# 3 blocks no friction forces question

• cmwilli
In summary, the conversation discusses the calculation of forces acting on two blocks, one with a mass of 4 kg and the other with a mass of 8 kg. The equation F=ma is used to calculate the resultant force on the 4 kg block, which is found to be 5.143 N. The conversation also mentions a disagreement with the homework system and clarifies that the correct answer is 13.14268 N, with the 8 kg block exerting 13.14 N on the 4 kg block and vice versa.
cmwilli

## Homework Statement

[PLAIN]http://img834.imageshack.us/img834/6920/captureqt.jpg

f=ma

## The Attempt at a Solution

Fsum=Fl + Fr=(m1+m2+m3)a
26+-8=(2+8+4)a
a=18/14=1.28571

f=ma
f=4(1.28571)=5.14268N

I know this is incorrect. The force of 4kg block pushes on the 8kg the same as the 8kg pushes on the 4kg, but the way I calculated it doesn't work with that but I know of no other way.

Last edited by a moderator:
You are right that the resultant force acting on the 4 kg block is F=4a=5.143 N. See the figure: What forces act on this block?

ehild

The homework systems seems to disagree.

Call the blocks from left to right A,B,C

The way the teacher explained it in class, the force of C acting on B is the same as the force of B acting on C. But the way I calculated it Fc=4a but Fb=8a

Look at the 4 kg mass. There are two forces acting on it: the force from the 8kg mass (F8) ponting to the right, and Fr=8 N, pointing to the left. The resultant force points to the right, and it is F8-Fr=4*a. So F8=4a+Fr.

ehild

Is the correct answer then 13.14268N?

Yes, the 8 kg mass exerts 13.14 N on the 4 kg one, and a force of the same magnitude and opposite direction is exerted by the 4 kg mass on the 8 kg one.

ehild

## 1. What are the key components of the "3 blocks no friction forces question"?

The key components of this question are three blocks of varying masses that are connected by strings and being pulled by a force at one end. There is no friction between the blocks and the surface they are on.

## 2. How does the lack of friction impact the movement of the blocks?

Without friction, the blocks will continue to move in the direction of the applied force without any slowing down or change in direction. This is because there is no opposing force to counteract the applied force.

## 3. What is the relationship between the masses of the blocks and their acceleration?

The acceleration of each block is directly proportional to its mass. This means that the block with the largest mass will experience the least amount of acceleration, while the block with the smallest mass will experience the most acceleration.

## 4. Can the force applied to the blocks be increased to make them move faster?

No, the force applied to the blocks will not affect their speed as there is no friction to overcome. The blocks will continue to move at a constant speed in the direction of the applied force.

## 5. How does this question relate to Newton's Laws of Motion?

This question relates to Newton's First Law of Motion, which states that an object will remain at rest or in motion with a constant velocity unless acted upon by an external force. In this case, the blocks will continue to move at a constant velocity in the direction of the applied force unless acted upon by an external force, such as friction.

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