Question

Match the block diagrams of block of mass (m) with their corresponding free body diagram (every surface is frictionless)

(bodies are at rest with respect to ground)

• A. P = (ii); Q - (ii); R - (iii)
• B. P = (iii); Q - (iii); R - (iii)
• C. P = (iv); Q - (i); R - (iii)
• D. P = (iii); Q - (ii); R - (iii)

Answer 1

The correct option is D

P = (iii); Q - (ii); R - (iii)

For surface 2 to apply a force on the object, the object must apply a force on the wall. Also if surface 2 applied a force on the object, then its Free Body Diagram would be,

$N_2$ will have a horizontal component that will not be balanced by any force.

$\therefore$ net force will non zero and the body cannot be at rest.

$\therefore$ correct Free Body Diagram is

(q) In this case, the object will be applying a force on both walls as it is pulled down by gravity and is at rest.

If only wall 1 were applying a force

Free Body Diagram $\to$

N, will have an unbalanced horizontal component.

Similarly, if only wall 2 were applying a force, then,

Which is unbalanced?

This both have to apply a force

(r) This is similar to case (p). if object 2 applied a force on m, then Free Body Diagram will be

This is again not possible as $N_2$ will then have an unbalanced component.

So (iii) is the right free body diagram as well in this case.