A barrier

Counter balanced barriers can be found in many places, such as the one shown in Fig. 1-7. The weight fixed to the shorter arm counter balances much of the weight of the barrier arm allowing the barrier to be opened easily with a relatively small downward force near the counter balance weight.

The barrier can be opened when the moment induced by the counter balance weight and the applied downward force is larger than the moment induced by the weight of the barrier about the support point.