Physics help please ~~ Multiple choice?

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Boxes A and B in (Figure 1) both remain at rest.

Part A

Is the friction force on A larger than, smaller than, or equal to the friction force on B?

A) The friction force on A is larger than the friction force on B.

B) The friction force on A is smaller than the friction force on B.

C) The friction force on A is equal to the friction force on B.

https://session.masteringphysics.com/problemAsset/…

• C) The friction force on A is equal to the friction force on B

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You can do it in your head quickly once you understand the physics. But here is a full explanation …

Take g=10m/s².

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The normal reaction on A is Fₙ = mg = 20×10 = 200N

The limiting (maximum possible) frictional force for A is μₛFₙ = 0.4×200 = 80N

So A won’t start slipping unless 80N (or more) is applied horizontally.

The horizontal force being applied is 30N; this is less than the limiting frictional force, so A is not on the point of slipping.

The 30N being applied is being balanced by friction.

So the frictional force is 30N.

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The normal reaction on B is Fₙ = mg = 10×10 = 100N

The limiting (maximum possible) frictional force for B is μₛFₙ = 0.5×100 = 50N.

So B won’t start slipping unless 50N (or more) is applied horizontally.

The horizontal force being applied is 30N; this is less than the limiting frictional force, so B is not on the point of slipping.

The 30N being applied is being balanced by friction.

So the frictional force is also 30N.

Both frictional forces are equal (both 30N).

If that’s not clear, try the video in the link.

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