When the cart is at rest at the top of the ramp, its velocity is zero. As it starts to roll down the ramp, its velocity increases. This is because the force of gravity is pulling the cart down the ramp, causing it to accelerate. The acceleration of the cart is constant, so its velocity will increase at a constant rate.

At any given instant, the cart's velocity can be calculated by using the following equation:

```

v = u + at

```

where:

* v is the final velocity of the cart in meters per second (m/s)

* u is the initial velocity of the cart in meters per second (m/s)

* a is the acceleration of the cart in meters per second squared (m/s²)

* t is the time in seconds (s)

In the case of a cart rolling down a ramp, the initial velocity is zero and the acceleration is due to gravity. The acceleration due to gravity is approximately 9.8 meters per second squared (9.8 m/s²).

So, if a cart is rolling down a ramp, its velocity will increase at a constant rate of 9.8 meters per second squared (9.8 m/s²).