In this lesson, students analyze how objects can transfer energy in a collision, tracing how energy changes from one form to another in an energy system and then exploring the relationship between an object’s mass and its speed.
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Forces and Motion
In order for a space rocket to be launched, it needs an unbalanced force to cause it to move. Unbalanced forces cause movement because they transfer energy into or out of objects or systems. Remember that energy is the ability to do work.
When an unbalanced force is applied to an object, energy transfers to that object. This transfer of energy changes the motion of the object. Imagine that a ball is sitting at the top of a hill. Because of its position on the hill, the ball has a form of potential energy called gravitational energy. Gravitational energy is the energy stored in an object as a result of its vertical position or height.
If someone comes along and pushes the ball, that person applies a force to the ball that transfers kinetic energy to the ball. The ball now has kinetic energy, which is what powers its movement from the top of the hill to the bottom of the hill.
If no other forces acted on the ball once it started moving, it would continue moving because of inertia. However, there are other forces acting on the ball that cause it to eventually slow down. For example, friction is a force that slows motion when two objects rub against each other.
Friction slows motion because it causes some of the energy of the moving object to change into heat. Friction is why your hands feel hot after you rub them together. Drag, also called air resistance, is similar to friction except that it occurs between a solid substance and a fluid such as air.
These same principles that caused the ball to move apply to rockets. Because of this, engineers launching rockets need to apply an unbalanced force on the rocket to cause it to begin to move. However, engineers also need to think about the mass of the rocket they are launching.
This takes us to Newton’s second law, which says that the amount of force needed to move an object any distance depends on its mass. It is written as an equation: “force = mass times acceleration.” Acceleration is an increase in speed over time. It is measured in meters per second squared (m/s2). Speed is the rate at which an object covers distance in a period of time. It is measured in meters per second (m/s). In other words, the more massive the rocket is, the more force will be required to change its motion and launch it into space.
Newton’s third law is the action-reaction law. This law states that for every action, there is an equal and opposite reaction. Action-reaction pairs occur whenever two objects come into contact with each other.
You are experiencing action-reaction forces right now. Remember that gravity is constantly pulling down on you. In reaction to the force of gravity pulling down, the ground has its own force that pushes back up with the same amount of force. This keeps you from sinking into the ground. If the ground did not push back with the same amount of force, you would fall into the ground. In this case, the forces would be unbalanced. Gravity would pull you down with a greater force than the force of the ground pushing back up.
A deflating balloon is another example of action-reaction forces. As the air escapes out the open end of the balloon, the force causes the rest of the balloon to move forward with an equal and opposite force.
Rockets are designed to move because of action-reaction forces. The rocket pushes combustion exhaust downward—the action. This is also called thrust. In reaction, the exhaust pushes the rocket upward.
Because of Newton’s second law, the greater the thrust, the more the rocket will accelerate upwards. Similarly, the more massive the rocket is, the more powerful the thrust will need to be. This is why the International Space Station couldn’t be launched after it was already built. There is no rocket that can produce a powerful enough thrust to get an object that massive into space.
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