Kart Racer I — Scratch L4 Lesson 33

Learnt28 min · friction, a push, a top speed you can work out
Basic25 min · the kart on the keys
Challenge 1–327 min · a top speed of 8, reverse, a speedometer
Extrabonus · two karts, one keyboard

WHAT IT SHOULD LOOK LIKE

Hold ↑ and the kart builds up speed. Let go and it rolls on, slower and slower. On the grass it crawls.

the finished Kart Racer I

Review — Brick Breaker 5 min

The ball was two numbers, vx and vy. Each tick it moved by both, and a wall flipped one of them.
Where the ball landed on the paddle set its vx, so you could aim it.
The wall was rows of letters in a list, each letter a brick’s hit points, so a new level was a new save code, not new blocks.

Quick-fire

Back in Level 2’s Car Race, pressing ↑ did not move the car at all. So what did it do?

Reveal the answer

It made a variable called speed bigger. One move (speed) steps did all the moving, and friction took a little off speed every tick. Today we look at the numbers inside that — and find a top speed nobody typed in.

Today’s Topic 3 min

One tick of a kart: friction first, then the push
The top speed: push ÷ leak
A drive My Block that takes two numbers: gas and steer
Steering that only works while the kart is moving
Grass: not a wall, just a stronger friction

Learning outcome

By the end of this lesson you will be able to:

Predict a kart’s top speed from its push and its friction.
Drive a sprite from a My Block with inputs.
Make a surface change how a sprite moves, with one colour check.

Learnt — a kart with weight 28 min

1 · One tick of a kart

A real kart does not jump to full speed. It builds up. And when you let go, it rolls on and slows down. Two rules, every tick, in this order:

Friction: keep 95 % of the speed. speed × 0.95
Push: if ↑ is held, add 0.5.

Tick	Speed before	× 0.95	+ 0.5
1	0	0	0.5
2	0.5	0.475	0.975
3	0.975	0.926	1.426
4	1.426	1.355	1.855

Look at the last column. It grows by 0.5, then 0.475, then 0.451, then 0.429. Each tick the kart gains a little less. Why? The faster it goes, the more friction takes away.

2 · The top speed falls out of the sums

Keep going and the speed stops growing. That happens when friction takes away exactly what the push puts back.


set [speed v] to ((speed) * (0.95))
change [speed v] by (0.5)

At speed 10: friction takes 0.5, the push adds 0.5. Stuck at 10.

Friction leaks 5 % a tick, so the leak is 0.05. The rule is short:

top speed = push ÷ leak = 0.5 ÷ 0.05 = 10

WHAT IT SHOULD LOOK LIKE

Speed, tick by tick. It climbs fast, then slower, then flattens out. Red is the road, green the grass.

100 ticks holding ↑

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart draws its own speed as a graph.

150 ticks of gas on the road, then on the grass. The kart says both top speeds at the end: 10 and 2.5.

3 · The drive block

All of a kart’s moving goes in one My Block. It takes two inputs. gas is 1 (↑), −1 (↓) or 0. steer is 1 (→), −1 (←) or 0.


define drive (gas) (steer)
if <touching color [#3FA34D]?> then
  set [speed v] to ((speed) * (0.8))
else
  set [speed v] to ((speed) * (0.95))
end
change [speed v] by ((gas) * (0.5))
if <([abs v] of (speed)) > (0.5)> then
  turn cw ((steer) * (5)) degrees
end
move (speed) steps

One tick of the kart. Friction, push, steer, move — in that order, every time.


when flag clicked
go to x: (-37) y: (137)
point in direction (90)
set [speed v] to (0)
forever
  set [gas v] to (0)
  if <key (up arrow v) pressed?> then
    set [gas v] to (1)
  end
  if <key (down arrow v) pressed?> then
    set [gas v] to (-1)
  end
  set [steer v] to (0)
  if <key (right arrow v) pressed?> then
    set [steer v] to (1)
  end
  if <key (left arrow v) pressed?> then
    set [steer v] to (-1)
  end
  drive (gas) (steer) :: custom
end

The keys turn into two numbers. The My Block does the rest.

