Lesson 5 Grades 6–8
Motors, impulse and energy
- Timeabout 60 minutes
- WorksheetW5 Motor codes and impulse (W5b thrust curve for extension)
- Rocket BuilderBeginner mode, How high will it go?
Worksheets (PDF) Open Rocket Builder
Standards
| Code | In plain words |
|---|---|
| MS-PS2-2 | A change in motion depends on the total force and the mass (thrust-to-weight, F = m × a). |
| MS-PS3-5 | When an object's kinetic energy changes, energy was transferred to or from it. |
| MS-PS3-2 (connection) | Higher up means more gravitational potential energy stored in the Earth–rocket system. |
| 6.RP.A.3, 6.EE.A.2 (math) | Ratios; evaluate impulse = average thrust × time. |
Practice: using mathematics and computational thinking. Crosscutting concept: energy and matter.
I can…
- I can name the parts of a model rocket motor and what each does.
- I can decode a motor code like C6-5 and calculate total impulse.
- I can trace energy through a flight: stored in the propellant, moving (kinetic), height (potential), and lost to drag as heat.
Words to know
- Propellant, nozzle, delay, ejection charge the parts of a motor, bottom to top.
- Igniter the wire that lights the motor. Adults only.
- Total impulse (N·s) average thrust × burn time; the class letter (A, B, C…) gives its range. Each letter is up to twice the one before.
- Kinetic energy energy of motion. Gravitational potential energy: energy stored by height.
Materials
Calculators, W5. Optional: a sealed motor pack (adult shows it, unopened). Optional adult-led demo: film canisters with inside-snap lids, effervescent tablets, water, safety glasses.
Steps
- 1Teach: inside a motor10 min
- 2Decode and calculate20 min
- 3Energy through a flight15 min
- 4Rocket Builder10 min
- 5Wrap up5 min
1Teach: inside a motor10 min
Read aloudA model rocket motor is a factory-made paper tube: a nozzle, propellant that burns for about a second, a delay that makes a smoke trail while the rocket coasts, and an ejection charge that pops the nose and pushes out the parachute near apogee. Only adults handle motors and igniters.
In C6-5: C is the impulse class (5.01–10 N·s), 6 is the average thrust in newtons, 5 is the delay in seconds.
2Decode and calculate20 min
Work through W5: decode four codes, find impulse = average thrust × burn time and the class, and check thrust-to-weight (100 g weighs about 1 N; it must be 5 or more).
3Energy through a flight15 min
Draw a flight path. Label where energy is:
- On the pad: stored in the propellant (chemical).
- Boost: the motor transfers energy to the rocket: its speed and height grow, so its kinetic and potential energy grow.
- Coast: kinetic energy turns into potential energy as it slows and climbs; drag turns some into heat in the air.
- Apogee: the most potential energy, almost no kinetic.
- Descent: the parachute's drag takes energy away, so it lands slowly.
4Rocket Builder10 min
Build your rocket. Find How high will it go?: the recommended motor and predicted altitude. Use the speed and altitude there for the energy story.
5Wrap up5 min
Why do bigger motors need bigger fields?
Check for understanding
- In B6-4, what do the B, 6 and 4 mean?
- 5 N average for 2 s: total impulse and class?
- Rocket + motor weighs 1.2 N; average thrust 5 N. Pass the 5× rule?
- During the coast the rocket slows down. Where does its kinetic energy go?
- Who puts the igniter in the motor?
Answer key
- Class B (2.51–5 N·s), about 6 N average thrust, 4 s delay.
- 10 N·s, class C.
- No: 5 ÷ 1.2 ≈ 4.2.
- Mostly into gravitational potential energy (height); some into heat through drag.
- An adult. Always.
Teacher notes
- Motor classes and NAR minimum fields: 1/2A and smaller 15 m (50 ft); A 30 m; B 60 m; C 120 m; D 150 m; E–F 300 m. Typical printed rockets: M13 15–150 m high, M18 60–300 m, M24 150–350 m, M29 300–750 m.
- Optional adult-led fizzy demo (third law and energy from a reaction): an adult drops half a tablet into a film canister with a teaspoon of water, snaps the lid on, sets it lid-down and steps back; outside, glasses on, nobody leans over it; if it doesn't pop, wait 30 s. The adult keeps the tablets (Alka-Seltzer contains aspirin).
- Extension (W5b): plot the B-motor thrust curve and find the area under it (≈ 4.3 N·s). F = m × a at liftoff: 0.086 kg rocket, 4.7 N average thrust: weight ≈ 0.84 N, net ≈ 3.9 N, a ≈ 45 m/s². Kinetic energy at 50 m/s: ½ × 0.086 × 50² ≈ 108 J; potential energy at 120 m: 0.086 × 9.8 × 120 ≈ 101 J. Where did the other energy go? (Drag.)
- Students under 12 do the paper and math parts; the motor itself stays with an adult.
- Energy from real numbers: the launch simulator's flight graphs show speed and altitude against time, and Download flight data (CSV) gives the numbers. Students compute kinetic energy (½mv²) and potential energy (mgh) at a few moments in a spreadsheet and graph both.