[michael7]

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

[Unknown008]

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.

[michael7]

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 )

[michael7]

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

[michael7]

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

[Unknown008]

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:



Now, F = ma.

The net force is: (F - 19.6)

F - 19.6 = 4(3)

And you'll get the answer :)