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — a little script presses the keys for this kart.

Eight seconds round the circuit, braking for every corner. Ticks on the grass: 0.

Why a My Block? Because next lesson a second driver — a script, not you — will press the same two “keys”. It calls the same drive (gas) (steer) :: custom block, so it gets exactly the same kart.

4 · No steering while parked

A parked kart cannot turn: its wheels are not rolling. So the turn sits inside if <([abs v] of (speed)) > (0.5)> then. The abs makes it work going backwards too: −3 is moving, just the other way.

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — both karts try to steer right while standing still.

Without the check: 225° of spinning on the spot. With it: 0°.

5 · Grass is just stronger friction

In Car Race, grass slowed you down. Here it is the same rule with a different number. The road keeps 95 %. The grass keeps 80 %, so its leak is 0.2. Top speed on grass: 0.5 ÷ 0.2 = 2.5.

WHAT IT SHOULD LOOK LIKE

The same 40 ticks of ↑. On the road: 234 steps. On the grass: 90.

40 ticks of gas each

faded sprite = where it ends up

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — both karts get full gas for the same 40 ticks.

Road: 234 steps, speed 8.7 and still climbing. Grass: 90 steps, speed 2.5 and stuck.

Basic — the kart on the keys 25 min

Start a new project. You will build the kart from the Learnt yourself.

Step 1 — the circuit and the kart

Save our circuit, kart-circuit.svg, and use Upload Backdrop. Delete the white backdrop.
Delete the cat. Save our kart, car-red.svg, and use Upload Sprite. Call it Kart.
Set the Kart’s size to 40. Its nose points right, so direction 90 drives along the top straight.

Step 2 — the variables

For all sprites: speed.
On the Kart, for this sprite only: gas and steer.

Step 3 — the drive block

In My Blocks, click Make a Block. Type drive.
Click Add an input and name it gas. Add another called steer. Click OK.
Build the body from Learnt section 3. Drag gas and steer out of the pink define block when you need them.

Step 4 — the keys

Build the green-flag script from Learnt section 3. It ends with drive (gas) (steer) :: custom inside the forever. Tick the speed box so you can watch it.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Hold ↑ to go, ↓ to brake and reverse, ← and → to steer.

What you are aiming at: a kart that builds up speed, rolls when you let go, and crawls on the grass.

The kart spins round when it is standing still.

The turn cw ((steer) * (5)) degrees is not inside the if <([abs v] of (speed)) > (0.5)> then.

The kart never stops after I let go.

The friction is missing, or it is × 1. Every tick must keep a little less than all of the speed.

The kart is just as fast on the grass.

The colour in <touching color [#3FA34D]?> is not the grass. Click the swatch, pick the eyedropper and click the green on the Stage.

CHALLENGE 1 · L4-33-M1🪙 30 Adcoins8 min

A top speed of exactly 8

Your kart is too fast for a beginner. Slow it down — with sums, not guesses.

The top speed on the road becomes exactly 8.
Keep the friction at 0.95. Change only the push.
Work the number out on paper first.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — same friction, a smaller push. The curve flattens at exactly 8.

push 0.4

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart draws the new speed curve.

Push 0.4: the curve flattens at 8.

Done when: on a plain white backdrop (no grass anywhere), holding ↑, the speed box creeps up to 8 and stops there.

Teacher reveal

top speed = push ÷ leak, so push = top speed × leak = 8 × 0.05 = 0.4.


change [speed v] by ((gas) * (0.4))

Why a plain backdrop? The circuit’s straight is too short to reach top speed from a standing start, and a kart pinned against the edge of the Stage keeps working out its speed anyway.

