Action-Reaction Force for external forces

In summary, the conversation discusses the action-reaction forces between two boxes and the relevant equations, including Newton's Laws, used to find the acceleration of the boxes. The final conclusion is that the acceleration of both boxes is 2m/s^2 and the sum of the two forces acting on each box is 10N and -10N, respectively.
  • #1
Aryan Andaleeb
12
0

Homework Statement


upload_2017-3-4_1-25-52.png


1) Are F and F' as shown in figure, the action-reaction forces between the two boxes?

2) What is the action and reaction force between the two boxes?

Homework Equations

The Attempt at a Solution


I think F = 28N and F' = - 28 N Simply adding the action- reaction forces for each case of 4 N and 24 N external forces.
 
Last edited:
Physics news on Phys.org
  • #2
Aryan Andaleeb said:
1) Are F and F' as shown in figure, the action-reaction forces between the two boxes?
What do you think?
Aryan Andaleeb said:
2) What is the action and reaction force between the two boxes?
What kind of motion do you think these masses are undergoing? Are they moving, accelerating or what? Is there friction? Showing the full statement of the problem would be very helpful.
 
  • #3
The floor is friction less. and the boxes were at rest prior to force application. That's all.
 
  • #4
Aryan Andaleeb said:
The floor is friction less. and the boxes were at rest prior to force application. That's all.
OK, that's good. You left out the "Relevant equations" in the template. What equation(s) do you think are relevant here?
 
  • #5
Well I don't find a particular equation. It's just that I found out all the forces that can be acting using free-body diagram. And I made sum of forces
 
  • #6
Have you studied Newton's Laws?
 
  • #7
Yes
 
  • #8
Yes.
 
  • #9
Aryan Andaleeb said:
Yes.
Which ones of Newton's laws involve equations? Can you write them down? Do you know what they mean? They are very relevant to this problem.
 
  • #10
kuruman said:
Which ones of Newton's laws involve equations? Can you write them down? Do you know what they mean? They are very relevant to this problem.
For 1st law, it mathematically states,

if Fnet = 0 on any object, then it continues in its state of motion or rest foreverFor 2nd law,

F is proportional to rate of change of momentum (not only proportional, but also are equal. i.e. F =ma)

For 3rd law,

If A applies force F on B, B applies force F on A, (direction is opposite
 
  • #11
Good. More correctly the second law is ##\vec{F}_{net}=m \vec{a}##. The subscript "net" means that you have to add up all the forces and set the result equal to mass times acceleration. Also note that the net force is always in the same direction as the acceleration.

Knowing all this, can you find the acceleration of the blocks? Note that they move together as one which means that they have the same acceleration and velocity at all times.
 
  • #12
This way, it becomes that a = 2m.s^-2
 
  • #13
That is what I get
 
  • #14
Aryan Andaleeb said:
This way, it becomes that a = 2m.s^-2
OK. Now let's take one mass at a time. First the 3 kg mass. Can you write an expression showing the sum of the two forces acting on it? It must have two terms.
 
  • #15
Yes!

Let, 7kg mass be m1. 3kg mass be m2

For m2, we can write

$$ Fnet = Faction-reaction + Fapplied from right $$

$$ Or, 6 = Faction-reaction + (-4) $$

$$ Therefore, Faction-reaction = 10 N $$

Similarly, we get -10 N for the m1 using the strategy above !

Am I correct now?

THANK YOU SO SO MUCH! You really helped me a lot in this discussion.
 
  • #16
Aryan Andaleeb said:
Am I correct now?
You are correct. Good job. :smile:
 
  • Like
Likes PhanthomJay

What is an action-reaction force?

An action-reaction force is a pair of equal and opposite forces that occur when two objects interact with each other. This means that when one object exerts a force on another object, the second object will exert an equal and opposite force back on the first object.

How does Newton's third law of motion relate to action-reaction forces?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will exert an equal and opposite force back on the first object. This is known as the principle of action and reaction, and it is the basis for understanding action-reaction forces.

Can action-reaction forces cancel each other out?

No, action-reaction forces cannot cancel each other out. While they are equal and opposite, they are acting on different objects and therefore cannot cancel each other's effects. For example, if you push against a wall, the wall will push back on you with an equal and opposite force, but you will not move because the wall is anchored to the ground.

Can action-reaction forces exist without external forces?

No, action-reaction forces require external forces to exist. This is because action-reaction forces occur when two objects interact with each other, and these interactions are caused by external forces. For example, when you kick a ball, your foot exerts a force on the ball, and the ball exerts an equal and opposite force back on your foot. Both of these forces are external forces.

How can we calculate action-reaction forces?

To calculate action-reaction forces, we need to identify the two objects that are interacting and the external forces acting on them. Then, we can use Newton's third law of motion to determine the magnitude and direction of the action-reaction forces. Additionally, we can use Newton's second law of motion (F = ma) to calculate the forces based on the mass and acceleration of the objects.

Similar threads

  • Introductory Physics Homework Help
Replies
9
Views
829
  • Introductory Physics Homework Help
Replies
6
Views
1K
  • Introductory Physics Homework Help
Replies
1
Views
267
  • Introductory Physics Homework Help
Replies
5
Views
2K
  • Introductory Physics Homework Help
Replies
1
Views
927
  • Introductory Physics Homework Help
Replies
8
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
950
  • Introductory Physics Homework Help
Replies
10
Views
936
  • Introductory Physics Homework Help
Replies
3
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
4K
Back
Top