Newton's First Law of Motion, also known as the Law of Inertia, states that an object's velocity will not change unless it is acted on by an outside force. This means that an object at rest will stay at rest until a force causes it to move. Likewise, an object in motion will stay in motion until a force acts on it and causes its velocity to change.
Newton's Second Law of Motion states that 'when an object is acted on by an outside force, the strength of the force equals the mass of the object times the resulting acceleration'. In other words, the formula to use in calculating force is force = mass x acceleration. Opposing forces such as friction can be added or subtracted from the total to find the amount of force that was really used in a situation.
You can demonstrate this principle by dropping a rock or marble and a wadded-up piece of paper at the same time. They fall at an equal rate—their acceleration is constant due to the force of gravity acting on them. However, the rock has a much greater force of impact when it hits the ground, because of its greater mass. If you drop the two objects into a dish of sand or flour, you can see how different the force of impact for each object was, based on the crater made in the sand by each one.
Another way to show this is two push off two toy cars or roller skates of equal mass at the same time, giving one of them a harder push than the other. The mass is equal in both, but the acceleration is greater in the one that you exerted greater force on.
Stated simply, Newton's Third Law of Motion says that 'for every action, there is an equal and opposite reaction.' Use a pair of roller skates and a ball to show how this works. What happens when you're standing still in skates and then throw a ball hard? The force of throwing the ball pushes your skates (and you) in the other direction.
You can also demonstrate this using Newton's Cradle. This apparatus consists of steel balls suspended on a frame. When the ball on one end is pulled back and then let go, it swings into the other balls. The ball on the opposite end then swings up with an equal force to the first ball, as shown in the illustration on the right. The force of the first ball causes and equal and opposite reaction in the ball at the other end.