# Documentation

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# der

Return time derivative of operand

## Syntax

`der(x)x.der`

## Description

The `equations` section may contain `der` operator, which returns the time derivative of its operand:

`der(x)` = `x.der` = $\stackrel{˙}{x}$= $\frac{dx}{dt}$

`der` operator takes any numerical expression as its argument:

• `der` applied to expressions that are continuous returns their time derivative

• `der` applied to `time` argument returns 1

• `der` applied to expressions that are parametric or constant returns 0

• `der` applied to countable operands returns 0. For example, `der(a<b)` returns 0 even if `a` and `b` are variables.

The return unit of `der` is the unit of its operand divided by seconds.

You can nest `der` operators to specify higher order derivatives. For example, `der(der(x))`is the second order time derivative of x.

The following restrictions apply:

• You cannot form nonlinear expressions of the output from `der`. For example, `der(x)*der(x)` would produce an error because this is no longer a linearly implicit system.

• For a component to compile, the number of differential equations should equal the number of differential variables.

## Examples

This example shows implementation for a simple dynamic system:

`$\stackrel{˙}{x}=1-x$`

The Simscape™ file looks as follows:

```component MyDynamicSystem variables x = 0; end equations x.der == (1 - x)*{ 1, '1/s' }; % x' = 1 - x end end ```

The reason you need to multiply by `{ 1, '1/s' }` is that `(1-x)` is unitless, while the left-hand side (`x.der`) has the units of 1/s. Both sides of the equation statement must have the same units.

`equations`