Penalty Shootout II — Scratch L2 Lesson 38

Learnt27 min · swing, stop, kick, too hard
Basic25 min · add the meter
Challenge 1–327 min · swing, stop, judge
Extrabonus · faster every goal

WHAT IT SHOULD LOOK LIKE

Where you aim is half a penalty. How hard you hit it is the other half — and today a meter decides.

the meter swings

SPACE at 96 — too hard

panels run left to right

Review — where we got to 5 min

The marker sits on the mouse. The arrow uses point towards [mouse-pointer v] to face it.
At the click we save (mouse x) and (mouse y) into aim x and aim y.
Four comparisons, joined by three ands, decide GOAL or miss.

Quick-fire

A shot lands at x = 100 exactly — on the right post. Goal or miss?

Reveal the answer

Miss. <(aim x) < (100)> asks for less than 100, and 100 is not less than 100. The post itself is out.

Today’s Topic 3 min

A variable that climbs to 100 and falls back to 0, for ever
A meter that grows with it
A key that stops it
Shots that sail over the bar, or dribble short

Learning outcome

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

Make a number swing between two limits, using a second variable for its step.
Stop a loop with repeat until <key [space v] pressed?>.
Change what a shot does depending on a number.

Learnt — a meter you have to time 27 min

1 · A number that swings

We want (power) to climb to 100, fall back to 0, and climb again. One variable cannot remember which way it is going. So we use two.

Variable	What it holds
`(power)`	How hard the kick is, 0 to 100.
`(step)`	How much power changes each time round. 2 going up, −2 going down.

Make power and step, For all sprites.
In a forever, add change [power v] by (step). Drop the (step) reporter into the number slot.
Add if <(power) > (99)> then with set [step v] to (-2) inside. That is the top.
Add if <(power) < (1)> then with set [step v] to (2) inside. That is the bottom.
Tick both variables so their boxes show. Click the flag and watch.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  change [power v] by (step)
  if <(power) > (99)> then
    set [step v] to (-2)
  end
  if <(power) < (1)> then
    set [step v] to (2)
  end
  set size to (power) %
end

The Power sprite. change [power v] by (step) never changes. Only (step) does.

WHAT IT SHOULD LOOK LIKE

On the way up, step is 2. At the top it turns into −2, and the same block now pulls power down.

climbing · step = 2

falling · step = −2

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the meter climb to 100 and fall back to 0. Nothing to press.

Watch power climb and fall. Watch step flip from 2 to −2 at the top.

Why > 99 and not = 100? With a step of 2 from 0, power does land on 100. Change the step to 3 and it goes 99, 102 — it never equals 100, and the meter climbs for ever. > 99 catches both.

Why not call it direction? Scratch already has a (direction) reporter in Motion. Two things with one name is a mix-up waiting to happen. step says what it is.

2 · A meter that grows upwards

set size to (power) % turns the number into a picture. But Scratch grows a sprite around its centre. A bar with its centre in the middle grows up and down.

Paint a new sprite. Name it Power.
Draw a tall, thin triangle, point down. Put the point exactly on the little centre cross in the paint editor.
Keep all of the triangle above the cross.
Move the sprite to the side of the goal: go to x: (200) y: (-150).
Run the swing script. The wedge grows up from its tip.

WHAT IT SHOULD LOOK LIKE

Scratch grows a sprite around its centre. Put the wedge’s tip on the centre and it grows upwards, like a meter should.

size 30 %

size 90 %

panels run left to right

A frame helps. Paint a second sprite, Frame: the same triangle as an outline, with the top part coloured red. Put it at the same x and y, one layer behind. Now you can see where 100 is, and where the danger starts.

3 · SPACE stops it

Swap the inner loop for repeat until <key [space v] pressed?>. It swings until you press SPACE, then carries on below.

