Name: _____________________ Class: Physics 214
SSN/ID:   _____________________ Section & Group: ____________
Lab 6 - Levers and Pulleys

Objective
The objective of this lab is to show the operation of simple machines - a lever (Part A) and fixed & moveable pulleys (Part B) in several configurations.

Equipment
See the Java applets below.

Definition
To achieve rotational motion about an axis, it is necessary to apply a force at some point which is at a distance d from the fulcrum (or the center of the rotation). The distance d is called the lever arm. The product of the force times the distance is called the torque.

Simple machines like levers and pulleys exploit the ideas of torque and work. For example, if you have a can of soda that weighs 10-N and you try to lift it straight up, you have to exert exactly that amount of force: 10-N. In this case, your mechanical advantage, the ratio of the weight you lift divided by the force you exert in lifting it, is 1 (10N/10N).

But, if you use a lever or a pulley, you can lift it using only 5-N of force (the trade-off, from our discussion of work, is of course the distance over which the smaller force acts). In this case, your mechanical advantage is (weight_lifted) / (force_exerted) = 10N/5N = 2.

Procedure

Part A: Pulleys

URL: http://home.a-city.de/walter.fendt/phe/pulleysystem.htm

  1. Starting with the 2-pulley system:
    1. Choose a weight, G, that you want to lift
    2. Set G' = 3-N (this is the weight of the loose pulleys) and hit Enter
    3. Record F
    4. Set G' = 1-N and hit Enter
    5. Record F
  2. Repeat #1 (above) for the 4-pulley and the 6-pulley system
  3. Figure out the mass of the weight, G, you set in #1 above (please note: g = 9.8-m/s2)
  4. Calculate the Mechanical Advantage for each of the 3 configurations above where 
    
                            weight_lifted
    Mechanical_Advantage = ---------------
                              Fexerted
  5. Calculate the efficiency of the system for each of the 3 configurations above where 
    
                  Mechanical_Advantage(calculated)
    efficiency = -------------------------------------
                  Mechanical_Advantage(theoretical)
    and where Mechanical_Advantage(theoretical) = number of pulleys = number of supporting strings.
You can raise or lower the load with the mouse. If you click on the mouse button, a spring balance will appear showing the tension in the string. You can change the weight of the load and the hanging pulley(s) by using the appropriate boxes. Inputs higher than the spring scale limit (10 N) are automatically changed.

# of Pulleys Weight G [N] Weight G' [N] Force F [N] massG [kg] Mechanical Advantage Efficiency
2 Pulleys   3 N        
2 Pulleys   1 N        
4 Pulleys   3 N        
4 Pulleys   1 N        
6 Pulleys   3 N        
6 Pulleys   1 N        

Part B: Lever

URL: http://home.a-city.de/walter.fendt/physengl/lever.htm

  1. Hang a 3-N weight at a distance of 0.1-m
  2. How much weight (and at what distance) do you have to add to the other side to balance it?
  3. Repeat #2 for 3-N at a distance of 0.3-m
  4. Repeat #2 for 3-N at a distance of 0.8-m
  5. Compute the Mechanical Advantage for each of the 3 measurements where 
    
                                weight_to_be_balanced (3N)
    Mechanical_Advantage = --------------------------------------
                            weight_added_to_balance (Fbalancing)
    and compare this number to the ratio of the two distances (1st distance = where the weight is located; 2nd distance = where the Force is applied, or weight is added, to balance the lever)

This applet shows a symmetrical lever with some mass pieces each of which has a weight of 1.0 N. The lever arms can be read from the coloured rectangles; one rectangle corresponds to 0.10 m. The lever is in balance when the applet is started.

You can attach a new mass piece or put it to another place with pressed mouse button. In a similar way you can remove a mass piece by clicking on it.

A lever is in balance if the total left side torque is equal to the total right side torque.

To Balance: Add Weight [N] Place at Distance [m] Mechanical Advantage
3-N at 0.1-m      
3-N at 0.3-m      
3-N at 0.8-m      

Questions

  1. Is the efficiency (that you computed in Part A) what you'd expect? Why or why not?

Notes

  1. In Part A, As you go from G'=3-N to G'=1-N, the calculated Mechanical Advantage approaches closer and closer to the theoretical Mechanical Advantage; i.e., the efficiency gets closer and closer to 1.
  2. In Part B, you can add or remove weights simply by clicking on the applet with the mouse button (you can remove the weights by dragging them off the edge of the screen).
  3. In Part B, if you don't add the weight in one, single column, then you need to compute the average distance in step #5 (where you compute the ratio of the distances).