Grade 6-8 | 7 (45 min) Classes
In this lesson, students will answer the question, Does the mass of a model rocket affect the altitude, velocity and duration of the rocket’s flight? Your students are part of a group of engineers working for Rockets R US which has been contracted to build rockets that will carry the PocketLab Data Analyzer to astronauts on the International Space Station (ISS). They must be sure that their rockets reach 200ft and contain a payload of no more than 10 pennies to reach the ISS. Students will determine which team member below is correct.
- Martin Moneysaver wants the rocket to carry only the data payload (PocketLab) to the ISS because he feels that would save money for fuel.
- Sha’Niece Supersupplier says the rocket needs to be packed with as many things as possible so that the astronauts on the ISS have fresh food, games, clothing, and comfort items from home. She doesn’t think the amount of mass in the rocket really makes much of a difference.
- Wes Wishywashy is afraid that if they don’t send any supplies for the astronauts the rocket will be too light to fly.
Students will hypothesize how the mass of the payload will affect the rocket’s final altitude. After a review of weight vs. mass, students will practice what they learned and build a straw rocket. After analyzing the data, students will apply those learnings to build the Green Eggs Rocket and plan their payload for their flight. They will state a hypothesis and identify the independent and dependent variables.
After the flight, the students will combine their data into a class data chart to allow better analysis. They will compare the results and determine the answer to the essential question, “Does Mass Matter?”.
The student’s final product will be to complete a Claims- Evidence- Reasoning writing piece supporting their final conclusions. A traditional multiple-choice quiz is included for use if needed.
Targeted Performance Expectation(s):
Next Generation Science Standards (NGSS)
Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.
Construct and present arguments using evidence to support the claim that gravitational interactions are attractive and depend on the masses of interacting objects.
Construct and interpret graphical displays of data to describe the relationships of kinetic energy to the mass of an object and to the speed of an object.
Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
Evaluate competing design solutions based on jointly developed and agreed-upon design criteria using a systematic process to determine how well they meet the criteria and constraints of the problem.
Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.
Write and evaluate numerical expressions involving whole-number exponents.
Use variables to represent two quantities in a real-world problem that change in relationship to one another.
The rate at which an object increases its speed.
The peak altitude or highest point of a rocket’s flight.
A push or pull upon an object resulting from the object’s interaction with another object.
The amount of matter in an object.
Cargo (equipment, goods, or materials) carried by a rocket.
A first model from which other models are developed.
The rate of motion or speed in a given direction. Measured in terms of distance moved per unit time, in a specific direction.
The amount of gravitational pull exerted onto an object.
Sir Isaac Newton
What is known about rocketry today can be traced back to the time of Sir Isaac Newton (1642 – 1727). Newton described the motion of objects falling to the Earth in his book Philosophiae Naturalis Principia Mathematica where he outlined three laws of motion. Although Newton was merely describing principles of nature, Newton’s Laws apply to the physics of rocketry. His laws are simple statements regarding the physics governing motion and can be used to provide precise explanations of rocket flight.
Newton's First Law of Motion
Objects at rest will remain at rest and objects in motion will remain in motion in a straight line unless acted upon by an unbalanced force
During a model rocket flight, forces become balanced and unbalanced all the time. A rocket on the launch pad is in a state of rest. This is called inertia. A rocket is balanced all the time because the surface of the pad pushes the rocket up while the force of gravity tries to pull it down. An unbalanced force must be exerted for a rocket to lift-off from the launch pad. A rocket blasting off the launch pad changes from a state of rest to a state of motion. It will keep moving in a straight line at the same speed unless it is acted upon by an unbalanced force (gravity and drag).
There are four forces (drag, gravity, thrust, lift) that act on all objects that travel through the air. Drag and gravity are the two unbalanced forces that act on a model rocket. Drag is the resistance or frictional force between the surface of a moving object and air. Drag increases with speed. Gravity is the force pulling an object back to the surface of the Earth. The amount of this force is proportional to the mass of the object.
When the rocket lifts off the launch pad it is guided by the launch rod in a straight line upward. The unbalanced forces (drag and gravity) cause it to arch and fall to the ground.
Newton's Second Law of Motion
Force is equal to mass times acceleration. (F = ma)
The amount of thrust (force produced by a rocket engine) will be determined by the mass of gases created and how fast the gas escapes the rocket. The greater the rate at which the rocket fuel is burned and the faster the velocity of the escaping gas, the greater the thrust of the rocket engine.
Newton's Third Law of Motion
For every action there is an equal and opposite reaction.
With rockets, the action is the expelling of gas out of the engine. The reaction is the movement of the rocket in the opposite direction. The rocket is pushed by the escaping gases produced by the chemical reaction of fuel and oxidizer combining in the combustion chamber.
Center of Gravity / Center of Pressure
The basic principle is that the center of gravity must be ahead of the center of pressure for the rocket to be stable. The center of gravity (CG) is the point at which the mass of the rocket is balanced because the weight forward from this point is equal to the weight to the rear of this point. (Think of this as balancing a pencil on your finger. The pencil will balance when there is an equal amount of mass on both sides of your finger.) The center of pressure (CP) is the point on the rocket at which half of the aerodynamic surface area is located forward and half to the rear.
Fins make the rocket fly straight. A rocket without fins will tumble around its CG (also called the balance point) when flying through the air like a balloon that is inflated and then let go. The balloon will fly erratically because it is uncontrolled. With fins, a rocket has more surface area behind the CG than in front. When the rocket is flying through the air, the air has more surface area to push against behind the balance point than in front because of the greater surface area provided by the fins. Therefore, the rocket tends to stabilize itself. The rocket will rotate until the nose is pointing forward in the air and the fins are pointing backward.
If you point a rocket straight up, the CP should be below the CG. This will allow the rocket to have a stable flight. You can accomplish this by adding fins (as noted above) or by adding mass to the front of the rocket. meaning if the rocket is pointed upward, the CP should be below the CG. Neutral stability is when the CG and CP are at the same point and the rocket’s flight will be random. If the CP is in front of the CG (in other words, the surface area is greater towards the front or top of the rocket), then the flight will be unstable.
Each Student Needs:
Student Design Portfolio
Paper Clips (5-10)
Paper 8.5″ x 11″
The Class Needs:
Weight vs. Mass PowerPoint
Green Eggs Rocket Kit
Pennies (5-10 per group)
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