11.13 Torque and levers

The mechanical advantage is:

Since mechanical advantage is a ratio, it does not have any units.

Pulleys

Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. In addition, pulleys can be “added together” to create mechanical advantage, by having the flexible material looped over several pulleys in turn. More loops and pulleys increases the mechanical advantage.

Classes of levers

Class 1 levers

In a class 1 lever the fulcrum is between the effort and the load. Examples of class 1 levers are the seesaw, crowbar and equal-arm balance. The mechanical advantage of a class 1 lever can be increased by moving the fulcrum closer to the load.

Class 2 levers

In class 2 levers the fulcrum is at the one end of the bar, with the load closer to the fulcrum and the effort on the other end of bar. The mechanical advantage of this type of lever can be increased by increasing the length of the bar. A bottle opener or wheel barrow are examples of class 2 levers.

Class 3 levers

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

Exercise

1. Riyaad applies a force of 120 N on a spanner to undo a nut.
   1. Calculate the moment of the force if he applies the force 0,15 m from the bolt.
   2. The nut does not turn, so Riyaad moves his hand to the end of the spanner and applies the same force 0,2 m away from the bolt. Now the nut begins to move. Calculate the force. Is it bigger or smaller than before?
   3. Once the nuts starts to 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 from the nut.
2. Calculate the clockwise and anticlockwise moments of force in the figure below to see if the see-saw is balanced.

   

3. Jeffrey uses a force of 390 N to lift a load of 130 kg.

   

   1. Calculate the mechanical advantage of the lever that he is using.
   2. What type of lever is he using? Give a reason for your answer.
   3. If the force is applied 1 m from the pivot, calculate the maximum distance between the pivot and the load.
4. A crowbar is used to lift a box of weight 400 N. The box is placed 75 cm from the pivot. A crow bar is a class 1 lever.
   1. Why is a crowbar a class 1 lever? Draw a diagram to explain your answer.
   2. What force F needs to be applied at a distance of 1,25 m from the pivot to balance the crowbar?
   3. If force F was applied at a distance of 2 m, what would the magnitude of F be?
5. A wheelbarrow is used to carry a load of 200 N. The load is 40 cm from the pivot and the force F is applied at a distance of 1,2 m from the pivot.
   1. What type of lever is a wheelbarrow?
   2. Calculate the force F that needs to be applied to lift the load.
6. The bolts holding a car wheel in place is tightened to a torque of 90 N $·$ m. The mechanic has two spanners to undo the bolts, one with a length of 20 cm and one with a length of 30 cm. Which spanner should he use? Give a reason for your answer by showing calculations and explaining them.