Newton's Third Law: Two Blocks Situation 1 & 2

In summary, when the purple block is standing still on top of the green block with a force F, the net force on the green block is F plus the static friction of the purple block. In the second situation, when the static friction is replaced by kinetic friction, the net force on the green block is F minus the kinetic friction of the purple block. This is due to Newton's third law, which states that the friction forces on the blocks will be equal and opposite, and the friction will depend on the combined weights of the blocks. The arrows representing the friction force between the blocks should both point to the right, and the friction that the top block exerts on the bottom block will be to the left in both cases.
  • #1
Lisa...
189
0
Two blocks standing on oneanother are pulled with a force F. There's friction between them. With a certain force F the purple block is standing still on the green one (situation 1) because of static friction. Is the netto force working on the green block F+ static friction of the purple block (because Newtons third law implies the static friction on the purple block= the opposite static friction of the purple block on the green block)?

http://img363.imageshack.us/img363/3448/situations9qp.gif [Broken]

And in the second situation the static friction is overcome and replaced by kinetic friction. Is the netto force working on the green block now F- kinetic friction of the purple block (again thanks to Newtons third law)?
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
When the purple block does not move - there is no work done by the purple on the green block (static situtation, whereas as work is a product of force and distance). However, the purple block adds its weight to the green block, and if friction is involved between the green block and supporting surface, that is where the weight of the purple block will be manifest, i.e the friction depends on the combined weights of the purple and green blocks.

In the second situation, the Fk acts on the purple block accelerating it to the right, but it acts oppositely on the green block, opposing the motion of the green block. Again, friction between the green block and its supporting surface would still involve the combined weight of the purple and green block.
 
  • #3
I don't understand how you've drawn your arrows representing the friction force between the blocks. Since the friction arrows are on the top block, I assume you mean to show the friction acting on that top block. If so, those friction arrows should point to the right in both cases.

And, from Newton's 3rd law, you realize that the top and bottom blocks will exert equal and opposite friction forces on each other. Thus the friction that the top block exerts on the bottom block will be to the left in both cases.
 

1. What is Newton's Third Law?

Newton's Third Law, also known as the Law of Action and Reaction, states that for every action, there is an equal and opposite reaction.

2. How does Newton's Third Law apply to the Two Blocks Situation 1 & 2?

In the Two Blocks Situation 1 & 2, the force applied by one block on another block is equal and opposite to the force applied by the second block on the first block. This is an example of Newton's Third Law in action.

3. Why is Newton's Third Law important?

Newton's Third Law is important because it helps us understand the fundamental relationship between forces. It explains how objects interact with each other and how forces are always paired in equal and opposite directions.

4. What are some real-life examples of Newton's Third Law?

Some examples of Newton's Third Law in everyday life include pushing off the ground to jump, the recoil of a gun when fired, and the movement of a boat when the oars push against the water.

5. Can Newton's Third Law be applied to non-contact forces?

Yes, Newton's Third Law can be applied to non-contact forces, such as gravity and magnetism. For example, the gravitational force between two objects is always equal and opposite, as stated in Newton's Third Law.

Similar threads

  • Introductory Physics Homework Help
Replies
13
Views
861
  • Introductory Physics Homework Help
Replies
16
Views
1K
  • Introductory Physics Homework Help
Replies
17
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
423
  • Introductory Physics Homework Help
Replies
23
Views
1K
  • Introductory Physics Homework Help
Replies
6
Views
1K
  • Introductory Physics Homework Help
Replies
14
Views
325
  • Introductory Physics Homework Help
Replies
15
Views
2K
  • Introductory Physics Homework Help
2
Replies
45
Views
5K
  • Introductory Physics Homework Help
Replies
6
Views
1K
Back
Top