What is the Correct Concept for Solving the Stacked Blocks Problem?

[Arun Raja]

Homework Statement

Homework Equations

F= ma[/B]

The Attempt at a Solution

X does not slip. so friction exists. Friction should be equal to the force F applied so that according to Newton's 3rd law, the friction and F cancel out and X remains on Y.
Is this the right concept?

 

[ehild]

Homework Statement

Homework Equations

F= ma[/B]

The Attempt at a Solution

X does not slip. so friction exists. Friction should be equal to the force F applied so that according to Newton's 3rd law, the friction and F cancel out and X remains on Y.
Is this the right concept?

No. F is applied on Y. The friction between X and Y acts also on X. So the net horizontal force exerted on Y is F-friction. If F and the friction cancel out Y does not accelerate. As X moves together with Y the whole system stays in rest or moves without acceleration. But the problem stated that it accelerates.
X does not slip on Y means that they move together, with the same acceleration.
ehild

 

[Simon Bridge]

That would be fine if the boxes moved at a constant speed ... but they don't.
Instead, draw a free body diagram for each block.