From Control, drag a repeat until inside the forever.
Move the four swinging blocks into it.
From Sensing, drop <key [space v] pressed?> into its slot.
Under the repeat until, add say (join [Power: ] (power)) for (1) seconds.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to (-2)
    end
    if <(power) < (1)> then
      set [step v] to (2)
    end
    set size to (power) %
  end
  say (join [Power: ] (power)) for (1) seconds
end

The meter freezes, says its number, and starts swinging again from where it stopped.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the Stage once, then press the space bar to stop the meter. It says the power, then swings again.

Try to stop it on exactly 50. It is harder than it looks.

4 · The meter takes the kick

Last lesson the ball kicked on a click. Now the kick happens when SPACE stops the meter. So the meter saves the aim, then tells the ball to go.


set [aim x v] to (mouse x)
set [aim y v] to (mouse y)
broadcast [kick v] and wait

These replace the say, under the repeat until. Your right hand aims; your left hand times it.

Why broadcast [kick v] and wait? It holds the meter still until the ball has landed and gone back. A plain broadcast [kick v] would start the meter again at once. A second SPACE would then kick a ball that is still in the air.

5 · Too hard, too soft

Two new rules, at the top of the ball’s kick. They move where the ball is going.

On the ball, start a new script with when I receive [kick v].
Add if <(power) > (85)> then with set [aim y v] to (150). That is up in the crowd.
Add if <(power) < (30)> then with set [aim y v] to (-70). That is on the grass, short of the line.
Then the glide and the goal test from last lesson.


when I receive [kick v]
if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end
glide (0.5) secs to x: (aim x) y: (aim y)
if <<<(aim x) > (-100)> and <(aim x) < (100)>> and <<(aim y) > (-25)> and <(aim y) < (85)>>> then
  say [GOAL!] for (1) seconds
else
  say [Miss!] for (1) seconds
end
go to x: (0) y: (-118)

The Ball. The goal test is untouched: 150 is above the bar and −70 is short of the line, so both come out as a miss.

WHAT IT SHOULD LOOK LIKE

Same aim, three powers. The goal test from Lesson 37 judges all three without a single change.

power 95 — over the bar

power 20 — too soft

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 and watch. Three kicks are taken for you at the same spot: power 95, then 20, then 60. Nothing to press.

One aim, three powers: 95 sails over, 20 stops short, 60 goes in.

Basic — add the meter 25 min

Open last lesson’s project. The Aim and Arrow sprites stay exactly as they are.

Step 1 — the Power sprite

Paint the wedge, tip on the centre cross.
Make power and step.
Build the swing from Learnt 1. Watch three swings.

Step 2 — the frame (optional, but worth it)

Paint the outline sprite. Colour the top band red.
Place it at the same x and y as Power.
Drag Power in front if the frame covers it.

Step 3 — SPACE to kick

Wrap the swing in repeat until SPACE.
Under it: save the aim, then broadcast kick and wait.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to (-2)
    end
    if <(power) < (1)> then
      set [step v] to (2)
    end
    set size to (power) %
  end
  set [aim x v] to (mouse x)
  set [aim y v] to (mouse y)
  broadcast [kick v] and wait
end

The whole Power sprite. Fourteen blocks, the most a Level 2 stack should hold.

Step 4 — the ball listens

Delete the ball’s old wait until <mouse down?> loop.
Keep a small flag script: goals to 0, back to the spot.
Build the kick from Learnt 5, with the goals counter back in it.


when flag clicked
set [goals v] to (0)
go to x: (0) y: (-118)

when I receive [kick v]
if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end
glide (0.5) secs to x: (aim x) y: (aim y)
if <<<(aim x) > (-100)> and <(aim x) < (100)>> and <<(aim y) > (-25)> and <(aim y) < (85)>>> then
  change [goals v] by (1)
  say [GOAL!] for (1) seconds
else
  say [Miss!] for (1) seconds
end
go to x: (0) y: (-118)

The Ball’s two scripts.

Step 5 — play it

Ten kicks. Write down your goals. Then ten more, trying to beat it.

The meter grows down as well as up.

The wedge’s tip is not on the centre cross. Open the costume, select the whole drawing, and drag it until the tip sits on the cross.

