In physics, a force is defined as something that changes an
object's state of motion. If you push something, or pull something, you are exerting a force on it. Forces are found all over. Everything on Earth is affected by the force of gravity, which is always pulling down on everything to keep it on the surface of the Earth. We use the terms direction and strength to describe a force. For example, you hit a ball with a bat in a specific direction. The strength of the force would be how hard you hit the ball. The harder you hit the ball, the farther it is going to go.
A scientist named Isaac Newton (1643 to 1727) contributed many things to our scientific understanding of the world. He developed the theory of gravity. He invented a new kind of mathematics which he called "fluxions," but which today we call calculus. He also discovered the laws of motion, which describe how things move. Collectively, we know these laws as Newton's Laws of Motion.
The First Law of Motion says that an object that is in motion will stay in motion, moving in the same direction and at the same speed, until some force acts upon it. For example, the ball you hit across your back yard doesn't stay in the air forever because as soon as it leaves the bat, the force of gravity begins to pull it back down towards the ground. It also slows down because of a second force, air resistance.
Newton's Second Law of Motion says that objects of greater mass require greater force to accelerate. You can observe this law by hitting a ball using different amounts of strength. The ball you hit the hardest goes the farthest.
Newton's Third Law of Motion says that for every action, there is an equal and opposite reaction. This means that there are always at least two forces at work on an object at any one time, and they are the same. For example, when the bat hits a ball, the ball is exerting an equal force on the bat in the opposite direction.