Chapter 16 Section 7 How does a rocket work? Objective: Describe how a rocket works.

Newton’s Third Law of Motion The Chinese invented rockets almost 800 years ago. They used them for fireworks and weapons.

Newton’s Third Law of Motion To understand how a rocket works, you must know Newton’s third law of motion. This law says that for every action, there is an equal and opposite reaction. For example, suppose you are sitting in a rowboat on a lake. You throw a rock in the lake. At the same time, the rowboat moves backward slightly. This is the reaction.

STATE: What is Newton’s third law of motion?

Rocket Engines The force that pushes a rocket forward is called thrust. The greater the thrust, the higher and faster the rocket will travel.

Rocket Engines What causes thrust? Fuel is burned inside a rocket engine. The fuel can be a solid or a liquid. As the fuel burns, inside the engine it produces hot gases that begin to expand.

Rocket Engines The expanding gases create pressure inside of the engine. This pressure forces the hot gases out of the back of the rocket. This is the action force. The rocket moves forward, in the opposite direction. This is called the thrust, or reaction force.

2. DEFINE: What is thrust?

Escaping Earth’s Gravity A lot of thrust is needed for a rocket to escape Earth’s gravity. To get into Earth orbit, a rocket must reach a speed of more than 40,000 km/h. Large amounts of fuel are needed to produce enough thrust to reach this speed.

Escaping Earth’s Gravity Rocket engines need oxygen to burn fuel. In space, there is no air to supply the oxygen. Rockets carry their own.

Escaping Earth’s Gravity As a rocket moves farther away from Earth, the pull of Earth’s gravity on the rocket becomes weaker. Once the rocket is in space, there is little to slow it down. The rocket does not need to burn fuel to keep moving. It keeps moving in the same direction at a constant speed. Fuel is needed in space only to change the rocket’s speed or direction.

3. EXPLAIN: Why must rockets carry oxygen?

16-7 Classwork CHECKING CONCEPTS Rockets were invented by the ___ people. Newton’s third law of motion says that for every action, there is an equal and ___ reaction. The force that pushes a rocket forward is called ___, Rocket fuel needs ___ to burn. As a rocket moves farther away from Earth, the pull of gravity becomes ___.

16-7 Classwork 6. ANALYZE: What is fuel needed for in space? 7. PREDICT: Could a rocket traveling at a speed of 10,000 km/h get into space? Explain. 8. INFER: Why is thrust also known as the reaction force? 9. EXPLAIN: Why does a rocket not need to burn fuel to keep moving once it is in space?