Power climbs past 100 and never comes back.

The top test is = 100, or the step is not 2. Use <(power) > (99)>.

The meter sticks at 0.

step is 0. Check the flag script sets it to 2 before the loop.

One press of SPACE kicks twice.

You held SPACE through the whole kick, so the meter stopped again at once. Add wait until <not <key [space v] pressed?>> after the broadcast.

A click still kicks the ball.

The old wait until <mouse down?> loop is still on the ball, so two scripts are kicking it. Delete the old loop.

CHALLENGE 1 · L2-38-M1🪙 30 Adcoins8 min

A meter that swings

Power climbs to 100, falls to 0, and climbs again, for ever.
The meter grows up from its tip, never down.
The power box on the Stage never shows less than 0 or more than 100.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — up to 100, down to 0, for ever.

at the top

at the bottom

panels run left to right

Done when: you watch three full swings and the power box stays between 0 and 100 the whole time.

CHALLENGE 2 · L2-38-M2🪙 50 Adcoins9 min

Stop it and shoot

SPACE freezes the meter.
The ball flies to where the mouse was at that moment.
The meter waits until the ball is back on the spot.
Then it swings again.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — SPACE freezes the meter and the ball flies.

SPACE at 52

Done when: you stop the meter between 40 and 60 three times in a row, and each ball lands under the marker.

Teacher note

Ask what happens with a plain broadcast instead of broadcast and wait. Let them try it and press SPACE twice quickly. Two kicks at once, one ball, a mess. The fix is one word.

CHALLENGE 3 · L2-38-M3🪙 70 Adcoins10 min

Too hard, too soft

Power above 85 sails over the bar.
Power below 30 stops short of the goal line.
Anything in between flies where you aimed.
Goals count, and start at 0 each flag.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the middle of the meter is the only safe place.

power 60

Done when: you get all three — over the bar, short, and a goal — on purpose, in an order you choose before you start.

Teacher reveal — two ifs before the glide


if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end

They change aim y and nothing else. A pupil who adds new tests to the goal check instead has built something that works but reads badly — worth showing both side by side.

A good follow-up: make the ball say Over the bar! or Too soft!. That needs the Ball to remember which rule fired — a third variable, or a say inside each if before the glide.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and click the Stage once. Aim with the mouse, then press the space bar to stop the meter and shoot.

Today’s game. Aim with the mouse, time it with SPACE.

EXTRA MISSION · L2-38-MX🪙 100 Adcoinsbonus

Faster every goal

A game should get harder as you get better. Make the meter swing faster after every goal.

WHAT IT SHOULD LOOK LIKE

Extra — every goal makes the next one harder to time.

goal 1 · step 2

goal 5 · step 7

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and click the Stage once. Aim with the mouse, press the space bar to shoot. Every goal makes the meter swing faster.

Score, and watch meter speed go up. The swing gets harder to catch.

Teacher reveal — the meter, reading meter speed


when flag clicked
set [power v] to (0)
set [step v] to (meter speed)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to ((0) - (meter speed))
    end
    if <(power) < (1)> then
      set [step v] to (meter speed)
    end
    set size to (power) %
  end
  set [aim x v] to (mouse x)
  set [aim y v] to (mouse y)
  broadcast [kick v] and wait
end

((0) - (meter speed)) is how Scratch writes “minus meter speed”. The Ball sets meter speed to 2 at the flag and adds 1 inside its GOAL half.

Summary 3 min

A number that swings needs two variables: the number, and its step.
Two ifs flip the step at the ends. Test with > and <, not =.
Scratch grows a sprite around its centre. Put the meter’s tip there.
repeat until <key [space v] pressed?> runs until a key stops it.
broadcast [kick v] and wait keeps the meter still until the kick is over.

Power: How hard the kick is, from 0 to 100.
Step: How much a number changes each time round a loop.
Swing: A number going up to a limit, then down to another, over and over.
Centre: The point a sprite turns and grows around. The cross in the paint editor.

