Acceleration of two different blocks.

Yeah which means there is no motion. Our teacher mentioned this question.

Its a really harsh question, wouldn't be out of place on M3 apparently.

Oh, cool. I'm doing AS Level and my teacher gave it as an M1 question. Well, THIS is really a harsh question. I hope anyone here can help me...

Could anyone please help me with the last part? I just want to know the way it can be solved. My equations, etc lead me to deceleration of one of the blocks.

It is M1, it was on my paper.

Yeh, it was on my paper last year... you should end up with a deceleration value for the bottom if you used F=ma (it is wrong obviously!). For the upper block.. if my memory serves me right it should have an acceleration of 4. something or another possibly?

For the bottom block, from previous parts in the question you should have worked out friction and various co-efficients of frictions. The bottom block will have 2 frictional forces acting on it - one from the top block and another from the surface. If you work out what these are correctly and add them you should find that it exceeds the 4N force being applied to it and so we can deduce that this bottom block will never move. With the friction force being greater than the 4N, it is why you ended up with a deceleration (an acceleration in the opposite direction), due to the friction, but since the bottom box isn't moving, it's absurd to think that the friction would cause the box to move, this is because friction (assuming the box is not moving and is not in limiting equilibrium) only ever provides what is "needed" to oppose the force pulling it forward. Long story short, the bottom box will never move.

Which exam board and which year, please?

Yay! I also get the acceleration of upper block as 4.09 ms^-2. Plus I get negative acceleration of the lower block. Does that mean that my answers are right?

OCR June 2010


That sounds right but there's probably a mark for explaining that the bottom box doesn't move.