Second and third class levers

• Second and third-class levers

As a first step, before hanging masses on the balance, perform the so-called “tare-operation”. In the general theory of levers, the balance is hypothetically massless; in our everyday experience, however, we must take into account the weight of the balance and add a counterweight to find equilibrium.

This operation becomes extremely important when the use of unequal-arm balances is required. The difference in the arms’ length characterizing these balances, in fact, makes the tare operation necessary to perform the experiment in the right way.

The equilibrium position is determined by setting to zero the resultant of external momentum of forces. Once the lever fulcrum is fixed you just have to measure masses and their position with respect to the pivot.

Since the resistance is between the fulcrum and the power, this configuration realizes a second class lever. If we built another configuration, with power and resistance positions exchanged, we would obtain a third class lever.

The gardener uses a wheel barrow to lift tools and garden waste. The load is in the centre of the barrow.

The fisherman catches the fish which becomes the load at the end of the lever.

• Rod diam. 3 x length 72.5 with cylinder — 4861.10-020
• Pulley with hook — 4112.10
• Rod diam. 10 x length 127 with hook — 4113.53
• Polyester inelastic cord — 4113.20
• Cylindrical mass 50, 25 and 10g with hook — 4111.08-10-12
• Multifunction support rod, rounded ends, with holes, length 35 mm — 5408.81-L350
• Lever holed rod with pair of threaded cylinders — 4114.13

• What is the total force momentum at equilibrium?
• What is the physical meaning of the sensitivity of a balance?