have more than one force acting on it. If we add up all of the forces acting on an object, we call this the net force. It can be a tricky business, since the forces can be acting in different directions. An object can have two forces acting on it and still have a net force of zero if the two forces are acting in different directions and have the same magnitude.
If we sum up all of the forces acting on an object, and divide by the mass of the object, we will have calculated its acceleration. Similarly, we can say
S F = ma
The Greek letter sigma(S) indicates to sum the forces.