Question

A plank is supported on the steps of a staircase as
shown in the figure. How many forces are acting on
the plank?

Answer 1

This is a question asked in NSO class 9.

The correct diagram has been attached.

To find:

How many forces are acting on the plank?

Calculation:

First of all, we need to draw the FREE BODY DIAGRAM of the plank placed on the staircase as attached in 2nd diagram.

• Here , we can see that the plank is in contact with three edges of the staircase, hence it will experience 3 normal reaction forces namely N1, N2 and N3.

• Also, the weight of the plank (W = mg) will be directed downwards.

• Also, due to the normal reaction forces, the plank is going to experience three frictional forces which will prevent it from sliding down namely f1, f2 and f3. It is marked in 3rd diagram.

So, total forces are :

• N1
• N2
• N3
• mg
• f1
• f2
• f3

$\boxed{ \bf \:7 \: forces \: in \: total \: }$

Answer 2

Answer:

7 forces

Explanation:

3 gravitational

3 frictional

1 by its weight