CHALLENGE 2 · L4-33-M2🪙 50 Adcoins9 min

Reverse like a real kart

Right now ↓ makes the kart reverse as fast as it goes forwards. Real karts do not.

While the kart is moving forwards, ↓ is a brake.
Once it is (nearly) stopped, ↓ reverses — slowly. Top reverse speed: 2.5.
Reversing with → turns the nose left, like a real car backing up.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — hold ↓ to brake, keep holding to reverse. Reversing with → swings the nose LEFT.

braking, then reversing

reverse + →

faded sprite = where it ends up · panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart drives forward, brakes, reverses, then reverses while steering right.

Reverse top speed: 2.5. Reversing with → turned the nose 60° to the left.

Done when: backing up with → held swings the kart’s nose to the left, and the speed box never goes below −2.5.

Teacher reveal


if <<(gas) < (0)> and <(speed) < (0.5)>> then
  set [speed v] to ((speed) + ((gas) * (0.125)))
else
  set [speed v] to ((speed) + ((gas) * (0.5)))
end
if <([abs v] of (speed)) > (0.5)> then
  if <(speed) < (0)> then
    turn cw ((steer) * (-5)) degrees
  else
    turn cw ((steer) * (5)) degrees
  end
end

A reverse push of 0.125 with a leak of 0.05 gives 0.125 ÷ 0.05 = 2.5. The same rule as forwards, with a smaller push.

CHALLENGE 3 · L4-33-M3🪙 70 Adcoins10 min

A speedometer

A number in a box is not how racing games show speed. Build a dial.

A Speedo sprite: a half-circle dial. Ours is speedo.svg.
A Needle sprite on top of it, pointing up from its middle (needle.svg).
Speed 0: the needle points left. Speed 5: straight up. Speed 10: right.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — a speedometer. The needle points left at 0, up at 5, right at 10.

flat out on the straight

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — a script drives the kart and the needle follows its speed.

Fastest this run: 9.2. The needle swung most of the way to the right.

Done when: the needle rests on 0 at the start, sweeps right down the straight, and dips in every corner.

Teacher reveal


when flag clicked
go to x: (0) y: (10)
go to [front v] layer
forever
  point in direction (((speed) * (18)) - (90))
end

Speed 0 must give −90 (left), and speed 10 must give 90 (right): a 180° swing over 10 speed, so 18° for each 1 of speed, starting at −90.

EXTRA MISSION · L4-33-MX🪙 100 Adcoinsbonus

Two karts, one keyboard

Race a friend at home — or race yourself with both hands.

A second kart, blue, on the lower grid box.
It drives on W (gas), S (brake), A and D (steer).
Each kart has its own speed. The grass slows each one on its own.

WHAT IT SHOULD LOOK LIKE

Extra — two karts, one keyboard. Arrows drive red, W A S D drive blue.

two drivers

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Arrows drive the red kart; W, S, A and D drive the blue one.

Two karts, two speeds, one keyboard.

Teacher reveal

On Kart 2, make a variable speed 2 (for this sprite only), and use it everywhere in its drive block instead of speed:


define drive (gas) (steer)
if <touching color [#3FA34D]?> then
  set [speed 2 v] to ((speed 2) * (0.8))
else
  set [speed 2 v] to ((speed 2) * (0.95))
end
change [speed 2 v] by ((gas) * (0.5))
if <([abs v] of (speed 2)) > (0.5)> then
  turn cw ((steer) * (5)) degrees
end
move (speed 2) steps

Its key script asks <key (w v) pressed?> and so on. If both karts share speed, pressing ↑ moves both of them.

Summary 5 min

Every tick: friction first, then the push.
The top speed is push ÷ leak. Nobody types it in.
One drive (gas) (steer) My Block does all the moving. The keys only choose the two numbers.
Steering only works while the kart moves.
Grass is a stronger friction, so a lower top speed: 2.5.

