Newton’s Laws of Motion The Earth revolves around the sun in an elliptical orbit. The moon orbits the Earth in the same way. But what keeps the Earth and the moon in orbit? Why don’t they just fly off into space? Or crash into each other? The first person to answer these questions was the scientist Isaac Newton. Newton realized that there must be a force acting between Earth and the moon that kept the moon in orbit. What is a force? [ A force is a push or a pull ]. When one object push es or pulls on another, you say that the first object exerts a force on the second object. In science, we represent forces by arrows. The direction of a force is represented by the direction of the arrow. The length of the arrow tells you the strength of the force – the longer the arrow, the greater the force. Demonstration: Pull a chair across the floor of the room, then try to pull a chair across the room with a student sitting in it. W hat’s the difference? [ the more mass the object has, the more it resists the change/force].
What is the scientific name for that resistance to change? [ inertia ] Did the chair move on its own? [ No ] Someone had to push or pull it — in other words, exert a force on it. If the two forces are balanced, would the chair move? [ No ] If the forces were unbalanced, in other words, if I pushed harder on the chair than the inertia of the chair, would it move? [ Yes ] When the object is at rest, does it have a speed? [ yes — but it’s zero ] When the object is moving, does it hav e a speed? [ yes ] What do we call that change from a speed of zero to a speed of something? [ acceleration ]
So, let’s bring it back to the initial question: what keeps the Earth and the moon in orbit, preventing them from crashing or flying off away from eac h other? If the sun and Earth are constantly pulling on one another because of gravity, why doesn’t Earth fall into the sun? Why doesn’t the moon crash into Earth? The fact that these collisions haven’t happened means there must be some other factor we ha ven’t yet taken into account. What are Newton’s first two laws of motion ? [First Law: Objects tend to stay at rest or move in a straight line at a constant speed unless acted upon by an outside force.
- Fall '10
- Isaac Newton