# What causes gas particles to accelerate so that P = F / A

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1. Dec 15, 2012

### vanmaiden

Dear Physics Forums,

Pressure is force / area, and force is mass * acceleration. When you have a gas in a container, it's said to exert a pressure on its container. Therefore, the particles are accelerating toward the container's walls. What's causing these gas particles to accelerate toward the container? Why can't pressure be momentum / area? Momentum, like force, has a tendency to transfer energy as well to a stationary object (the container's walls).

Thank you for addressing my confusion,

Vanmaiden

2. Dec 15, 2012

### Studiot

You are nearly there.

Force = rate of change of momentum.

When a gas particle collides with a container wall it is deflected, suffering a change of momentum (vector change).

This is experienced as a force and a reaction.

The pressure is the aggregate of many such collisions, taken over area and time.

3. Dec 19, 2012

### Philip Wood

Studiot has it. Where you were getting confused is in demanding that the molecule must be accelerating towards the wall. [For an ideal gas we ignore any such effect.] The acceleration occurs when the molecule hits the wall and changes its direction, and therefore its velocity.

4. Dec 19, 2012

### Studiot

Excellent point.