Friction: What takes a little speed away every tick.
Leak: How much friction takes, as a part: 0.05 means 5 %.
Push: How much the gas adds every tick.
Top speed: The speed where friction takes exactly what the push adds.

Hand in

Save your kart. Next lesson we open up the move (10) steps block and find two smaller moves hiding inside it — and then make the kart drift.

Learnt28 min · friction, a push, a top speed you can work out
Basic25 min · the kart on the keys
Challenge 1–327 min · a top speed of 8, reverse, a speedometer
Extrabonus · two karts, one keyboard

WHAT IT SHOULD LOOK LIKE

Hold ↑ and the kart builds up speed. Let go and it rolls on, slower and slower. On the grass it crawls.

the finished Kart Racer I

Review — Brick Breaker 5 min

The ball was two numbers, vx and vy. Each tick it moved by both, and a wall flipped one of them.
Where the ball landed on the paddle set its vx, so you could aim it.
The wall was rows of letters in a list, each letter a brick’s hit points, so a new level was a new save code, not new blocks.

Quick-fire

Back in Level 2’s Car Race, pressing ↑ did not move the car at all. So what did it do?

Reveal the answer

Today’s Topic 3 min

One tick of a kart: friction first, then the push
The top speed: push ÷ leak
A drive My Block that takes two numbers: gas and steer
Steering that only works while the kart is moving
Grass: not a wall, just a stronger friction

Learning outcome

By the end of this lesson you will be able to:

Predict a kart’s top speed from its push and its friction.
Drive a sprite from a My Block with inputs.
Make a surface change how a sprite moves, with one colour check.

Learnt — a kart with weight 28 min

1 · One tick of a kart

A real kart does not jump to full speed. It builds up. And when you let go, it rolls on and slows down. Two rules, every tick, in this order:

Friction: keep 95 % of the speed. speed × 0.95
Push: if ↑ is held, add 0.5.

Tick	Speed before	× 0.95	+ 0.5
1	0	0	0.5
2	0.5	0.475	0.975
3	0.975	0.926	1.426
4	1.426	1.355	1.855

Look at the last column. It grows by 0.5, then 0.475, then 0.451, then 0.429. Each tick the kart gains a little less. Why? The faster it goes, the more friction takes away.

2 · The top speed falls out of the sums

Keep going and the speed stops growing. That happens when friction takes away exactly what the push puts back.


set [speed v] to ((speed) * (0.95))
change [speed v] by (0.5)

At speed 10: friction takes 0.5, the push adds 0.5. Stuck at 10.

Friction leaks 5 % a tick, so the leak is 0.05. The rule is short:

top speed = push ÷ leak = 0.5 ÷ 0.05 = 10

WHAT IT SHOULD LOOK LIKE

Speed, tick by tick. It climbs fast, then slower, then flattens out. Red is the road, green the grass.

100 ticks holding ↑

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart draws its own speed as a graph.

150 ticks of gas on the road, then on the grass. The kart says both top speeds at the end: 10 and 2.5.

3 · The drive block

All of a kart’s moving goes in one My Block. It takes two inputs. gas is 1 (↑), −1 (↓) or 0. steer is 1 (→), −1 (←) or 0.


define drive (gas) (steer)
if <touching color [#3FA34D]?> then
  set [speed v] to ((speed) * (0.8))
else
  set [speed v] to ((speed) * (0.95))
end
change [speed v] by ((gas) * (0.5))
if <([abs v] of (speed)) > (0.5)> then
  turn cw ((steer) * (5)) degrees
end
move (speed) steps

One tick of the kart. Friction, push, steer, move — in that order, every time.


when flag clicked
go to x: (-37) y: (137)
point in direction (90)
set [speed v] to (0)
forever
  set [gas v] to (0)
  if <key (up arrow v) pressed?> then
    set [gas v] to (1)
  end
  if <key (down arrow v) pressed?> then
    set [gas v] to (-1)
  end
  set [steer v] to (0)
  if <key (right arrow v) pressed?> then
    set [steer v] to (1)
  end
  if <key (left arrow v) pressed?> then
    set [steer v] to (-1)
  end
  drive (gas) (steer) :: custom
end

The keys turn into two numbers. The My Block does the rest.

