Three blocks are pushed up a frictionless plane that is inclined at an angle of 30 degrees with respect to the horizontal as shown below. The blocks move together with constant acceleration a = 3.0 m/s2. The masses of the blocks are: MA = MC = 1 kg, and MB = 2 kg.

M=Mass

The magnitude of the force that block A exerts on Block B is greater than the magnitude of the force that block B exerts on Block A.

TRUE
FALSE

Compare FAonB, the force Block A exerts on Block B, to FConB, the force Block C exerts on Block B,

FAonB < FConB
FAonB = FConB
FAonB > FConB

What is the value of F, the external force that is exerted on Block A?

F = 3.0 N
F = 4.9 N
F = 12.0 N
F = 31.6 N
F = 39.2 N

https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys100/fall10/hwb/06/01/fig4.gif

It would be good if you gave us what you got, what do you think is good. We'll then try to see how to correct things, so that you understand better.

So for the first question, I think it should be force because the magnitude should be the same since they are action-reaction pairs. For the second question, I think there is a force given by the force of gravity parallel to the ramp, so F (A on B )

Should be bigger than F (C on B). But I am not sure how I should do for the last question.

I know that the outside force should be 31.6N, but how can I get it, and what concept apply to this question

1. Right, hence, the answer is "false".

2. It's true that F(A on B) is bigger than F(C on B), but it's because the net force on B is such that B is accelerating up the plane. Hence, the force up the plane should be greater than the force down the plane, on B.

3. Use F = ma.

Treat the three blocks as a whole. The total mass is 2 + 1 + 1 = 4 kg.

The force acting down the plane is the component of the gravitational force acting down the plane. It is given by:

mg sin\theta = (4\times 9.8) sin(30) = 19.6 N

Now, F = ma.

The net force is: (F - 19.6)

F - 19.6 = 4(3)

And you'll get the answer :)