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# Jump from the ground
Arcade advanced skills · Platforms
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## Today
**We are learning to** put the jump inside an if that asks whether the player is on the floor, so it only works from the ground.
- I can make a jump that only works from the ground.
- I can explain why a jump needs that check.
- I can build a runner with gravity, a floor, a jump and obstacles.
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## Why it matters
> In every platformer you can only jump when your feet are on something. Without that one rule, the player could climb the sky.
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💬 Discussion
## Predict
**You press A three times, quickly, one press after another. What does the player do?**
- Jumps three times, landing between each
- Jumps once; the other presses do nothing
- Jumps higher and higher, and can fly off the top of the screen
- Falls through the floor
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✅ Check
## What happened?
* Each press sets vy to -150 again, even in mid-air, so the player keeps rising. Nothing checks whether the player is on the ground.
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✍️ Activity
## Run it
Open the lesson at this step and try it together.
[studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-jump/?full=step-2#step-2](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-jump/?full=step-2#step-2)
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💬 Discussion
## Investigate
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✅ Check
## Leo's jump
**Leo's player flies off the top when he presses A again and again. Why?**
- Nothing checks the player is on the ground, so every press jumps again, even in mid-air
- ay should be negative
- The floor pushes him up
- -150 is too big a number
* Answer: Nothing checks the player is on the ground, so every press jumps again, even in mid-air
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✅ Check
## When does A jump?
**This version jumps only if player bottom ≥ 100. Fill in what a press of A does.**
| player bottom when A is pressed | A does |
|---|---|
| 100 (on the floor) | ? |
| 70 (rising) | ? |
| 95 (falling, nearly down) | ? |
* Answer: jumps, nothing, nothing
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✅ Check
## Why 100?
**Why is the floor's bottom exactly 100 when the player is standing?**
- 100 is the bottom of the screen
- The jump sets it
- The floor if sets it back to 100 every frame the player sinks below it
- Gravity stops at 100
* Answer: The floor if sets it back to 100 every frame the player sinks below it
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✍️ Activity
## Feet on the floor (3 min)
Students jump on the spot. Ask: can you jump again while you are in the air?
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✍️ Activity
## Modify
- **No flying:** This is Leo's game. Fix it: **A** only jumps when the player is **on the ground**.
- **Moon jump:** Change the jump speed and the gravity. Pick the pair you like best.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-jump/?full=step-4#step-4)
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💬 Discussion
## Investigate: Mix it up
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✅ Check
## Mix it up
**A sprite starts still with ay 200. What is its vy after 1.5 seconds?**
* Answer: 300
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✍️ Activity
## Make
Make a **runner**. The player stays on the left; the world comes to them:
- the player has **gravity** and stands on a **floor** at y 100,
- **A** jumps, but only from the ground,
- a **rock** slides in from the right every 1.5 seconds,
- a hit costs a **life** (three lives, then game over).
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-jump/?full=step-6#step-6)
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✍️ Activity
## Change a kangaroo
This kangaroo hops across a field, but only from the ground. Make it a moon kangaroo: weaker gravity and a gentler jump. Can it still only jump from the ground?
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-jump/?full=step-7#step-7)
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✍️ Activity
## Runner race (4 min)
Play the next pair's runner. How many rocks can you jump before you lose?
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✅ Check
## Exit ticket
**Why does a jump need an if that checks the player is on the ground?**
* A jump only sets vy upwards. Without the check, every press of A jumps again, even in mid-air, so the player can fly.