Find acceleration between two blocks (bottom block)

[isukatphysics69]

1. Homework Statement
Problem in pictures

Homework Equations

f=ma

The Attempt at a Solution

So the tension force is 20N, i took the frictional force on the bottom of the block as 3*9.8*.4 because the bottom of the block has a total mass of top block and bottom block = 3kg, so i have 20N - friction between top and bottom block - friction between bottom block and surface = 20N - 11.76N - 3.92N = 4.32N DIVIDED BY A TOTAL OF 3KG (TOP AND BOTTOM BLOCK) = 1.44 M/S^2.. why is this wrong?

 

[haruspex]

DIVIDED BY A TOTAL OF 3KG (TOP AND BOTTOM BLOCK)

Why?

 

[isukatphysics69]

Why?

because the total mass is 3kg, the 2kg block has a 1kg block on top. so if i pull with 20N i have to take into account the top block an additional 1kg

 

[Orodruin]

because the total mass is 3kg, the 2kg block has a 1kg block on top. so if i pull with 20N i have to take into account the top block an additional 1kg

Which system did you compute the forces on? What is the total mass of that system?

 

[isukatphysics69]

Which system did you compute the forces on? What is the total mass of that system?

hey i figured that problem out, do you understand ranking forces?

 

[isukatphysics69]

 

[isukatphysics69]

ahh i see one thing i did wrong here...

 

[isukatphysics69]

i never read the problems carefully because i am an idiot, i could have gotten a B on my calc 2 exam but i got a C because i was looking for volume rather than surface area, i didn't realize until i only had 20 minutes left for the exam and i ended up with a C

 

[isukatphysics69]

But this ranking forces stuff is still very confusing

 

[isukatphysics69]

Vertical forces, Force on block A from hand > force on block b from block A > force gravity on block A from Earth > force gravity on block b from Earth > force friction between both blocks that are equal

Incorrect

 

[haruspex]

View attachment 223106
Vertical forces, Force on block A from hand > force on block b from block A > force gravity on block A from Earth > force gravity on block b from Earth > force friction between both blocks that are equal

Incorrect

That last attempt is clearly wrong since you have FFBA > ... FFBA.
It might help if you start by writing out the usual force balance equations for each block.

 

[fresh_42]

@isukatphysics69 Please open new threads for new problems.

Thread closed.