Next lesson: the keeper stops standing still.

Learnt27 min · swing, stop, kick, too hard
Basic25 min · add the meter
Challenge 1–327 min · swing, stop, judge
Extrabonus · faster every goal

WHAT IT SHOULD LOOK LIKE

Where you aim is half a penalty. How hard you hit it is the other half — and today a meter decides.

the meter swings

SPACE at 96 — too hard

panels run left to right

Review — where we got to 5 min

The marker sits on the mouse. The arrow uses point towards [mouse-pointer v] to face it.
At the click we save (mouse x) and (mouse y) into aim x and aim y.
Four comparisons, joined by three ands, decide GOAL or miss.

Quick-fire

A shot lands at x = 100 exactly — on the right post. Goal or miss?

Reveal the answer

Miss. <(aim x) < (100)> asks for less than 100, and 100 is not less than 100. The post itself is out.

Today’s Topic 3 min

A variable that climbs to 100 and falls back to 0, for ever
A meter that grows with it
A key that stops it
Shots that sail over the bar, or dribble short

Learning outcome

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

Make a number swing between two limits, using a second variable for its step.
Stop a loop with repeat until <key [space v] pressed?>.
Change what a shot does depending on a number.

Learnt — a meter you have to time 27 min

1 · A number that swings

We want (power) to climb to 100, fall back to 0, and climb again. One variable cannot remember which way it is going. So we use two.

Variable	What it holds
`(power)`	How hard the kick is, 0 to 100.
`(step)`	How much power changes each time round. 2 going up, −2 going down.

Make power and step, For all sprites.
In a forever, add change [power v] by (step). Drop the (step) reporter into the number slot.
Add if <(power) > (99)> then with set [step v] to (-2) inside. That is the top.
Add if <(power) < (1)> then with set [step v] to (2) inside. That is the bottom.
Tick both variables so their boxes show. Click the flag and watch.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  change [power v] by (step)
  if <(power) > (99)> then
    set [step v] to (-2)
  end
  if <(power) < (1)> then
    set [step v] to (2)
  end
  set size to (power) %
end

The Power sprite. change [power v] by (step) never changes. Only (step) does.

WHAT IT SHOULD LOOK LIKE

On the way up, step is 2. At the top it turns into −2, and the same block now pulls power down.

climbing · step = 2

falling · step = −2

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the meter climb to 100 and fall back to 0. Nothing to press.

Watch power climb and fall. Watch step flip from 2 to −2 at the top.

Why > 99 and not = 100? With a step of 2 from 0, power does land on 100. Change the step to 3 and it goes 99, 102 — it never equals 100, and the meter climbs for ever. > 99 catches both.

Why not call it direction? Scratch already has a (direction) reporter in Motion. Two things with one name is a mix-up waiting to happen. step says what it is.

2 · A meter that grows upwards

set size to (power) % turns the number into a picture. But Scratch grows a sprite around its centre. A bar with its centre in the middle grows up and down.

Paint a new sprite. Name it Power.
Draw a tall, thin triangle, point down. Put the point exactly on the little centre cross in the paint editor.
Keep all of the triangle above the cross.
Move the sprite to the side of the goal: go to x: (200) y: (-150).
Run the swing script. The wedge grows up from its tip.

WHAT IT SHOULD LOOK LIKE

Scratch grows a sprite around its centre. Put the wedge’s tip on the centre and it grows upwards, like a meter should.

size 30 %

size 90 %

panels run left to right

3 · SPACE stops it

Swap the inner loop for repeat until <key [space v] pressed?>. It swings until you press SPACE, then carries on below.

From Control, drag a repeat until inside the forever.
Move the four swinging blocks into it.
From Sensing, drop <key [space v] pressed?> into its slot.
Under the repeat until, add say (join [Power: ] (power)) for (1) seconds.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to (-2)
    end
    if <(power) < (1)> then
      set [step v] to (2)
    end
    set size to (power) %
  end
  say (join [Power: ] (power)) for (1) seconds
end

The meter freezes, says its number, and starts swinging again from where it stopped.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the Stage once, then press the space bar to stop the meter. It says the power, then swings again.

