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 pushes 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. What 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 have 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 each 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 haven 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. Second Law: The force acting on a body is directly proportional to, and in the same direction as, its acceleration] Newton s first law of motion is, essentially, the definition of inertia the tendency of an object to resist a change in motion. The more mass an object has, the greater its inertia. An object with greater inertia is more difficult to start or stop.
Newton s second law tells us that the force (F) on an object is directly proportional to it s acceleration, and that is dependent on its mass. So, the second law can be summed up as F=ma. Or, acceleration depends on the objects mass and on the net force acting on the object: Force Acceleration = ------------ Mass Orbital Motion: Why do Earth and the moon remain in their orbits? Newton concluded that two factors: inertia and gravity combine to keep the Earth in orbit around the sun and the moon in orbit around the earth. What exactly, is the force of gravity? 9.8 m/s 2. Where have we seen that unit of measurement before, and what does it describe? [it s the acceleration] The Earth s gravity keeps pulling the moon toward it, preventing the moon from moving in a straight line. At the same time, the moon keeps moving forward because of inertia. If not for Earth s gravity, the moon would shoot off into space in a straight line, and if not for the moon s inertia, the moon would collide with Earth due to the pull of Earth s gravity Earth Gravitational Force of Earth Moon Actual Orbit Moon s motion without Gravity (due to inertia) Image created by the author. License: Public Domain
Newton s Laws of Motion (cont d) Newton proposed that whenever one object exerts a force on a second object, the second object exerts a force back on the first. The force exerted by the second object is equal in strength and opposite in force to the first force. If we think of one force as the action and the other force as the reaction, then Newton s third law states that for every action, there is an equal and opposite reaction. Can anyone think of an example of an action-reaction pair? [jumping, rowing...] In those examples, there was always a motion as a result of the forces acting against each other, right? But, can we always detect a motion when paired forces are in action? The answer is no. For example, if I drop my pen, we see gravity pull the pen towards the ground. At the same time, we know from Newton s third law, that the pen must be pulling Earth upward with an equal and opposite reaction force. But we don t feel a giant jolt of Earth moving in that direction because Earth s inertia is so great that its acceleration is too small to notice. You may be asking yourselves whether or not action-reaction forces simply cancel each other out. Before, we talked about the fact that if two equal forces act in opposite directions on an object, the forces are balanced and no motion results. So, why don t the action-reaction forces cancel out as well? The action-reaction forces don t cancel because they re acting on different objects. For instance, in the ice skater image, Figure Skater 1 exerts a right-ward force onto Figure Skater 2 and Figure Skater 2 exerts a left-ward force onto Figure Skater 1. The action-reaction forces act on different objects, namely the two different skaters. Force on Skater 2 Force on Skater 1 Original Image by Benjamin Crowell. Source: http://en.wikipedia.org/wiki/file:skaters_showing_newtons_third_law.svg License: Public Domain. Arrows and text added by Meredith Beaton.
So, what does Newton s Third Law have to do with Astronomy and space exploration? Well, actually, a lot. Remember when we talked about our first solid proof of Earth being a sphere coming from our images taken on the Apollo 17 mission? Those images, the satellites we send into space, the telescopes that we send into space, the unmanned space missions to outer planets, and our manned exploration of the Moon all relied heavily on our knowledge of action-reaction forces. Modern rockets were first developed in the early 1900 s. They owe much of their development to a few scientists. In the early 1990s, the Russian physicist Tsiolkovky described how rockets work and proposed designs for their construction. Robert Goddard, an American Physicist, also designed rockets, and in 1915 Goddard began to build rockets and test his designs. Rocket engines are reaction engines. The basic principle driving a rocket engine is Newton s 3 rd Law, which is what? ("for every action there is an equal and opposite reaction.") A rocket engine pushes its mass in one direction and moves via the reaction that occurs in the other direction. [Demonstration: When you blow up a balloon and let it go so that it flies all over the room before running out of air, you have created a rocket engine. In this case, what is being thrown is the air molecules inside the balloon. When you throw them out the nozzle of a balloon, the rest of the balloon reacts in the opposite direction.] Rockets are controlled by two of Newton s laws: F = ma and for every action, there is an equal and opposite reaction The Second Law The mass of an object is what again? So, when considering a space shuttle or rocket, we need to consider the mass of the rocket. If you have ever seen the Space Shuttle launch, you know that there are three parts: * The Orbiter * The big external tank * The two solid rocket boosters (SRBs) The whole vehicle shuttle, external tank, solid rocket booster casings and all the fuel has a total weight of 4.4 million pounds at launch that s equivalent to about 45 humpback whales! 4.4 million pounds to get 165,000 pounds in orbit is a pretty big difference! The fuel weighs almost 20 times more than the Orbiter. Just how fast does the rocket need to go? In order to lift off the ground, a rocket must have more upward thrust than downward force of gravity. Once a rocket is off the ground, it must reach a certain velocity to go into orbit. This velocity is known as orbital velocity. If the rocket has an even greater velocity, it flies off into outer space. Escape velocity is the velocity a rocket must reach to fly beyond a planet s gravitational pull. The escape velocity a rocket needs to leave Earth is about 40,200 kilometers per hour.
We need enough force to get something that s 4.4 million pounds to travel at 40,200 kilometers per hour! How can we do that considering the laws? As fuel burns during a rocket s ascent into space, what happens to the mass? [It is reduced] And, with increased distance from Earth, what happens to the pull of gravity? [It s not as great] So, the mass decreases, and the pull of gravity decreases. But how do we increase the acceleration? The Third Law The force that lifts the launcher comes from burning the fuel and converting it to energy. The gases produced by the conversion of gas to fuel escape through the nozzle at the base of the rocket. The gases exert an upward force that is equal and opposite to the force of the escaping exhaust. So, by the reactive force of the gases combusting and escaping from the rocket, we increase the acceleration and decrease the mass of the rocket and the pull of gravity as we continue to move up that we are able to continue to accelerate into space even with the burning/use of fuel as we go.