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — a little script presses the keys for this kart.

Eight seconds round the circuit, braking for every corner. Ticks on the grass: 0.

4 · No steering while parked

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — both karts try to steer right while standing still.

Without the check: 225° of spinning on the spot. With it: 0°.

5 · Grass is just stronger friction

In Car Race, grass slowed you down. Here it is the same rule with a different number. The road keeps 95 %. The grass keeps 80 %, so its leak is 0.2. Top speed on grass: 0.5 ÷ 0.2 = 2.5.

WHAT IT SHOULD LOOK LIKE

The same 40 ticks of ↑. On the road: 234 steps. On the grass: 90.

40 ticks of gas each

faded sprite = where it ends up

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — both karts get full gas for the same 40 ticks.

Road: 234 steps, speed 8.7 and still climbing. Grass: 90 steps, speed 2.5 and stuck.

Basic — the kart on the keys 25 min

Start a new project. You will build the kart from the Learnt yourself.

Step 1 — the circuit and the kart

Save our circuit, kart-circuit.svg, and use Upload Backdrop. Delete the white backdrop.
Delete the cat. Save our kart, car-red.svg, and use Upload Sprite. Call it Kart.
Set the Kart’s size to 40. Its nose points right, so direction 90 drives along the top straight.

Step 2 — the variables

For all sprites: speed.
On the Kart, for this sprite only: gas and steer.

Step 3 — the drive block

In My Blocks, click Make a Block. Type drive.
Click Add an input and name it gas. Add another called steer. Click OK.
Build the body from Learnt section 3. Drag gas and steer out of the pink define block when you need them.

Step 4 — the keys

Build the green-flag script from Learnt section 3. It ends with drive (gas) (steer) :: custom inside the forever. Tick the speed box so you can watch it.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Hold ↑ to go, ↓ to brake and reverse, ← and → to steer.

What you are aiming at: a kart that builds up speed, rolls when you let go, and crawls on the grass.

The kart spins round when it is standing still.

The turn cw ((steer) * (5)) degrees is not inside the if <([abs v] of (speed)) > (0.5)> then.

The kart never stops after I let go.

The friction is missing, or it is × 1. Every tick must keep a little less than all of the speed.

The kart is just as fast on the grass.

The colour in <touching color [#3FA34D]?> is not the grass. Click the swatch, pick the eyedropper and click the green on the Stage.

CHALLENGE 1 · L4-33-M1🪙 30 Adcoins8 min

A top speed of exactly 8

Your kart is too fast for a beginner. Slow it down — with sums, not guesses.

The top speed on the road becomes exactly 8.
Keep the friction at 0.95. Change only the push.
Work the number out on paper first.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — same friction, a smaller push. The curve flattens at exactly 8.

push 0.4

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart draws the new speed curve.

Push 0.4: the curve flattens at 8.

Done when: on a plain white backdrop (no grass anywhere), holding ↑, the speed box creeps up to 8 and stops there.

Teacher reveal

top speed = push ÷ leak, so push = top speed × leak = 8 × 0.05 = 0.4.


change [speed v] by ((gas) * (0.4))

Why a plain backdrop? The circuit’s straight is too short to reach top speed from a standing start, and a kart pinned against the edge of the Stage keeps working out its speed anyway.

CHALLENGE 2 · L4-33-M2🪙 50 Adcoins9 min

Reverse like a real kart

Right now ↓ makes the kart reverse as fast as it goes forwards. Real karts do not.

