Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.6

effort

load

fulcrum

Figure 12.20: Class 2 levers

Class 3 levers
In class 3 levers the fulcrum is also at the end ofthe bar, but the effort isbetween the fulcrum and the
load. An example of this type of lever is the human arm.

effort

load

fulcrum

Figure 12.21: Class 3 levers

Exercise 12 - 11

1. Riyaad applies a force of 120 N on a spanner to undo a nut.

(a) Calculate the moment of the force if he applies the force 0,15 m fromthe bolt.

(b) The nut does not turn, so Riyaad moves his hand to the end of the spanner and applies the

same force 0,2 m awayfrom the bolt. Now thenut begins to move. Calculate the force. Is

it bigger or smaller thanbefore?

(c) Once the nuts startsto turn, the moment needed to turn it is less than it was to start it

turning. It is now 20 N·m. Calculate the new moment of force that Riyaad now needs to

apply 0,2 m away fromthe nut.

2. Calculate the clockwise and anticlockwise moments of force in thefigure below to see if the
   see-saw is balanced.

� �

1,5 m 3 m

900 N 450 N

3. Jeffrey uses a force of 390 N to lift a load of130 kg.