Week 2: Building a Robot That Moves

Session length: 65-80 minutes

Learning purpose

Now that students know what a robot is, they build one that actually moves. They learn how a motor spins an axle, how wheels and gears turn that spin into motion, and how gears trade speed for turning power (torque). They investigate why some rolling bases tip over and others stay steady - friction and traction for grip, and a low, wide center of mass for stability - and they meet differential drive, where driving two wheels at different speeds is what steers the robot. Then each student builds and tests a rolling base of their own.

Expected student outcomes

By the end of this week, students can:

Explain how a motor, axle, and wheels turn spinning into rolling
Identify the wheels, axles, and gears on a rolling base
Compare speed and torque, and how gears trade one for the other
Use friction and traction to explain why wheels grip or slip
Recognize stable and unstable chassis designs
Explain how wheel spacing and center of mass affect balance
Build or simulate a rolling base and steer it using differential drive
Test a design and improve it based on the results

Suggested pacing

About 65-80 minutes. Adjust to your group - these are guides, not limits.

Step	Focus	Minutes
How robots move	Motor, axle, wheels, gears trading speed for torque, friction and traction.	15 min
Staying upright	Balance, stability, center of mass, chassis design, and differential drive.	10 min
Virtual chassis investigation	Change one thing at a time and log its effect on speed, grip, and stability.	15 min
Predict the steadiest base	Guess which base stays upright longest before the tip test.	5 min
Rolling-base challenge	Build a base that rolls straight, steers, and stays upright.	20 min
Speed and tip-over tests	Record speed, straightness, and the tipping angle of each setup.	10 min
Knowledge check	Five questions on movement, torque, traction, stability, and steering.	8 min
Reflection	Write about gears, stability, and differential-drive steering.	7 min

Before you start

Set up

Set out wheels, gears, and chassis parts (kit), or cardboard, caps, skewers, and tape (unplugged), sorted so students can swap quickly.
Print the Chassis Test Log and the speed and tip-over test record, one per student or pair.
Mark a one-meter test lane on the floor and set up a ramp or a stack of books for the tip test.
If using the simulator, open the chassis-lab mission on each device.

Prepare ahead

Build one working rolling base yourself so you can show a stable example and spot common wobble problems.
Try the tip test with a tall base and a low, wide base so you can predict what students will see.
Have spare rubber bands ready to fix low-traction wheels.

Materials

Chassis Test Log worksheet (printable)(Kit, Simulator, Unplugged)
Pencil, paper, and a ruler or measuring tape(Kit, Simulator, Unplugged)
A short ramp or a stack of books to make a slope for the tip test(Kit, Unplugged)
A programmable robot kit with two motors, wheels, axles, and gears(Kit)
Assorted wheels and gears from the kit to swap and compare(Kit)
Computer or tablet with the browser simulator(Simulator)
Cardboard for the chassis, plus tape and scissors(Unplugged)
Bottle caps or jar lids for wheels and wooden skewers or straws for axles(Unplugged)
Rubber bands, coins, or clay to add grip and weightOptional(Unplugged)
A marble or small ball to model the center of mass shiftingOptional(Unplugged)

Safety

cautionKeep fingers, hair, and loose clothing away from spinning wheels and gears, and turn the motor off before swapping parts.(Kit)
cautionUse child-safe scissors with an adult to cut cardboard and axle holes, and keep skewer points blunted.(Unplugged)
cautionTest rolling bases in a clear area away from table edges, stairs, and drops.(Kit, Unplugged)
infoSave your work often so a browser refresh does not lose your chassis and test log.(Simulator)

Full safety briefing

Review these once with your group. They apply across the whole course; the notes above are what matters most this week.

Batteries and circuitsKit

Use only appropriate low-voltage educational equipment - no wall outlets and no household batteries wired by hand.
Never connect a battery's two terminals directly to each other.
Stop right away if a motor, wire, or battery becomes hot, and tell an adult.
Disconnect the power before changing wheels, motors, or other mechanical parts.

Motors and gearsKit

Keep fingers, hair, jewelry, sleeves, and loose clothing away from moving gears and wheels.
Do not force a stalled motor that has stopped turning - switch it off and find out why.
Make sure moving parts are secured before you run a test.

Moving robots

Test on the floor, not on a table where the robot could drive off an edge.
Keep robots away from stairs, roads, water, pets, and people's feet.
Use a clear, open test area.
Always include a safe-stop behavior so the robot halts when the job is done or something is in the way.

Tools and materialsKit, Unplugged

Ask an adult for help with cutting or any sharp tools.
Use safe classroom materials - no sharp, toxic, or breakable items.
Never launch or swing an attachment toward a person.

Running the session

1.Start with the motor-axle-wheel chain and gears trading speed for torque before any building begins.
2.Run the Virtual chassis investigation, insisting students change only one thing at a time and log each result.
3.Have students predict the steadiest base, then run the tip test to check the prediction.
4.Move into the rolling-base build, then the speed and tip-over tests, and finish with the knowledge check and reflection.