Try to stop it on exactly 50. It is harder than it looks.

4 · The meter takes the kick

Last lesson the ball kicked on a click. Now the kick happens when SPACE stops the meter. So the meter saves the aim, then tells the ball to go.


set [aim x v] to (mouse x)
set [aim y v] to (mouse y)
broadcast [kick v] and wait

These replace the say, under the repeat until. Your right hand aims; your left hand times it.

5 · Too hard, too soft

Two new rules, at the top of the ball’s kick. They move where the ball is going.

On the ball, start a new script with when I receive [kick v].
Add if <(power) > (85)> then with set [aim y v] to (150). That is up in the crowd.
Add if <(power) < (30)> then with set [aim y v] to (-70). That is on the grass, short of the line.
Then the glide and the goal test from last lesson.


when I receive [kick v]
if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end
glide (0.5) secs to x: (aim x) y: (aim y)
if <<<(aim x) > (-100)> and <(aim x) < (100)>> and <<(aim y) > (-25)> and <(aim y) < (85)>>> then
  say [GOAL!] for (1) seconds
else
  say [Miss!] for (1) seconds
end
go to x: (0) y: (-118)

The Ball. The goal test is untouched: 150 is above the bar and −70 is short of the line, so both come out as a miss.

WHAT IT SHOULD LOOK LIKE

Same aim, three powers. The goal test from Lesson 37 judges all three without a single change.

power 95 — over the bar

power 20 — too soft

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 and watch. Three kicks are taken for you at the same spot: power 95, then 20, then 60. Nothing to press.

One aim, three powers: 95 sails over, 20 stops short, 60 goes in.

Basic — add the meter 25 min

Open last lesson’s project. The Aim and Arrow sprites stay exactly as they are.

Step 1 — the Power sprite

Paint the wedge, tip on the centre cross.
Make power and step.
Build the swing from Learnt 1. Watch three swings.

Step 2 — the frame (optional, but worth it)

Paint the outline sprite. Colour the top band red.
Place it at the same x and y as Power.
Drag Power in front if the frame covers it.

Step 3 — SPACE to kick

Wrap the swing in repeat until SPACE.
Under it: save the aim, then broadcast kick and wait.


when flag clicked
set [power v] to (0)
set [step v] to (2)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to (-2)
    end
    if <(power) < (1)> then
      set [step v] to (2)
    end
    set size to (power) %
  end
  set [aim x v] to (mouse x)
  set [aim y v] to (mouse y)
  broadcast [kick v] and wait
end

The whole Power sprite. Fourteen blocks, the most a Level 2 stack should hold.

Step 4 — the ball listens

Delete the ball’s old wait until <mouse down?> loop.
Keep a small flag script: goals to 0, back to the spot.
Build the kick from Learnt 5, with the goals counter back in it.


when flag clicked
set [goals v] to (0)
go to x: (0) y: (-118)

when I receive [kick v]
if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end
glide (0.5) secs to x: (aim x) y: (aim y)
if <<<(aim x) > (-100)> and <(aim x) < (100)>> and <<(aim y) > (-25)> and <(aim y) < (85)>>> then
  change [goals v] by (1)
  say [GOAL!] for (1) seconds
else
  say [Miss!] for (1) seconds
end
go to x: (0) y: (-118)

The Ball’s two scripts.

Step 5 — play it

Ten kicks. Write down your goals. Then ten more, trying to beat it.

The meter grows down as well as up.

The wedge’s tip is not on the centre cross. Open the costume, select the whole drawing, and drag it until the tip sits on the cross.

Power climbs past 100 and never comes back.

The top test is = 100, or the step is not 2. Use <(power) > (99)>.

The meter sticks at 0.

step is 0. Check the flag script sets it to 2 before the loop.

One press of SPACE kicks twice.

