Question

A man uses a crowbar of length 1.5 m to raise a load of 75 kgf by putting a sharp edge below the bar at a distance 1 m from his hand. State the kind of lever and calculate the effort needed.

• A. Class II lever, 35 kgf
• B. Class I lever, 37.5 kgf
• C. Class III lever, 37.5 kgf
• D. Class I lever, 35 kgf

Answer 1

The correct option is B

Class I lever, 37.5 kgf

Given, length of the crowbar = 1.5 m
Length of effort arm, EF = 1 m
Load = 75 kgf

Considering the figure above,
Length of load arm, LF = Total length - Length of effort arm = 1.5 - 1 = 0.5 m

According to law of levers,
Mechanical advantage of a lever $=\frac{\text{Effort arm (EF)}}{\text{Load arm (LF)}}=\frac{1.0}{0.5}=2$

Also, $\text{Mechanical advantage}=\frac{\text{Load (L)}}{\text{Effort (E)}}$

$\Rightarrow \frac{75kgf}{\text{E}}=2$

$\Rightarrow \text{E}=\frac{75}{2}=37.5kgf$

It is a class I type lever as fulcrum (F) is between the effort (E) and the load (L).