# What is Weight?

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Are you a force?

Before discussing what a force plate is and how it works, let’s back up the truck and remind you of some physics, more specifically Newtons third law, the Law of Action and Reaction, which states that for every action there is an equal and opposite reaction.

So let’s start with a person with a mass of say 70 kg, if you multiply this by the acceleration due to gravity, 9.81 m/s2 you get the persons weight, which is ~700 Newtons. Straight away you can see that your body weight is a force as it is measured in Newtons i.e. force = mass x acceleration.

Quiet standing you’re exerting a force into the ground of ~700 N called an active force and the ground is exerting an upward reactive force of an equal amount. So for quiet standing the action and reaction forces are equal.So these action and reaction forces are what the force plate measures when you stand, move over or jump on them. More specifically the force plates measure ground reaction forces (GRF).