(These laws are paraphrased)
First law of motion: an object at rest stays at rest, and an object in motion stays in motion, unless an outside force acts upon it.

This is a pretty easy one. Sit on your couch, and keep sitting there. You're going to stay there. Then someone pushes you off the couch. You're going to move off the couch because an external force was applied to you. On the other hand, if you are driving in a car, you're going to keep moving, until an outside force (say, the brake pads on your wheels) is applied to stop you.

Second law of motion: Force equals mass times acceleration (F=ma)

This is a little more complicated. But let's say you have a V8 engine. That engine puts out the same horsepower no matter what car it's in. It's usually in a big pickup truck. But let's say you put that engine in a Volkswagen Beetle. With the same force, but a lower mass, the acceleration is going to be much higher than it was in the truck. On the other hand, put that same V8 engine in a 747 plane, and you're going to have extremely low acceleration, because the mass is so much higher.

Third law of motion: For every action, there is an equal and opposite reaction.

Let's say you have a bowling ball in a bowling alley. Instead of 10 pins, you have a bowling ball with the same mass sitting at the end of a lane. You roll your ball down the lane. When it hits that other ball, the third law of motion comes into effect. The ball that was just sitting there shoots out the end of the lane, whereas the ball that was moving comes to an abrupt halt. The moving ball acted upon the stationary ball by causing it to move. But as a reaction, the stationary ball caused the moving ball to stop. Equal and opposite.

Now, you said four laws. He had only three laws of motion. He also has a law of universal gravitation, which is more complicated than the laws of motion.

Law of gravitation: to get the force of gravity between two objects (say, planets), you have to fill in the following equation. The mass of planet one multiplied by the mass of planet two, divided by the square of the distance between the two planets. Multiply all that by the universal gravitation constant (again very complicated, but if you just accept that it is 6.67300 × 10^-11 m3 kg^-1 s^-2, we can move on). And that's the force of gravity.

So, let's just use one quick example. The sun is massive, which allows it to hold another very large body (the earth) in a constant orbit. But let's say instead of a huge, massive sun like we have, you replaced it with a 10 pound bowling ball. Because you eliminated a huge amount of mass, it would make the force of gravity much, much smaller, and the earth would no longer orbit the sun. It would shoot out into space instead.

I hope these are what you're looking for.
Source(s):
http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html


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