You held SPACE through the whole kick, so the meter stopped again at once. Add wait until <not <key [space v] pressed?>> after the broadcast.

A click still kicks the ball.

The old wait until <mouse down?> loop is still on the ball, so two scripts are kicking it. Delete the old loop.

CHALLENGE 1 · L2-38-M1🪙 30 Adcoins8 min

A meter that swings

Power climbs to 100, falls to 0, and climbs again, for ever.
The meter grows up from its tip, never down.
The power box on the Stage never shows less than 0 or more than 100.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — up to 100, down to 0, for ever.

at the top

at the bottom

panels run left to right

Done when: you watch three full swings and the power box stays between 0 and 100 the whole time.

CHALLENGE 2 · L2-38-M2🪙 50 Adcoins9 min

Stop it and shoot

SPACE freezes the meter.
The ball flies to where the mouse was at that moment.
The meter waits until the ball is back on the spot.
Then it swings again.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — SPACE freezes the meter and the ball flies.

SPACE at 52

Done when: you stop the meter between 40 and 60 three times in a row, and each ball lands under the marker.

Teacher note

Ask what happens with a plain broadcast instead of broadcast and wait. Let them try it and press SPACE twice quickly. Two kicks at once, one ball, a mess. The fix is one word.

CHALLENGE 3 · L2-38-M3🪙 70 Adcoins10 min

Too hard, too soft

Power above 85 sails over the bar.
Power below 30 stops short of the goal line.
Anything in between flies where you aimed.
Goals count, and start at 0 each flag.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the middle of the meter is the only safe place.

power 60

Done when: you get all three — over the bar, short, and a goal — on purpose, in an order you choose before you start.

Teacher reveal — two ifs before the glide


if <(power) > (85)> then
  set [aim y v] to (150)
end
if <(power) < (30)> then
  set [aim y v] to (-70)
end

They change aim y and nothing else. A pupil who adds new tests to the goal check instead has built something that works but reads badly — worth showing both side by side.

A good follow-up: make the ball say Over the bar! or Too soft!. That needs the Ball to remember which rule fired — a third variable, or a say inside each if before the glide.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and click the Stage once. Aim with the mouse, then press the space bar to stop the meter and shoot.

Today’s game. Aim with the mouse, time it with SPACE.

EXTRA MISSION · L2-38-MX🪙 100 Adcoinsbonus

Faster every goal

A game should get harder as you get better. Make the meter swing faster after every goal.

WHAT IT SHOULD LOOK LIKE

Extra — every goal makes the next one harder to time.

goal 1 · step 2

goal 5 · step 7

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and click the Stage once. Aim with the mouse, press the space bar to shoot. Every goal makes the meter swing faster.

Score, and watch meter speed go up. The swing gets harder to catch.

Teacher reveal — the meter, reading meter speed


when flag clicked
set [power v] to (0)
set [step v] to (meter speed)
forever
  repeat until <key [space v] pressed?>
    change [power v] by (step)
    if <(power) > (99)> then
      set [step v] to ((0) - (meter speed))
    end
    if <(power) < (1)> then
      set [step v] to (meter speed)
    end
    set size to (power) %
  end
  set [aim x v] to (mouse x)
  set [aim y v] to (mouse y)
  broadcast [kick v] and wait
end

((0) - (meter speed)) is how Scratch writes “minus meter speed”. The Ball sets meter speed to 2 at the flag and adds 1 inside its GOAL half.

Summary 3 min

A number that swings needs two variables: the number, and its step.
Two ifs flip the step at the ends. Test with > and <, not =.
Scratch grows a sprite around its centre. Put the meter’s tip there.
repeat until <key [space v] pressed?> runs until a key stops it.
broadcast [kick v] and wait keeps the meter still until the kick is over.

Power: How hard the kick is, from 0 to 100.
Step: How much a number changes each time round a loop.
Swing: A number going up to a limit, then down to another, over and over.
Centre: The point a sprite turns and grows around. The cross in the paint editor.

Next lesson: the keeper stops standing still.