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A block of mass m is placed on a smooth inclined wedge ABC of inclination θ as shown in the figure. The wedge is given an acceleration a towards the right. The relation between a and θ for the block to remain stationary on the wedge is

Asked in NEET 2018 · Accelerating wedges

Figure: Accelerating wedges
Answer: (4) a=g tan θ

Step-by-step solution

Given: block on a smooth wedge of angle θ, the wedge pushed horizontally with acceleration a.

Idea: work in the wedge's frame, where the block is at rest and feels a pseudo-force ma backwards, and balance forces along the slope, where the normal force has no component.

Along the slope, gravity pulls the block down with mg sin θ.

Along the slope, the pseudo-force pushes it up with ma cos θ.

For the block to stay put these must match: ma cos θ=mg sin θ.

The mass and cos θ cancel: a=g(sin θ)/(cos θ).

a=g tan θ.

Why the other options are wrong

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