Lesson 2 Grades 6–8
The forces in a rocket flight
- Timeabout 55 minutes
- WorksheetW2 Forces and launch angles
- Rocket BuilderBeginner mode
Worksheets (PDF) Open Rocket Builder
Standards
| Code | In plain words |
|---|---|
| MS-PS2-2 | A change in motion depends on the sum of the forces and the mass. |
| MS-PS2-4 (connection) | Gravity pulls masses together; a rocket's weight is Earth's pull on its mass. |
| 6.RP.A.3 (math) | Ratio reasoning: thrust-to-weight. |
Practice: developing and using models (force diagrams). Crosscutting concept: systems and system models.
I can…
- I can draw force diagrams for each part of a flight: on the pad, boost, coast, parachute.
- I can tell balanced from unbalanced forces and say what each does to motion.
- I can check a thrust-to-weight ratio of at least 5 to 1.
Words to know
- Thrust, weight, drag, lift the four forces. Weight always points down; drag points opposite to the motion; fins make lift that steers the nose back into line.
- Net force what's left when you add all the forces.
- Apogee the highest point of the flight.
- Thrust-to-weight ratio thrust ÷ weight.
Materials
Paper strips (8 × 20 cm), pencils, straws, cardstock, tape, scissors, a protractor taped to a box, a tape measure, safety glasses, W2.
Steps
1Teach10 min
Read aloudOn the pad, the weight pulls down and the pad pushes up: balanced, so nothing changes. At liftoff the thrust is far bigger than weight plus drag, so the rocket accelerates. Model rockets need thrust at least 5 times their weight, so they leave the rod fast enough for the fins to steer. After burnout there's no thrust; weight and drag both slow it as it coasts to apogee. Under the parachute, drag up equals weight down: balanced again, so it falls at a steady speed.
2Model it10 min
On W2, draw and label the forces in the four flight pictures. Longer arrow = bigger force. Mark which pictures are balanced.
3Investigate: straw rockets and launch angle25 min
- Roll a paper tube around a pencil, tape it, close the nose airtight, add 3 small fins.
- Glasses on. Launch at 15°, 30°, 45°, 60° and 75°, 2–3 trials each, the same breath each time (or a squeeze bottle).
- Record and graph mean distance against angle.
4Rocket Builder5 min
▶ Watch it launch: name each phase aloud. In the report, read the power bar (thrust ÷ weight). Is it 5× or more?
5Wrap up5 min
Why does the rocket keep going up after the motor stops? (Inertia: it's moving up; weight and drag slow it, they don't stop it at once.)
Check for understanding
- During the coast, which forces act on the rocket?
- Thrust 6 N, weight 1 N, drag 0.5 N. What's the net force, and which way?
- Is thrust 6 N on a 1.2 N rocket at least 5 times the weight?
- Under a parachute at steady speed, are the forces balanced?
Answer key
- Weight and drag (no thrust).
- 6 − 1 − 0.5 = 4.5 N up.
- 6 ÷ 1.2 = 5: just passes.
- Yes: drag up equals weight down.
Teacher notes
- What to expect: about 40–45° usually goes farthest. Real model rockets launch nearly straight up so they land inside the field.
- Extension: net force 4.5 N on a 0.1 kg rocket: a = 45 m/s², about 4½ times gravity. Draw arrows to scale (1 cm = 1 N).
- About MS-PS2-4: this lesson only connects to it (weight = gravity on mass); the full PE needs an argument about masses and gravitational attraction.
- Safety: glasses on for everyone; never aim at people; fetch only on "clear".
- Force arrows and graphs: in the launch simulator, turn on the force arrows and open the flight graphs (altitude, speed and acceleration against time). Ask students to find boost, coast and apogee on the speed graph.