Basic Forces and Motion Problem (friction)

[Nragis]

1. Homework Statement
You have two masses moving across a frictionless surface, yet there is friction between the two masses. Mass 2 is on top of Mass 1 A force is applied to m1 which causes It to accelerate at 6 m/s² and the friction between the two masses causes m2 to accelerate at 4 m/s²

What is F_f? F_push? F_net,x?

m1 = 100kg
a1 = 6 m/s²
m2 = 20kg
a2= 4 m/s²

Homework Equations

F = ma
Newton’s third law

The Attempt at a Solution

Both my teacher and my classmates told me that this was wrong and you just plug the formula F_push = (100kg+20kg)(6m/s) = F_net,x, but this didn't make sense because the frictional force on m2 had to come from m1, so it was subtracted (added to opposite direction) of m1

 

[haruspex]

1. Homework Statement
You have two masses moving across a frictionless surface, yet there is friction between the two masses. Mass 2 is on top of Mass 1 A force is applied to m1 which causes It to accelerate at 6 m/s² and the friction between the two masses causes m2 to accelerate at 4 m/s²

What is F_f? F_push? F_net,x?

m1 = 100kg
a1 = 6 m/s²
m2 = 20kg
a2= 4 m/s²

Homework Equations

F = ma
Newton’s third law

The Attempt at a Solution

Both my teacher and my classmates told me that this was wrong and you just plug the formula F_push = (100kg+20kg)(6m/s) = F_net,x, but this didn't make sense because the frictional force on m2 had to come from m1, so it was subtracted (added to opposite direction) of m1

View attachment 213714

You are correct. The applied force is 680N.
You could try asking your classmates: what if there were no friction between the blocks?

 

[Nragis]

Okay, thanks for the reply. I just didn’t want to bring it up further if I was actually wrong