The ground reaction force is the result of two forces acting together on the foot from the location of the ground. First, the normal force acting in the vertical direction keeps the body up. The other force is friction, which pushes the foot forward.[br][br]Pushing harder on the ground leads to an increase in the normal force, which in turn allows for a better grip (i.e. a larger friction force). How slippery the ground is also affects the nature of the friction force, as a more slippery surface (or a lesser grip) will reduce the amount of frictional force. The ratio between the frictional force and the normal force is called the [i]coefficient of friction [math]\mu[/math]:[br][br][math]\mu=\frac{f}{N}[/math].[/i][br][br]There are two kinds of frictional forces: [i]static friction[/i] and [i]kinetic friction[/i]. Static friction occurs when the two surfaces (i.e. the foot and the ground) are not sliding against each other. Kinetic friction occurs when they do slide against each other.[br][br]In the human gait, both kinds of friction act on the foot at different times. The two separate coefficients of friction ([math]\mu_s[/math] for static, [math]\mu_k[/math] for kinetic), will also vary. In this simulation, the coefficient of friction is simply the [i]upper bound[/i] of all coefficients of friction.[br][br]The goal of the simulation is to identify the components of the GRF and its equal and opposite counterpart which acts on the ground itself. Click and drag the text labels using the blue dots next to them. Move them to the heads of the vectors that represent the different forces acting on the foot or the ground. In the diagram, red arrows act on the foot and blue arrows act on the ground. Click "submit" to see if you have identified them correctly. Click "reset" to move them back to the default positions.

Created by Lewis Hicks and Priya Jamkhedkar for the Portland State University Physics Department, 2021.