Forces are pushes or pulls on an object. A force is a vector; it has both a magnitude (a number value) and a direction. The fact that forces have directions becomes very important in the study of aerodynamics. For example, if you push straight down on a book on a table, no matter how hard you push down, the book will not move to the side along the table. If you push hard on the side of the book parallel to the table, however, it will move across the table!

Thrust is the force that causes an airplane or a rocket to move in the direction of the thrust. It is usually generated by burning some type of fuel in an engine to cause air and gases to move (exhaust) at high velocities out an exit. Unfortunately, when an object such as a rocket is thrust through the air, there is a resistance force, called drag, acting on the object in the opposite direction from the thrust. The drag is a function of the shape of the object. A round, flat object generates more drag than a long, pointed object, for example. If the drag is equal to the thrust, the object will not move! Usually, rockets and airplanes are designed to decrease the drag so that the thrust is much larger than the drag and the object moves through the air. This design process is often called streamlining.

Velocity is a measure of how fast something is moving, and it, too, is a vector. The direction of the movement is equally as important as its speed! Thrust is calculated by evaluating the rate the hot gases are shooting out the end of the rocket times the velocity of the gases as they exit (exhaust). The thrust will have the same direction as the velocity. To generate a high amount of thrust, the gases are raised to a high pressure (higher than outside the rocket).

Pressure in a fluid is the measure of small forces by the molecules, all over a specific area. Air and gas molecules will automatically move from an area of high pressure to an area of low pressure. In a rocket engine, the gases are burned at a high pressure. The rocket moves through space, where the pressure is very low, so the gases exhaust out the end of the rocket! Because of the pressure difference and the small exit area, the gases exhaust at a high velocity and a high thrust.

In the balloon rocket demonstration, when the balloon was inflated, the air inside was at a higher pressure than in the room. The pressure inside pushed equally in all directions against the balloon's inner surface (rubber), but it could not get out. When the end of the balloon was released, the higher pressure air rushed out the small opening, creating thrust and causing the balloon to shoot across the string!

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If the balloon rocket was fired with a different amount of air in it, the thrust would have been different. The amount of air in the balloon controls how much air exhausts from the balloon. If less air was stored in the balloon (or if a smaller size balloon were used), the rate at which the air came out would be smaller, and then the thrust would have been smaller. If this had happened in the demonstration, the balloon would have moved a shorter distance along the string.

If the flight of a longer, skinnier balloon rocket is compared against a rounder balloon rocket with the same volume of air inside, the rounder balloon rocket would also have traveled a shorter distance along the string! This is because the drag of the round rocket was higher than that of the longer rocket. Since the drag was in the opposite direction to the thrust, it decreased the thrust and thus the distance traveled!

Sir Isaac Newton (see the "Inertia" lesson for details) developed three laws which define and explain the dynamics (and aerodynamics) of motion.

In his first law, he stated that an object at rest remained at rest and an object in motion remained in motion until some outside force acted to change the situation.

His second law is usually stated that an object acted upon by a force experiences an acceleration in the same direction as the force with a magnitude that is proportional to it. We usually state this law as an equation, F = ma, where m is the mass of the object. The boldface is used to remind us that both the force and the acceleration are vectors.

This balloon rocket demonstration is an example of his third law: for every action there is an equal and opposite reaction. The air exhausting out the back of the balloon is the action, and the opposite reaction pushes the balloon rocket along the string. The astronauts also demonstrate this law when they are in space and there is no gravity. If they make a motion such as pressing a button on the wall, they are pushed away from the wall by the same force they used to push the button! That's why they are often seen tied or belted down when they do their work.