The weight of the rocket acting downwards and the reaction force of the launcher on the rocket keeping it upright and ready to go. When the engines are fired, an unbalanced force accelerates the rocket into the sky. The upward force of the thrust from the rocket engines is greater than the downward weight of the rocket. This results in an unbalanced upward force, causing the rocket to accelerate upwards. This unbalanced force accelerates the rocket. The star is moving through a rotating galaxy that is, itself, moving through the universe.
While sitting "still," you are, in fact, traveling at a speed of hundreds of kilometers per second. Motion is also a relative term. All matter in the universe is moving all the time, but in the first law, motion here means changing position in relation to surroundings.
A ball is at rest if it is sitting on the ground. The ball is in motion if it is rolling. A rolling ball changes its position in relation to its surroundings. When you are sitting on a chair in an airplane, you are at rest, but if you get up and walk down the aisle, you are in motion. A rocket blasting off the launch pad changes from a state of rest to a state of motion.
The third term important to understanding this law is unbalanced force. If you hold a ball in your hand and keep it still, the ball is at rest. All the time the ball is held there though, it is being acted upon by forces. The force of gravity is trying to pull the ball downward, while at the same time your hand is pushing against the ball to hold it up. The forces acting on the ball are balanced. Let the ball go, or move your hand upward, and the forces become unbalanced.
The ball then changes from a state of rest to a state of motion. In rocket flight, forces become balanced and unbalanced all the time. A rocket on the launch pad is balanced.
The surface of the pad pushes the rocket up while gravity tries to pull it down. As the engines are ignited, the thrust from the rocket unbalances the forces, and the rocket travels upward. Later, when the rocket runs out of fuel, it slows down, stops at the highest point of its flight, then falls back to Earth. Objects in space also react to forces. A spacecraft moving through the solar system is in constant motion. The spacecraft will travel in a straight line if the forces on it are in balance.
This happens only when the spacecraft is very far from any large gravity source such as Earth or the other planets and their moons. If the spacecraft comes near a large body in space, the gravity of that body will unbalance the forces and curve the path of the spacecraft.
This happens, in particular, when a satellite is sent by a rocket on a path that is parallel to Earth's surface. If the rocket shoots the spacecraft fast enough, the spacecraft will orbit Earth. As long as another unbalanced force, such as friction with gas molecules in orbit or the firing of a rocket engine in the opposite direction from its movement, does not slow the spacecraft, it will orbit Earth forever.
Now that the three major terms of this first law have been explained, it is possible to restate this law. If an object, such as a rocket, is at rest, it takes an unbalanced force to make it move. If the object is already moving, it takes an unbalanced force, to stop it, change its direction from a straight line path, or alter its speed.
Newton's Third Law For the time being, we will skip the second law and go directly to the third. This law states that every action has an equal and opposite reaction. Throw a ball up at an angle and it will go up and then come down at the same angle some distance away from you. When you launch your toy rockets, they will come back down to earth at the same angle at which they were launched.
When you launch a pop bottle rocket, it is moving both upward and forward. We know from experience that what goes up must come down. Once the initial thrust is complete, it is gravity that brings our rockets back down. Imagine throwing a ball in a place without gravity. But with gravity pulling down on the ball, the ball will slow down, stop, and fall back toward Earth. How about the forward motion? Now, imagine you kick a ball off a cliff while a friend drops a ball straight down from the same cliff.
The same gravity is acting on both balls, so they fall at the same rate and hit the ground at the same time. However, the ball that you kicked travels forward as it falls, so it lands some distance away from the cliff. In practice, the rockets experience drag caused by air resistance. Rockets are designed with fins and nose cones to minimize drag as they fly.
The launch angle is also important as it governs both the distance and height that a rocket will attain. The distance a projectile travels is referred to as its range. And if an additional external force is applied, the velocity will change because of the force. A model rocket lifting off from the launch pad is a good example of this principle. Just prior to engine ignition, the velocity of the rocket is zero and the rocket is at rest. If the rocket is sitting on its fins, the weight of the rocket is balanced by the re-action of the earth to the weight as described by Newton's third law of motion.
0コメント