Isaac Newton

Newton's Laws of Motion

Isaac Newton added the details with his laws of motion and gravity. He found that an object's condition of motion (whatever it is doing) tended to remain constant unless an outside force acted on the object. This is contrary to the Aristotelian view that all objects tend to remain at rest. In the Newtonian view rest is just one condition of motion.

  1. An object's condition of motion will not change unless an outside (unbalanced) force causes a change.
  2. F=ma
  3. For every action there is an equal and opposite reaction.

Any change in the condition of motion is an acceleration. Recall that an object's motion has both a speed and a direction, the combination being called the velocity. Any change in the velocity (speed or direction) is an acceleration.

Orbital motion is a stable condition of motion. An object will keep doing it until an outside force makes it change. An orbiting object is being constantly accelerated. Its direction of motion is changing constantly, and that is an acceleration. We did a demonstration with a tennis ball on a string to see what happens when the force holding the ball in a circular path goes away. All orbiting objects revolve around the center of mass of the system. We can see this with the models.

Also - we discussed the difference between mass and weight. Mass is the stuff something is made of. Weight is the force of Earth's gravity on mass. These two guys on the space station certainly have mass, but their weight is zero. When they return to Earth their weight goes from zero in space up to their normal weight but their mass is not changing in the process.

Their weight is zero because they (and the station) are in free fall. Free fall means that the falling object is doing whatever gravity wants it to without any other forces interfering. You could experience free fall by jumping off a building, although this experiment is NOT recommended. Here's something to examine. Is this guy really in free fall?

As a scientific model, Newton's laws of motion are frequently tested and are widely used. If those laws were wrong, NASA would not have managed to put Curiosity down on Mars almost exactly in the center of the target area.

We briefly discussed the idea of scientific models and the way they are tested. You use the model to make a prediction and then test the prediction. Good models make useful predictions and explain many things. The very simplest explanation for the motion of the planets is "God makes them move that way." Simple enough, but just try to use that to get a spacecraft to Mars. This is a construct, an unfalsifiable hypothesis, an untestable model.

Newton's Adjustments to Kepler's Laws

Newton added one term to Kepler's third law:


He added a term representing the sum of the masses orbiting each other, measured in Solar Masses (Sun = 1). Kepler didn't find this because the mass of the Sun is so huge compared to the planets that the Newtonian term retains a value VERY close to 1, making it insignificant.

There's another property of orbits we need to be aware of. Take the Moon orbiting the Earth. What point does the Moon orbit around? Is it the center of the Earth? Nope. It's the center of mass of the Earth-Moon system. The Earth's mass is about 83 times the Moon's, so this center of mass is about 1,000 miles below Earth's surface. The Earth's center orbits around the Moon in a path about 3,000 miles in radius. You have a center of mass - it's roughly in the center between your hips (assuming you are stretched out straight). We did some demos with the centers of mass of some interesting objects.