While the kart is moving forwards, ↓ is a brake.
Once it is (nearly) stopped, ↓ reverses — slowly. Top reverse speed: 2.5.
Reversing with → turns the nose left, like a real car backing up.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — hold ↓ to brake, keep holding to reverse. Reversing with → swings the nose LEFT.

braking, then reversing

reverse + →

faded sprite = where it ends up · panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the kart drives forward, brakes, reverses, then reverses while steering right.

Reverse top speed: 2.5. Reversing with → turned the nose 60° to the left.

Done when: backing up with → held swings the kart’s nose to the left, and the speed box never goes below −2.5.

Teacher reveal


if <<(gas) < (0)> and <(speed) < (0.5)>> then
  set [speed v] to ((speed) + ((gas) * (0.125)))
else
  set [speed v] to ((speed) + ((gas) * (0.5)))
end
if <([abs v] of (speed)) > (0.5)> then
  if <(speed) < (0)> then
    turn cw ((steer) * (-5)) degrees
  else
    turn cw ((steer) * (5)) degrees
  end
end

A reverse push of 0.125 with a leak of 0.05 gives 0.125 ÷ 0.05 = 2.5. The same rule as forwards, with a smaller push.

CHALLENGE 3 · L4-33-M3🪙 70 Adcoins10 min

A speedometer

A number in a box is not how racing games show speed. Build a dial.

A Speedo sprite: a half-circle dial. Ours is speedo.svg.
A Needle sprite on top of it, pointing up from its middle (needle.svg).
Speed 0: the needle points left. Speed 5: straight up. Speed 10: right.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — a speedometer. The needle points left at 0, up at 5, right at 10.

flat out on the straight

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — a script drives the kart and the needle follows its speed.

Fastest this run: 9.2. The needle swung most of the way to the right.

Done when: the needle rests on 0 at the start, sweeps right down the straight, and dips in every corner.

Teacher reveal


when flag clicked
go to x: (0) y: (10)
go to [front v] layer
forever
  point in direction (((speed) * (18)) - (90))
end

Speed 0 must give −90 (left), and speed 10 must give 90 (right): a 180° swing over 10 speed, so 18° for each 1 of speed, starting at −90.

EXTRA MISSION · L4-33-MX🪙 100 Adcoinsbonus

Two karts, one keyboard

Race a friend at home — or race yourself with both hands.

A second kart, blue, on the lower grid box.
It drives on W (gas), S (brake), A and D (steer).
Each kart has its own speed. The grass slows each one on its own.

WHAT IT SHOULD LOOK LIKE

Extra — two karts, one keyboard. Arrows drive red, W A S D drive blue.

two drivers

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Arrows drive the red kart; W, S, A and D drive the blue one.

Two karts, two speeds, one keyboard.

Teacher reveal

On Kart 2, make a variable speed 2 (for this sprite only), and use it everywhere in its drive block instead of speed:


define drive (gas) (steer)
if <touching color [#3FA34D]?> then
  set [speed 2 v] to ((speed 2) * (0.8))
else
  set [speed 2 v] to ((speed 2) * (0.95))
end
change [speed 2 v] by ((gas) * (0.5))
if <([abs v] of (speed 2)) > (0.5)> then
  turn cw ((steer) * (5)) degrees
end
move (speed 2) steps

Its key script asks <key (w v) pressed?> and so on. If both karts share speed, pressing ↑ moves both of them.

Summary 5 min

Every tick: friction first, then the push.
The top speed is push ÷ leak. Nobody types it in.
One drive (gas) (steer) My Block does all the moving. The keys only choose the two numbers.
Steering only works while the kart moves.
Grass is a stronger friction, so a lower top speed: 2.5.

Friction: What takes a little speed away every tick.
Leak: How much friction takes, as a part: 0.05 means 5 %.
Push: How much the gas adds every tick.
Top speed: The speed where friction takes exactly what the push adds.

Hand in

Save your kart. Next lesson we open up the move (10) steps block and find two smaller moves hiding inside it — and then make the kart drift.