# FreeBody

> A block with forces (Force::new(name, x, y), in newtons, y up) drawn from its middle, scale units per newton.

Page: https://fframes-kit.pages.dev/docs/physics/free-body/
Cargo feature: `physics` (`fframes-kit = { version = "..", features = ["physics"] }`)

A block with `forces` (`Force::new(name, x, y)`, in newtons, `y` up) drawn from its middle,
`scale` units per newton. The arrows grow one after another, each labelled at its tip with
its name and magnitude. Then the other forces dim and their sum grows as a dashed arrow
labelled `net = … N`, or `net = 0` under the block when they balance. `net_force()` gives
the sum. Gallery scene: `FreeBody`.

Gallery clip of `FreeBody`: https://fframes-kit.pages.dev/media/scenes/FreeBody-f1733dad.mp4

```rust
use fframes_kit::prelude::*;

fn figure<'a>(ui: &mut Ui<'a, '_, '_>) -> Svgr<'a> {
    ui.show(FreeBody {
        forces: vec![
            Force::new("W", 0.0, -5.0),
            Force::new("N", 0.0, 5.0),
            Force::new("F", 6.0, 0.0),
            Force::new("f", -2.0, 0.0),
        ],
        center: (0.0, -0.4),
        scale: 0.45,
        delay: 0.5,
        ..ui.defaults()
    })
}
```

## API

Components are plain structs. Set the fields you need and fill the rest with `..ui.defaults()`: the defaults below come from the current theme (`theme.` values are fields of `Theme`).

### FreeBody

| Field | Type | Default | Description |
| --- | --- | --- | --- |
| `forces` | `Vec<Force>` | `Vec::new()` | The forces on the block, each with a name and components in newtons (`y` up). |
| `center` | `(f64, f64)` | `(0.0, 0.0)` | The middle of the block, where the arrows start, in units of `ui.space`. |
| `scale` | `f64` | `0.5` | Units of `ui.space` per newton. |
| `size` | `f32` | `160.0` | Side of the square block in pixels. |
| `delay` | `f32` | `0.0` | Seconds after the local start before the first arrow starts to grow. |
| `grow` | `f32` | `0.6` | Seconds each arrow takes to grow. |
| `stagger` | `f32` | `0.5` | Seconds between the start of one arrow and the next. |
| `net` | `bool` | `true` | Draws the sum of the forces as a dashed arrow after the others, or `net = 0` under the block when they balance. |
| `net_name` | `&str` | `"net"` | The name in the net force's label, "net = 4 N". |
| `labels` | `bool` | `true` | Writes each force's name at its tip. |
| `magnitudes` | `bool` | `true` | Adds the magnitude in newtons to each label. |
| `block_color` | `Color` | `theme.colors.muted` | Colour of the block. |
| `force_color` | `Color` | `theme.colors.accent` | Colour of the force arrows that have no colour of their own. |
| `net_color` | `Color` | `theme.colors.highlight` | Colour of the net force arrow and its label. |
| `text_color` | `Color` | `theme.colors.text` | Colour of the force labels; the net force's label takes `net_color`. |
| `stroke_width` | `f32` | `theme.stroke.regular` | Width of the arrow shafts in pixels. |
| `enter` | `Option<EnterAnim>` | `None` | How the block draws in; `None` shows it from the start. |
| `exit` | `Option<ExitAnim>` | `None` | How the block leaves at the end of the container; `None` stays. |
| `parts_enter` | `Option<FadeIn>` | `None` | How the arrows and labels fade in; `None` for none. |
| `parts_exit` | `Option<FadeOut>` | `None` | How the arrows and labels fade out before the end; `None` for none. |

Methods:

- `fn net_force(&self) -> (f64, f64)`: The sum of the forces in newtons.
- `fn start_of(&self, i: usize) -> f32`: Seconds after the local start when force `i` starts to grow; `forces.len()` gives the start of the net force.

Feature `physics` · [Source](https://github.com/khanhthanhdev/fframes-kit/blob/main/fframes-kit/src/kits/physics/free_body.rs#L52)