One way in which forces can be represented is by the use of labelled arrows; for example look at this swimmer:
As you can see he propels himself through the water using his arms and legs, pushing the water out of the way which creates RESISTANCE to the forward motion in the form of friction.
These forces are represented by a THRUST arrow as the swimmer pushes himself forward and then a FRICTION arrow acting in the opposite direction, pushing against the swimmer.
Notice that the THRUST arrow is bigger than the FRICTION arrow, meaning that the swimmer must be speeding up as he is overcoming the water resistance acting against him. If the two arrows were the same size the forces would be equal, so he would be moving at a steady speed.
OK, use this idea in the questions on this worksheet.