Common misconceptions

'Bigger wheels always mean a better robot' - bigger wheels add speed but need more torque to start and can raise the base.
'Gearing down makes the robot slower and weaker' - it is slower but stronger; it trades speed for torque.
'A spinning wheel means the robot is moving' - on a slick floor it can spin in place with no traction.
'You turn a robot with a steering wheel' - differential-drive robots steer by driving the wheels at different speeds.
'Taller robots are sturdier' - a low, wide base with a low center of mass is far harder to tip.

Questions to ask

Where does the motor's spin have to travel to reach the ground?
If you want this robot to climb, do you need more speed or more torque?
Why is this wheel slipping instead of gripping?
Which way will the robot turn if the left wheel slows down?
Where is the weight, and how could you make this base harder to tip?

Classroom & group adaptations

Make it easier

Give students a pre-built base and have them only swap wheels and test the tip-over ramp, instead of building the chassis from scratch.

Make it harder

Challenge students to gear their base for the best climb up the ramp and to drive an accurate square using only differential-drive turns.

Groups & whole class

In pairs or small groups, give each student a role that rotates - driver (builds or types), navigator (reads the plan), and recorder (fills the worksheet) - so everyone participates. For a whole-class demo, run one shared robot or simulator on the board, have students predict together, then let groups repeat it on their own path. Groups can also mix paths: one builds on the kit while another checks the same idea in the simulator or unplugged, then they compare results.

Hardware and no-hardware notes

Every activity this week runs three ways - all three teach the same core idea, so pick whichever fits your room. No specific product is required.

Kit

Chassis Test Log worksheet (printable)
Pencil, paper, and a ruler or measuring tape
A short ramp or a stack of books to make a slope for the tip test
A programmable robot kit with two motors, wheels, axles, and gears
Assorted wheels and gears from the kit to swap and compare

Simulator

Chassis Test Log worksheet (printable)
Pencil, paper, and a ruler or measuring tape
Computer or tablet with the browser simulator

Unplugged

Chassis Test Log worksheet (printable)
Pencil, paper, and a ruler or measuring tape
A short ramp or a stack of books to make a slope for the tip test
Cardboard for the chassis, plus tape and scissors
Bottle caps or jar lids for wheels and wooden skewers or straws for axles
Rubber bands, coins, or clay to add grip and weight
A marble or small ball to model the center of mass shifting

Troubleshooting

Common problems from this week's activities and what to try.

The base curves instead of going straightKitTry: Check both wheels are on tight and the motors run at the same speed; a loose wheel or uneven motors cause curving.
A wheel spins but the base does not moveKitTry: The wheel is slipping - low traction. Try a grippier wheel or a rougher surface.
Every setup drives the sameSimulatorTry: Make sure you changed the setting and re-ran; change only one setting at a time so the effect is clear.
The base always tips in a turnSimulatorTry: Lower the center of mass or widen the base, then turn a little more slowly.
The base rolls crookedUnpluggedTry: Line up the axles so they are parallel and the wheels are the same size on each side.
The wheels slide instead of rollingUnpluggedTry: The axle may be stuck. Make the hole a little bigger so the axle spins freely.
It always drifts to one sideKitTry: One motor may be faster or a wheel loose; match the motor speeds and tighten both wheels.
It tips forward when it stopsKitTry: The center of mass is too high or too far forward; lower the weight and move it back over the wheels.
The base only ever drives straightSimulatorTry: Set the two wheel speeds to different values; equal speeds always go straight.
The base tips on turnsSimulatorTry: Lower the center of mass or widen the wheelbase, then turn more gently.
It rolls crookedUnpluggedTry: The axles are not parallel or the wheels differ in size; straighten the holes and match the wheels.
The wheels slip on the tableUnpluggedTry: Wrap a rubber band around each wheel to add traction.

Knowledge-check answer key

Answers are shown here for the adult only - the student worksheet keeps them hidden.

1. Diagnose the design problem.
Correct answer: The center of mass is too high
Why: Tall and top-heavy means a high center of mass, which tips in fast turns. Lower the weight or widen the wheelbase.
2. How does a spinning motor make a robot roll across the floor?
Correct answer: The motor spins an axle, and the axle turns the wheels
Why: The motor turns an axle, the axle turns the wheels attached to it, and the turning wheels roll the robot.
3. You gear a robot down so its wheels turn slower. What do you gain, and what do you give up?
Correct answer: You gain torque (turning power) and give up speed
Why: Gears trade speed for torque. Turning the wheels slower gives more turning power for climbing or pushing.
4. A robot's wheels spin fast but it barely moves on a smooth, slippery floor. What is the problem?
Correct answer: Low traction - the wheels are slipping instead of gripping
Why: Smooth floors give little friction, so the wheels have low traction and slip in place instead of rolling the robot forward.
5. Which robot base is the hardest to tip over?
Correct answer: A low, wide base with the heavy parts near the bottom
Why: A low, wide base keeps the center of mass low and inside the wheels, which makes it hard to tip over.
6. In differential drive, how does a two-wheeled robot turn left?
Correct answer: The left wheel turns slower than the right wheel
Why: In differential drive the robot turns toward the slower wheel, so slowing the left wheel steers it left.