Question

A patient is in severe pains and has asked you to help him raise his leg, as indicated in the diagram below. Use the diagram to answer the following questions.

Resistance Resistance Force Force Resistance arm Resistance arm RA FA Force arm RA Force arm FA A B

A. Which part of the leg would you hold? Would it be as in ‘A’ or ‘B’ in the figure?

B. Justify your choice

C.  What class of lever is the diagrams and why?

D.  Assuming force arm in diagram ‘A’ is 25 cm and the resistance arm is 15 cm, what is the mechanical advantage?

E. Assuming resistance arm in diagram ‘B’ is 15 cm, what is the force arm if the mechanical advantage is 3.6

F.  What is mechanical advantage of a system and what does the mechanical advantage of 3.6 imply?

Answer 1

A. I will hold the leg as shown in figure B.

B. In figure B, the Force arm is larger than that in figure A. So the force needed to hold the leg is smaller in case of figure B.

C. Here, the pivot point or the fulcrum is the knee, along which the leg rotates. Now, as the resistance arm is smaller than the force arm, and the resistance force act in between the fulcrum and the force, this is a class 2 lever.

Force arm (b) Resistance arm (a) Force (F) Fulcrum Resistance (R)

D. The mechanical advantage is given as

R b ΜΑ : F a

Here, R is the resistance, F is the force, b is the length of force arm, and a is the length of resistance arm.

Here,

The mechanical advantage in diagram 'A' is 1.67

E. Now it is given that, MA=3.6

So,

b 3.6 = 15

So the length of the force arm is 54 cm.

F. The mechanical advantage is the measure of the application of force in a mechanical system. A mechanical advantage of 3.6 means that, it needs 3.6 times less force to achieve the desired output of the machine.