Newton's laws of motion

Isaac Newton's three laws of motion describe the relationship that exists between an object, the forces that are acting on an object, and the object's motion in response to those forces.
Newton's first law of motion states that an object at rest remains at rest and an object in motion remains in motion at constant speed in a straight line unless acted upon by an unbalanced force. This law is sometimes called the law of inertia. If you break this law apart a little bit, you can see that there are two parts to the law. The first part of the law is the "objects at rest" part. An object at rest is not moving relative to whatever point of reference happens to be in use. The stationary object will remain that way unless an unbalanced force does something to the object. The second part of the law is very similar to the first part except that it deals with moving objects. It says that an object will forever move at a constant velocity unless an unbalanced force is applied that causes the object to speed up, slow down, or turn. This is probably the most difficult part of Newton's laws of motion to explain to students, because they always imagine objects on Earth. Students are familiar with thrown baseballs and fired bullets slowing down and dropping. Friction and gravity are the unbalanced forces in this situation. If those forces were not present, that thrown baseball would move at the initial velocity forever.
Newton's second law of motion states that the acceleration of an object depends on the mass of the object and the amount of force applied. In other words, an object will accelerate faster if you push it harder. If you cannot push it harder, you can accelerate the object faster by reducing the mass. Acceleration and force are a direct proportion. Acceleration increases as force increases. However, acceleration and mass are an inverse proportion. Acceleration increases as mass decreases. Newton's second law can be written mathematically. Acceleration = Force/Mass. Sometimes, the law is written to highlight force. Force = mass x acceleration.
Newton's third law of motion states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. This law is often reworded to say that every action has an equal and opposite reaction. I want to make it clear that the two forces involved do not act on the same object. When a person kicks a soccer ball, they apply a force to the ball. The ball then applies a force that is equal in size but in the opposite direction to the foot kicking the ball. Guns and bullet mechanics work well to explain this. When the gun is fired, the force of the powder charge propels the bullet forward, but an equal and opposite force is applied to the gun. This is why guns recoil. However, the recoil of the gun is not the same speed as the bullet. This is because of Newton's second law. The gun is much bigger than the bullet; therefore, the bullet has a large acceleration from that force due to the low mass of the bullet. Conversely, the gun's much larger mass prevents it from accelerating at the same rate that the bullet accelerated.
http://www.pas.rochester.edu/~blackman/ast104/newton3laws16.html