Question

A small block of mass m1 is placed on a larger block of mass m2. The blocks are placed on an inclined plane with an angle theta. Suppose a known force of magnitude P is applied on that string so that m2 accelerates and m1 slides on m2. If the coefficient of kinetic friction between m1 and m2 is u(k1) and the coefficient of kinetic friction between the m2 and the table is u(kT) what will the acceleration of m2 be?

Answer 1

Ok so I am just unsure as to how block m1 effects the acceleration of block m2. I know that (m2)*a=P-(m2)*g*sin(theta)-(m2)*g*u(kT)*cos(theta)

Answer 2

I think I am supposed to subtract the frictional force of m1 because it acts between m1 and m2 but am not 100% sure. Also is there anything else to add?

Answer 3

The equation that you derived isn't right

Answer 4

try drawing a free body diagram for each block and then labelling all forces before writing down the equation

Answer 5

well that isn't all of the equation

Answer 6

that is just for the bottom block. I was wondering how forces on m1 will effect block 2

Answer 7

even if its for the bottom block, its still wrong.

Answer 8

think about it

Answer 9

if there wasn't a block on top of m2 then that would be the equation

Answer 10

oh

Answer 11

so my question is what other forces do I need to add to the equation because of m1 sliding on top of m2

Answer 12

so basically, the two extra forces on lower block will be the friction and normal force from upper block

Answer 13

with the friction directed in the opp. direction to that acting on upper block

Answer 14

ok yea I thought the frictional force was probably one. So I add the normal force too even though it acts perpendicular to the motion?

Answer 15

yes. only because its an inclined surface

Answer 16

but if you're considering the normal force, you can't write (m1+m2)a=force

Answer 17

ok so I am kind of confused. Let's pretend that the top block isn't on there. Why wouldn't I also include the normal force of block m2 in my above equation?

Answer 18

because its perpendicular to the direction of acceleration. but in this case the normal force of block 1 affects friction. it wouldn't however be included in the motion along plane

Answer 19

the hint is friction always opposes relative motion.